The chicken coloboma mutation exhibits features similar to human congenital developmental malformations such as ocular coloboma, cleft-palate, dwarfism, and polydactyly. The coloboma-associated region and encoded genes were investigated using advanced genomic, genetic, and gene expression technologies. Initially, the mutation was linked to a 990 kb region encoding 11 genes; the application of the genetic and genomic tools led to a reduction of the linked region to 176 kb and the elimination of 7 genes. Furthermore, bioinformatics analyses of capture array-next generation sequence data identified genetic elements including SNPs, insertions, deletions, gaps, chromosomal rearrangements, and miRNA binding sites within the introgressed causative region relative to the reference genome sequence. Coloboma-specific variants within exons, UTRs, and splice sites were studied for their contribution to the mutant phenotype. Our compiled results suggest three genes for future studies. The three candidate genes, SLC30A5 (a zinc transporter), CENPH (a centromere protein), and CDK7 (a cyclin-dependent kinase), are differentially expressed (compared to normal embryos) at stages and in tissues affected by the coloboma mutation. Of these genes, two (SLC30A5 and CENPH) are considered high-priority candidate based upon studies in other vertebrate model systems.
Citation: Robb EA, Antin PB, Delany ME (2013) Defining the Sequence Elements and Candidate Genes for the Coloboma Mutation. PLoS ONE 8(4): e60267. https://doi.org/10.1371/journal.pone.0060267
Editor: Laszlo Orban, Temasek Life Sciences Laboratory, Singapore
Received: March 24, 2012; Accepted: February 26, 2013; Published: April 9, 2013
Copyright: © 2013 Robb et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Research was supported by the U.S. Department of Agriculture-National Institute of Food and Agriculture Multistate Research NC-1170 (CA-D*-ASC-6414-RR), the NRSP-8 National Animal Genome Research Support Program (CA-D*-ASC-7233-RR) and the UC Davis (UCD) John and Joan Fiddyment endowment (M.E.D.). The WEisH studies were supported by National Institutes of Health grant HD064559 (P.B.A.). The authors gratefully acknowledge the UCD Department of Animal Science for graduate student fellowship support and the UCD Jastro-Shields Graduate Research Award (E.A.R.). The authors appreciate the infrastructure and the poultry genetic resources supported by the UCD Department of Animal Science, College of Agricultural and Environmental Sciences, and the California Agricultural Experiment Station. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Three percent of the four million children born each year in the United States have a birth defect , . Of those congenital defects, ∼50% are due to genetic causes of which 30% are heritable . The UCD-Coloboma.003 (Co.003) chicken congenic line is an important animal biomedical model as it expresses features similar to human congenital defects including cleft palate and dwarfism, and eye, limb, digit, and visceral abnormalities , . Thus, identification of the underlying genetic mechanism causing the chicken coloboma phenotype provides a unique opportunity to elucidate the cause of similar multisystem syndromes in human. The classic chick embryo model (see Stern ), with its in ovo easy access, provides great value in this regard –. The utility of the chick embryo model was further improved by the availability of the chicken reference genome sequence , advanced tools and high-throughput technologies, as well as the existence of well-phenotyped developmental mutations , –. Such advantages led to the National Institute of Health recognition of the chicken as a model organism for biomedical research (http://www.nih.gov/science/models/gallus/).
A key characteristic of the UCD-Co.003 genetic line, ocular coloboma, i.e., reduction of tissue near and around (e.g., eyelid) or in structures of the eye (e.g., lens, macula, optic nerve, uvea), is present in ∼1 per 10,000 human births  and accounts for up to 10% of childhood blindness . Congential coloboma was first reported in human in 1870 ( and references therein) and in the chicken in 1958  and has been described in both organisms as an individual malformation or occurring in conjunction with a number of other multisystem syndrome abnormalities (e.g., craniofacial, skeletal, limb, and urogenital) and/or other ocular defects , , , –. Although most cases of human coloboma are considered sporadic, sex-linked, autosomal recessive, and autosomal dominant modes of inheritance have been reported . Of such, approximately a dozen genes have been associated with the coloboma defect – but account for only a small subset of reported cases of coloboma, thus research using animal models is essential to understand the etiology of the condition. The inherited malformations (i.e., coloboma of the eye, cleft-palate, dwarfism, truncated limbs, exposed viscera, the addition or loss of a digit on the feet) of the UCD-Co.003 genetic line provides the unique opportunity to uncover the genes and pathways involved in coloboma as well as the other associated malformations described above. Such comparative vertebrate knowledge will contribute to a greater understanding of genes involved in human development as well as improved knowledge of shared vertebrate developmental pathways.
Previously we reported on the chromosomal mapping and size of the causative (linked) region (CR) for the coloboma mutation using a 60 K SNP array . The discovered molecular markers were then used in new individuals to identify recombination events; this in turn further reduced the size of the linked region. Subsequently, RNA-bait sequence probes, complementary to the reduced 990 kb region, were generated for use in next-generation sequencing (NGS) of the region. That is, a targeted genomic capture enrichment technology (a.k.a. capture array (CA)) was employed to identify the element causing this mutation . Here we focus on the CA/NGS analysis of the genetic features discovered (e.g., SNPs, insertions, deletions) which include normal polymorphisms of the introgressed region as well as the mutant-specific variant element. Our goal was to discriminate between the two categories of variants within the region to find high priority candidates for future study. The Co.003-specific variant elements were evaluated for their contribution to the mutant phenotype using a variety of genetic, genomic, and bioinformatics techniques (e.g., sequence verification, splice site analysis, miRNA binding sites, etc.). Additionally, putative translocation events in the Co.003 genetic line, identified through genomic alignments, were assessed for their legitimacy. Finally, a set of genes encoded in the minimum causative region (176 kb) were studied for their expression in both mutant and normal embryos during several stages of early embryogenesis by whole-mount in situ hybridization (ISH) to evaluate their potential involvement. Combined, these studies reveal three high-priority candidates for future functional analysis.
Animals used for the study were under the care and supervision of trained staff and as per protocols approved by the University of California, Davis Institutional Animal Care and Use Committee.
The individuals utilized for this study were from two genetic lines, the developmental mutant-congenic inbred line UCD-Co.003 and its inbred (F>0.99) parent background line UCD-003 , . These two genetic lines are herein referred to as coloboma and control, respectively. The coloboma (co a.k.a cm) mutation is sex-linked, recessive (females are the heterogametic sex in birds, Z/W), and an embryonic lethal. Therefore, non-carrier males (Z+/Z+) and unaffected females (Z+/W) are genotypically identical to UCD-003, except for any spontaneous mutations. All affected females (Zco/W) are mutants (Figure 1).
(A) Normal, non-carrier (Z+/Z+) UCD-Coloboma.003 chicken embryo. (B) Mild expression. UCD-Coloboma.003 mutant embryo (Zco/W) displaying mild truncation of the limbs, mild cleft-palate, mild dwarfism, mild exposed viscera, oligodactyly (loss of digit - three toes displayed with the loss of the hallux, compared to the normal four toes, termed anisodactyly) on the embryo’s right leg, polyphalangy (longer-than-normal first digit in a 4-3-2-2′ digit conformation) on the embryo’s left leg, and very mild coloboma of the eye. (C) Moderate expression. UCD-Coloboma.003 mutant embryo (Zco/W) displaying moderate truncation of the limbs, moderate cleft-palate, severe dwarfism, severe exposed viscera, polyphalangy on the both legs, and mild coloboma of the eye. This embryo image was taken from Robb et al. (2011) Journal of Heredity 102(2):141–156. (D) Severe expression. UCD-Coloboma.003 mutant embryo (Zco/W) displaying moderate truncation of the limbs, severe cleft-palate, severe dwarfism, severe exposed viscera, oligodactyly (three digits, loss of the hallux) on the embryo’s left leg, polyphalangy on the embryo’s right leg, and severe coloboma of the eye. All individuals are shown at 10 days of embryogenesis.
Sample Collection for Fine-Mapping, Validation, and Translocation Assessment
Adult and embryonic blood was collected according to Robb et al.  and DNA (with an RNase step) was isolated using the DNeasy® Blood & Tissue kit (Qiagen). DNA samples were isolated from Co.003 normal homozygotes (Z+/Z+, n = 41), normal heterozygotes (Z+/Zco, n = 66), and mutant hemizygotes (Zco/W, n = 197) and archived for future use as needed. A subset of those samples listed above [n = 28 mutant (Zco/W) and 2 control (Z+/Z+) individuals] was used in the variant validation portion of this study (see below). To test for the putative translocation event (described in ), 2 control (Z+/Z+), 4 heterozygous (Z+/Zco), and 4 mutant (Zco/W; which included the two samples sequenced with the CA/NGS) samples from the archived coloboma DNA were employed.
SNP-Genotyping, Analysis, and Causative Region Identification for Fine-Mapping
Ten SNPs were selected for their linkage with the coloboma trait following an Illumina 60 K SNP array study  and used to fine-map the Co.003 CR. Primer sequences are listed in Table S1. The genotypes of the collected samples were determined and/or confirmed at the 10 loci using standard PCR conditions for Phire® Hot Start II DNA Polymerase (Finnzymes) and purified by QIAquick® Spin Kit (Qiagen). Amplicons were sequenced (Davis Sequencing, Davis, CA) using ABI 3730 DNA sequencers (Applied Biosystems) and analyzed for genotype-specific SNP differences. The SNP analysis and CR identification (maximum and minimum CR: CRmax and CRmin respectively) were defined as previously described . Two coloboma mutant samples (co-275F and co-276F) were specifically chosen for CA/NGS based upon their 10 SNP loci genotypes (Table 1).
Capture Array Technology, Sequence Variant Identification, and Reference-assisted Assembly
Previously, targeted genomic capture enrichment technology was utilized (SeqWright, Inc.) to sequence the entire 990,267 bp (a.k.a. 990 kb) coloboma candidate region on GGA Z (20,813,939-21,804,206 (November 2011, galGal4 assembly) formerly reported as 994,523 bp; (GGA Z: 20,368,747-21,363,270 (May 2006, galGal3 assembly)))  as part of a three-mutant sequencing screen. As described by Robb and Delany , mutant-specific DNA pools (50 µg total) were sent to SeqWright DNA Technology Services (Houston, TX) for barcoding followed by library production (base pair peak of 150–200), targeted enrichment (for chromosomes 1, 12, and the Z) and SOLiD ™ V3 Plus Platform sequencing (50 bp, single-end sequencing) (Applied Biosystems, Foster City, CA). A total of 3.64 Gbp of data were generated using this technology. Here we described the analysis of the coloboma NGS data including SNPs, micro-indels (1–3 nt), macro-indels (4–27 nt), sequence gaps, chromosomal rearrangements, and miRNA binding sites with further assessment of these elements for their potential contribution towards the coloboma phenotype.
Unique Variant Identification and Causative Element Analysis
To refine and reduce the number of potential coloboma - causative elements, multiple pairwise-line comparative genomic analyses were utilized (described in ). Shared variants were submitted to NCBI (Accessions: ss472340674–ss472343089) and were therein eliminated as possible candidates whereas any variant unique to coloboma alone (i.e., not found within control lines or not previously reported in NCBI and the UCSC genome browsers) was further studied here. Elements found within an exon, UTR, or splice site were validated to differentiate the introgressed-region normal polymorphisms (variants not causative but derived from the original mutant DNA segment, i.e., the source) from the causative element and those elements which remain linked to the coloboma mutation (see Table S2 for sequence primers). Table 2 describes the number of each element (e.g., SNP, micro-indel, gap) found at each step of the bioinformatic analyses.
Putative Translocation Analysis
In a prior study de novo assemblies were compared to identify possible chromosomal (structural) rearrangements, and of five reference-genome assisted assemblies, four indicated a translocation event on GGA Z in the Co.003 congenic line (see Supplemental Figures 1 and 2 in ). Here we studied the legitimacy of the putative translocations by designing PCR primers to span the rearranged regions in both control and Co.003 genetic lines. A control PCR amplification was also employed for each translocation assembly primer set to ensure a negative results was not just due to PCR failure. Genomic DNA amplification was carried out as described above. Primer sequence information and results for the putative translocation assessment can be found in Table S3.
A. Coloboma Z chromosome causative region (CR) identified by the 60 K SNP array: Six polymorphic SNP markers (red lines) mark the 1.004 Mb causative region (CRmin) identified by utilization of the Illumina 60 K chicken iSelect SNP genotyping array . SNP markers rs16101051 and GGaluGA349348 denote the boundaries of this region (GGA Z: 20,800,461-21,804,207). Eleven genes (predicted and confirmed) reside within the original 1.004 Mb region (indicated by the gray box). Chromosomal coordinate (bp) on GGA Z are based on the November 2011 Gallus gallus assembly (galGal4). B. Coloboma causative region identified by fine-mapping and analysis of the capture array data: Fine-mapping reduced the causative region to 175,915 bp (aka 176 kb). The following SNP markers designate the boundaries of this region: rs16101716 (red arrow) and GGaluGA349348 (GGA Z: 21,628,290–21,804,207). The 176 kb causative region contains 4 known chicken genes (pink box: SLC30A5; orange box: CENPH; blue box: MRPS26; green box: CDK7). Prior to fine-mapping and subsequent reduction to 176 kb, the use of the CA/NGS technology, specifically analysis of sample co-275 sequence data, allowed for a region reduction to 299,860 bp (this boundary is denoted with a blue arrow). Chromosomal coordinate (bp) on GGA Z are based on the November 2011 Gallus gallus assembly (galGal4).
MicroRNA Binding Site Identification in CR Genes
The predicted location of microRNA (miRNA) binding sites were identified using RNAhybrid . The following constraints were utilized: energy cut off (−e) ≤−20; helix constraint (−f) from 2 to 7 (i.e., perfect seed sequence hybridization from nucleotides 2 to 7). Results as to the number of known chicken miRNA-targeted 3′ UTR regions (of known genes in the 176 kb linked region) are listed in Table 3. Additionally, TargetScan 6.0 – was utilized to predict vertebrate-conserved miRNA binding sites (by searching for the conserved 7–8 mer miRNA seed region) encoded within the 3′ UTRs of those genes listed in Table 4. Both the reference genome 3′ UTR gene sequence (obtained from NCBI or UCSC Genome Browser) and Co.003 mutant genomic 3′ UTR gene sequences  were crossed checked.
Whole-mount in situ Hybridization (ISH)
Expression status of the candidate genes encoded within the coloboma fine-mapped CR (175,915 bp) were examined in normal, female coloboma line embryos (Z+/W) and mutant female embryos (Zco/W) at HH24–HH26, wherein the main morphological features affected in this mutation (e.g. limbs, craniofacial) are sometimes but not always elaborated as yet . A total of 87 newly collected mutant (n = 44) and normal (n = 43) coloboma female embryos (note that this number is not included in the archived coloboma DNA count listed prior) were utilized for ISH. A minimum of four embryos of each group (control and mutant) were tested for every gene. The mutant and normal female embryos used for ISH were identified and selected using the ZZ/ZW (male v female) sexing genotyping, see ) and the 10 SNP coloboma genotyping protocol (described above).
Four genes (two confirmed and two predicted) are found within the fine-mapped region. These include solute carrier family 30 member 5 (SLC30A5), centromere protein H (CENPH), mitochondrial ribosomal protein subunit 36 (MRPS36), and cyclin-dependent kinase 7 (CDK7). To assess their gene expression during development, RNA probes were created from cDNA (EST) clones (Table S4) acquired from BBSRC ChickEST Database (http://www.chick.manchester.ac.uk/) using methods adapted from Nieto et al. . Protocols for ISH were carried out following Darnell et al. (; http://geisha.arizona.edu/geisha/). ISH embryo images were captured digitally via microscopy (Wild Heerbrugg) by placing embryos on 1.5% agarose-PBS (1×) plates.
Coloboma (co) is a sex-linked recessive embryonic lethal mutation which affects females. The coloboma syndrome includes dwarfism, craniofacial defects, bilateral facial coloboma, exposed viscera, and absent or greatly reduced extremities due to disruption in cartilage formation , . The phenotypic expression of coloboma is variable and can be subdivided into three categories: mild, moderate, and severe (see Figure 1A–D). In the majority of cases, the most severe phenotype was displayed in those embryos that died earliest in development (E6–7). Mutants classified as mild (Figure 1B) have a slightly abnormal beak with the proximolateral parts of the maxilla missing and slightly reduced wings and legs; additionally the toe digit pattern can be one of the three conformations described in the subsequent paragraph. Embryos displaying moderate coloboma (Figure 1C) expression have phenotypes in between that described for the cases of mild and severe coloboma. Severe coloboma (Figure 1D) expression includes malformations in the skull and face, with the eyes set forward (due to missing tissue) and the beak and throat reduced such that a cavity in the head is observed. Interestingly, Abbott et al.  reported an extreme form with legs either absent or very reduced (described as spikes). However, this extreme leg phenotype was not observed in the 200 mutants phenotyped in this study. Such expression differences are likely due to background genotype, i.e., currently coloboma is in the highly inbred (UCD-003) background thereby having different regulatory cis- and trans-acting factors from the original stock described by Abbott et al. . Table 5 presents the observed cleft-palate expression ratios wherein mild cleft palate is the most prevalent among the mutants.
Mutants can also be characterized by the presence of several different toe digit conformations: (1) a loss of the hallux (inner-most medial digit) displaying a 4-3-2 digit pattern (termed oligodactyly); (2) a longer-than-normal first digit in a 4-3-2-2′ digit pattern (polyphalangy); or (3) an additional pre-axial digit (single-digit duplication) in a 4-3-2-1-2′ digit pattern (pre-axial polydactyly), wherein the duplicated digit is 2′. The variations of digit conformations (4-3-2, 4-3-2-2′, or 4-3-2-1-2′) can be expressed on both feet or in a unilateral fashion with one form on each foot (heterodactyly), with a preference towards 4-3-2 on the mutant embryo’s right leg. See Table 5 for foot digit variation ratios.
Fine-mapping Reduced the Size of Linked Region and Estimate Co.003 Z Recombination Rate
Robb et al.  found a CRmax and CRmin of 1.504 Mb and 1.004 Mb, respectively (sizes updated using the galGal4 assembly), as defined by 10 SNP loci on the GGA Z associating with the coloboma mutation. Following that study, additional mutants were assessed at the 10 SNPs (Table S1) in order to detect recombination events. Analysis of 197 mutants (Zco/W) and 66 heterozygotes (Z+/Zco) led to a reduction of the linked region to 175,915 bp (a.k.a. 176 kb), eliminating 828 kb of genomic DNA and 7 genes (Figure 2). Interestingly, only seven mutants and three heterozygous carriers inherited a recombined chromosome (data not shown). Thus, the recombination rate within the originally-linked 1.004 Mb region on chromosome Z for the Co.003 genetic line was determined to be 0.025–0.030 cM/Mb (LOD score (Z) = 61.4) indicating high linkage disequilibrium in this region. This recombination rate is lower relative to that predicted (1.5–3.5 cM/Mb) for the region by other studies , .
Targeted Genomic Capture Enrichment Technology
Although the CA/NGS sequenced a 990,267 bp region found linked to the coloboma mutation as identified by Robb et al. , the fine-mapping (described above) eliminated a large majority of the region and allowed us to then focus in this study on the reduced region (176 Kb) for the bioinformatics and developmental expression of encoded genes (see below).
Reference genome-assisted de novo assembly – translocation identification and validation.
Reference genome-assisted de novo assembly using Mauve software  on the 15.7 M reads generated for the Co.003 genetic line  indicated alignments suggesting two putative translocation events involving the mutant chromosome (Zco). One larger event consisted of a 53,749 bp DNA fragment translocated (in the 3′ direction) a distance of 258,103 bp on GGA Zco. A second, smaller translocation event (7,345 bp) was displaced by 340,463 bp on GGA Zco (in the 3′ direction). Supplemental Figures 1 and 2 in Robb and Delany  depict the set of Mauve alignments. Interestingly, in both cases, the translocated DNA fragment was found at GGA Z coordinate 21.59 Mb, just upstream of the SLC30A5 gene (Table 4).
Here we evaluated each Mauve-predicted translocation to validate the event and consider whether such structural rearrangement could be causative for (1) the low recombination rates calculated for the 1.004 Mb linked region and/or (2) effect the normal expression of a gene(s) within the region thereby possibly contributing to the coloboma phenotype. Normal (Z+/Z+), heterozygous (Z+/Zco), and mutant (Zco/W) samples were all examined for the translocation event using PCR designed to span the rearrangements. In all cases, amplicons were found as predicted by the normal (reference genome alignment) and positive control primers; primers designed to span a putative translocation gave no result (no amplicons) thereby indicating that the Co.003 Z translocation events predicted by Mauve were false (Table S3).
Identification of single nucleotide polymorphisms (SNPs) found within the coloboma CR.
Some 15.7 M Co.003 CA/NGS reads  were mapped to the entire chicken reference genome (May 2006, galGal3 assembly; ) and were used to identify SNPs relative to the reference sequence. Upon assessment of the 2,500 SNPs identified within the original 990 kb CR, 2,156 were homozygous and 344 were heterozygous. Any heterozygous SNP was eliminated as being the causative element as the coloboma mutation is sex-linked recessive and only females are affected (Zco/W) and thus carry only one allele. Of the SNPs found within this region, 1,454 are transition SNPs (A ↔ G or C ↔ T) and 1,046 are transversion SNPs (A ↔ C, A ↔ T, G ↔ C, or G ↔ T), resulting in a transition to transversion ratio of 1.4∶1. This ratio is lower compared to that reported (2.2∶1) in other work  and what was observed in two other lines (2.1∶1) used as Z-chromosome controls in the capture array setup . The average SNP density across the sequenced Zco chromosome was 2.4 SNP per kb, approximately two-fold lower than that reported in other domestic chicken lines (5.1–5.8 SNP per kb) .
One mutant sample (co-275F) was specifically chosen for use in the CA/NGS technology due to its reduced CR (Table 1). Analysis of the precise breakpoint by the CA/NGS heterozygous SNP genotyping allowed for even further reduction in the size of the co linked region to 299,860 bp (a.k.a. 300 kb; GGA Z: 21,504,346–21,804,206) (Figure 2B). Recombinants were identified by analysis of the archived mutant samples, further reducing the linked region to 175,915 nt (a.k.a. 176 kb; GGA Z: 21,628,291–21,804,206). This refined region includes 538 SNPs and 4 genes; 293 of those SNPs were eliminated as causative through multiple pairwise-line comparisons (Table 2).
Of the remaining 245 coloboma-unique SNPs within the 176 kb region, 172 are located external to a gene (non-genic) and 90 were found located within a gene inclusive of 77 inside introns, 4 in exons, 4 in 3′ UTRs, and 5 at splice sites (Table 6). The entire set of 245 SNPs were assessed for codon and predicted amino acid changes within the 6 possible reading frames (+1, +2, +3, −1, −2, −3). Reporting only on the first (+1) reading frame (other data not shown), 191 SNPs were synonymous substitutions while 12 SNPs generated nonsense mutations. The SNPs found within the exons, UTRs, and splice sites of known genes (Table 6) were further assessed for causation in the validation portion of this study (see below).
Micro-indels (1–3 nts) in the UCD-Coloboma.003 Zco chromosome.
Similar to the SNP analysis, the number and location of micro-indels (insertions or deletions relative to the reference genome) of 1 to 3 nucleotides in length were identified. A total of 55 micro-indels (27 insertions, 28 deletions) were identified within the 176 kb region. Of those, 14 and 18 insertions and deletions, respectively, are unique to coloboma (Table 2). In total, 12 indels are external to a gene while 20 reside within a gene (15 are intronic and 5 are at splice sites) (Table 6). Upon frame shift analysis, reporting only reading frame +1, 4 of the 32 micro-indels generated nonsynonomous mutations none of which generated a stop codon. The micro-indels exhibited an average distance of 5,578 bp. This value (0.18 micro-indels per kb), calculated for the identified micro-indels in the 176 kb, is lower than that observed on the entire Z chromosome in other chicken breeds (1.44 short indels per kb) . The average micro-indel size was 1.31 bp, three-fold lower than that seen in other chicken breeds (3.7 bp) . It is important to note that this 1.31 bp average micro-indel (≤3 nt) size was calculated for only for the 176 kb region, while the other values were those calculated for the entire Z sex chromosome (3.7 bp average micro-indel size). The micro-indels found within exons, UTRs, and splice sites were further assessed for causation in the validation portion of this study (see below).
Identification of gaps within the UCD-Coloboma.003 congenic line.
The alignment data was analyzed for the presence of sequence gaps (i.e., putative deletions of ≥4 nts). Within the reduced 176 kb CR there were 111 gaps, with an average size of 52 bp (range: 4 to 634 bp) and an average distance 1,569 bp (range: 51 to 6,926 bp). The gaps within exons, UTRs, and splice sites (Table 2 and 6) were assessed for legitimacy, i.e., true gaps versus sequencing errors, in the validation portion of this study (see below).
MicroRNA binding site identification.
Predicted miRNA binding sites were identified for both the 176 kb reference genome (Z+) and coloboma (Zco) Z chromosomes. Table 3 displays the number of predicted chicken miRNA binding sites identified within the 3′ UTRs of the genes within the 176 kb region as well as the number of binding sites conserved across vertebrates. Similar to the multiple pairwise-line comparison strategy utilized to eliminate shared sequence variants (SNPs, micro-indels), miRNA binding sites were eliminated as being causative if they shared the same nucleotide sequence with the reference genome. Although a binding site could be normal, a mutant miRNA could cause the coloboma phenotype. Further analysis of miRNAs from the region necessitates use of RNA-seq technology.
Validation of Elements Identified by the Capture Enrichment Technology and Assessment of Causation
The unique SNPs, micro-indels, and sequence gaps associated with the coloboma mutation were assessed for their position/coordinate (e.g., exon, intron, etc.) and predicted sequence modification (e.g., synonomous, nonsynonomous). Elements found within exons, UTRs, and splice sites of known and predicted genes across the entire 176 kb sequence were assessed. Additional control (n = 2) and mutant (n = 28) samples were used to verify if a particular variant remained linked to the coloboma mutation. Verified SNPs and micro-indels have been submitted to NCBI (Table S2; Accession Numbers: ss475871097–113; ss475871115; ss475871117–28; ss475871130–36; ss475871141–45; ss475871147–69; ss475875322; ss475875324; ss475875337; ss475875345–48; ss475875354; ss550120096–ss550120108).
Validation of Exonic, UTR, and Splice Site SNPs, Micro-indels, and Gaps
Although the variants within an exon, UTR, or splice site of genes in the 990 kb CR were assessed, only those elements within the 176 kb region are reported here. Table S2 lists additional verified elements (outside the 176 kb), all of which were ruled out thereby confirming the fine-mapping results. A total of 12 SNPs were found within exons, UTRs, or at splice sites. An exon-located SNP was identified in SLC30A5, MRSP36, and CDK7 (Table 6, S2) while four SNPs were located within the 3′ UTR of CENPH. Five SNPs were located within a splice site of one of the candidate genes (Table 6, S2). Twenty-eight new mutant samples were used to verify coloboma-linkage at each SNP. All SNPs were validated, i.e., all 12 SNP sequences were found to be legitimate and present in additional mutants. However, the genotyping pattern analysis (haplotype per individual mutant) eliminated all 12 SNPs as being causative of the coloboma mutation since at least one mutant individual exhibited the reference genome or the UCD-003 (genetic background) genotype at each SNP locus.
Five micro-indels, more specifically 5 deletions, were found at splice sites of genes within the 176 kb Co.003 GGA Zco sequence (Table 6, S2). The percent validity for these micro-indels was 40% as 2 of the 5 micro-deletions displayed the same genotype as control (UCD-003) samples tested thereby indicating a false-positive micro-indel identification. Upon analysis in additional mutant samples (n = 28) none of the deletions remain linked to the mutation, and therefore none of these can be causative.
Ten gaps were identified within an exon, UTR, or splice site relative to the reference genome (Table 6, S2). In all 10 cases, the gaps were found to be false in both mutant and control samples, i.e., PCR amplification using reference sequence-developed primers produced amplicons with sequence content. This new sequence information was evaluated for variation in mutants v controls. Sequencing identified new sequence content absent from the current assembly as well as new variants (e.g., SNPs and micro-indels). Upon analysis, none of the 10 new sequence variants remain linked to coloboma.
Expression Analysis of Candidate Genes by ISH
The 176 kb fine-mapped region (galGal4 assembly) encodes four genes. Of these, two genes (MRPS36 and CDK7) are predicted and have chicken ESTs or mRNAs aligned while the other two (SLC30A5 and CENPH) have been confirmed (Table 4). Expression patterns of the four genes were examined for their developmental expression in coloboma mutant (Zco/W) and coloboma normal (Z+/W) female embryos at E5 (HH24 to HH26) to assess their spatial and temporal expression. All anatomical features are at the expected stage of development in the normal coloboma female embryos. However, in the mutant the limbs are often reduced which is to be expected as mutants are dwarfed and display truncated limbs (Figure 1B–D). Largely, expression of the genes at the stages studied can be considered ubiquitous although variation among regions within embryos and between normal and mutants were indicated. Assessment of the mutant embryos at stages of development comparable to the normal (all were from the Co.003 line) indicates three genes as having differential expression levels within coloboma-affected tissues: SLC30A5, CDK7 and CENPH whereas MRPS36 showed no evidence of expression differences (see Figure 3). We also examined the gene expression profiles in normal, outbred embryos (HH13– HH25) and the results can be found at the Gallus Expression in situ Hybridization Analysis (GEISHA) – A chicken embryo Gene Expression Database (http://geisha.arizona.edu/geisha/) , .
Localization of chicken SLC30A5, CENPH, CDK7, and MRPS36 gene expression in HH stage 24–25 in normal and mutant chicken embryos using whole-mount in situ hybridization. See the Results and Discussion sections for expanded explanations. (A) Normal coloboma embryo (HH25) with SLC30A5 expression. (B) Mutant SLC30A5 embryonic expression (HH24) is similar to that of the normal embryo except for increased expression in the telencephalon, pharyngeal arches, wing bud, posterior limb bud, portions of the brain (diencephalon, metencephalon), and the anterior-most and medial somites. (C) A representative, normal CENPH coloboma embryo. (D) Mutant CENPH gene expression is similar to that of the normal embryo (C) except for increased expression in the limb buds. (E) Normal CDK7 expression in the coloboma embryo (HH25) is widespread. (F) CDK7 expression within the mutant coloboma embryo (HH24) is similarly ubiquitous with an increase in expression in the limb buds, pharyngeal arches, as well as the tail bud (relative to the normal (E)), and the mesencephalon and telencephalon while decreased CDK7 expression is observed in the abdominal mesoderm. (G) The normal coloboma embryo (HH25) displays widespread MRPS36 expression. (H) Similarly, the coloboma mutant embryo (HH25) displays ubiquitous expression with decreased levels of expression (relative to the normal coloboma embryo) in the telencephalon, diencephalon and the tail bud. All prominent anatomical locations are labeled (see below for abbreviations). Black indicates that expression levels are the same for both control and mutant embryos. In those embryos where expression is seen in both normal and mutant, the regions are labeled in red and higher expression is indicated by an underline. If a region does not have expression, the area is labeled with blue. Note that for a gene, a region could be marked with blue font in one embryo (no expression, staining at background level) but red-underlined in the other group (expression higher than background). am: abdominal mesoderm; as: anterior somites; av: auditory vesicle; d: diencephalon; h: heart; lb: leg bud; me: mesencephalon; ms: medial somites; mt: metencephalon; pa: pharyngeal arches; sc: spinal cord; t: telencephalon; tb: tail bud; wb: wing bud.
Within the normal developing embryo (HH24), moderate SLC30A5 expression was observed in the somites, mesencephalon, pharyngeal arches and auditory vesicle (inner ear) while mild SLC30A5 expression was observed in the heart and in both the posterior wing and leg bud regions (Figure 3A). Figure 3B displays a representative image of SLC30A5 expression in an HH24 coloboma mutant embryo. Strong SLC30A5 expression is observed in the most anterior somites (1–6) and medial somites (16–21), the pharyngeal arches, posterior wing bud region and in the endoderm of the telencephalon, diencephalon, mesencephalon and metencephalon. Moderate SLC30A5 expression can be seen in somites 7–19 and 22–27 as well as the distal edge of the wing bud and the posterior leg bud region. Comparing the two representative images for SLC30A5, differential expression levels can be seen in the pharyngeal arches, somites, posterior limb bud regions, wing bud, and in several regions of the brain (telencephalon, diencephalon, metencephalon).
The normal coloboma embryo (HH25) displays strong CENPH expression in the entire wing bud, the dorsal surface of the leg bud, the developing heart, abdominal mesoderm, auditory vesicle and pharyngeal arches. Moderate CENPH expression can also be seen along the somites, spinal cord, telencephalon, diencephalon, metencephalon, and both wing and leg buds (Figure 3C). Similar to the expression profile observed in the normal coloboma embryo, the mutant displays strong CENPH expression throughout the entire body, with an increase of expression in the limbs (compared to the normal embryo) (Figure 3D).
CDK7 (Figure 3E) expression at HH24 in normal embryos was observed in the diencephalon, metencephalon, pharyngeal arches, auditory vesicle, and the posterior wing and leg bud region as well as the somites, heart, and abdominal mesoderm. Like the normal coloboma embryo, moderate CDK7 expression in the mutant (Figure 3F) is seen in the diencephalon, metencephalon, mesencephalon, auditory vesicle, somites, and heart. However, increased expression can be found in the leg and wing buds, pharyngeal arches, mesencephalon, and the tail bud while decreased expression is observed in the abdominal mesoderm.
Gene expression of MRPS36 at HH25 (Figure 3G) in normal embryos shows localization in the head mesenchyme (including: telencephalon, diencephalon, metencephalon), pharyngeal arches, the auditory vesicle, cranial through caudal somites, developing heart and at the distal edge of the wing and leg bud while mild expression is exhibited in the limb buds. Expression of MRPS36 in mutant embryos is similar to that of the normal embryo except for a minor decrease of expression in the mutant telencephalon, diencephalon, and tail bud (Figure 3H) – these regions are not phenotypically altered in the coloboma mutant.
Major advancements have been achieved toward the goal of identifying the causative element for the coloboma mutation first described in 1970 . In 2009, a 60K SNP genotyping array established the genomic coordinates of a 990 Kb region and confirmed the sex-linked mode of inheritance on the Z chromosome . The combined power of breeding and SNP-genotyping of mutant (Zco/W) and heterozygous (Z+/Zco) samples identified a recombination event that aided in the reduction of the causative region to 176 kb thereby eliminating 7 genes and 828 kb of DNA sequence.
Advanced genomic technologies (targeted genomic capture enrichment technology) paired with NGS were employed to sequence the entire segregating region associated with coloboma. This method offered the opportunity to investigate the region without gene/exon bias (as opposed to exome sequencing) and at high coverage (as opposed to lower coverage obtained through whole genome sequencing) . Bioinformatic analyses identified shared and coloboma-specific polymorphic variants as well as previously unknown genomic sequence content, both of which have contributed to the continued refinement of the chicken genome sequence and extant polymorphisms.
The coloboma-specific genetic features (putative translocations, 5 micro-indels, 13 SNPs and 10 gaps associated with exons, UTRs and splice sites) were validated in an aim to identify the causative element and unfortunately none of the variations remain linked to the four genes in the region. Given these results, our technology choice was fortuitous in regard to identifying the causative element for this mutation as the CA/NGS data can continue to be mined. Current experiments are focusing on introns and intergenic sequence data as well as functional gene studies.
Given that the validation results showed a lack of linked variants within any coding region of the four genes, we next assessed the gene expression (MRPS36, CDK7, CENPH, SLC30A5) in both mutant and normal coloboma embryos in an aim to identify any differences in expression, possibly indicating a role for these genes related to mis- or dys-regulation of expression in the developmental mutant phenotype. MRPS36 function, gene-associated human/mouse diseases (Table 4), and lack of linked variants paired with its ISH expression profiles (i.e., absence of relevant spatial and temporal expression in coloboma-affected anatomical locations in both mutant and normal embryos) do not support MRPS36 as a candidate gene for coloboma.
CDK7 mutant embryos exhibited increased expression in the limb buds as well as the pharyngeal arches as compared to normal embryos; these anatomical regions are perturbed by the coloboma mutation. Note that both mutant and normal embryos showed widespread expression, a profile also conserved in the zebrafish embryo , . Functional studies in multiple model organisms, ranging from Drosophila to human, report that this gene is essential for cell division without which there is massive cell cycle arrest and apoptosis –. Furthermore, in Cdk7 knockout mice, there is early termination at peri-implantation . Although coloboma mutant embryos do not survive to post-hatch they do survive for a reasonably long time frame (to day 6–14 (depending on phenotypic severity) of the 21 day incubation period; interestingly, Abbott et al. noted coloboma mutant survival to near hatch ). Although it seems unlikely, CDK7 cannot be ruled out at this juncture.
Although CENPH expression is widespread in both the normal and mutant coloboma line embryos, increased expression is observed in the limbs of the mutant but appears ubiquitous (and at similar expression levels) in the other mutant-associated anatomical features (Figure 3C–D). As is the case for the other genes, all exon/splice sequence variants within CENPH were eliminated. One could conclude from these results alone that CENPH is not a high-priority candidate gene for coloboma; however, studies in zebrafish suggest further consideration is warranted. CENPH may play a role in mitotic failure and aneuploidy , which could result in pleiotropic effects relevant to abnormal tissue modeling. Interestingly, cenph expression in the stagnant and curly (stac) zebrafish mutant showed localization to the craniofacial region, heart, and spinal cord as well as early embryo lethality . The stac mutant phenotype includes smaller eyes, vague brain ventricle borders, rough skin, and an upward bent posterior trunk . Thus, we suggest CENPH remains a candidate gene. A potential hypothesis warranting further study is that the normal and mutant embryos display a similar CENPH mRNA transcript (gene expression) profile but the CENPH protein is truncated or non-functional. Perhaps a variant sequence element upstream or downstream of the gene or within a CENPH intron, as could be the case for all of the genes, is generating a miRNA which in turn is binding to the CENPH mRNA and results in a lack of protein product (caused by nonsense mediated decay) or a non-functional (truncated) protein (caused by a splice-blocking miRNA). Additional analysis of sequences in the intergenic regions and introns along with protein and functional assays will be necessary to test this and other hypotheses.
Previously we identified SLC30A5 as a priority candidate gene for the coloboma mutation based on comparative analysis of the gene/protein function and knockout mouse studies (heart defects and bone-density loss) . Here we show different expression profiles in the mutant coloboma embryo compared to the normal embryo. Within the normal developing coloboma embryo, only slight/low (or background) expression is observed. The coloboma mutant embryo, however, displays increased SLC30A5 expression in the pharyngeal arches, somites, and posterior wing and leg bud regions perhaps contributing to the craniofacial abnormalities (e.g., coloboma, cleft palate), dwarfism and truncated limbs, and digit malformations (e.g., oligodactyly, polyphalangy, polydactyly), respectively. Although similar to the other candidates, no exon/splice variants within SLC30A5 remain linked to coloboma. It is possible that a non-genic mutation (e.g., in a promoter, chromatin remodeling region, intron, or a copy number variant) is causing the altered expression. Elements (SNPs, micro-indels, gaps) present in the introns and upstream of SLC30A5 are under investigation.
Advanced sequencing technologies provide massive amounts of data which can lead to causative element identification of inherited developmental mutations. However, the reality of the reverse genetics approach either targeted (in the case of this study) or whole genome sequencing, toward understanding phenotype is extremely complex to decipher. Ultimately the hurdle of bioinformatics, e.g., identification, validation, elimination of artifacts, determination of true polymorphisms, and exploration of natural/normal variation versus causative variation (i.e., “THE element”), must partner with functional studies. As a result of the research reported here and with consideration in other model systems, we postulate three candidates for coloboma with an emphasis on one as a high-priority candidate gene. The top priority candidate genes for the coloboma phenotype, based on compiled results (embryonic expression profile, presence of intron and promoter sequence variants (which are currently under investigation), comparative vertebrate biology), are SLC30A5 and CENPH. However, CDK7 cannot be completely ruled out at this juncture. Current assessment of variants within and around these genes (i.e., within the introns, promoter regions, intergenic regions, etc.) causing a possible alteration in a transcribed miRNA, miRNA binding site, promoter, transcription factor binding site, and transcript splicing, to name a few, are currently being assessed. CNVs (specifically in the case of this mutation, an insertion) are an important consideration as well as they have been described to cause several human mutations and diseases including several craniofacial disorders –. Noteworthy, assessment of known heterozygous (Z+/Zco) coloboma metaphase spreads indicates no observable chromosomal size differences (data not shown) and the validation studies ruled out all putative gaps (aka possible deletions) within the coding sequence of the 4 genes. Thus a CNV (insertion (≤4 nt)) is plausible.
There is much yet to be discovered in regard to normal polymorphisms and mutant variants of both well-studied and novel genes as to their primary sequence and the role of genetic variation as it is elaborated (via protein function) in pathways and networks for developmental processes which impact phenotype. Sequence analysis of the variants (unique mutations) responsible for developmental syndromes (often involving multiple pathways and tissue layers) provide the opportunity to reduce incidence through screening tests and offer a path toward mitigation depending on the causes of the malformation. Expansion of experimental vertebrate models for molecular and cellular analysis is essential to advance the understanding of mechanisms utilized for specific developmental syndromes and diseases in humans. There exist a number of developmental mutations in chicken valuable in contributing to this effort  and coloboma will undoubtedly be one of those.
Coloboma 1 Mb fine-mapping primers used to identify carrier status, causative region size and recombination events. AChromosomal location (bp) of SNP on GGA Z; positions are based on the November 2011 Gallus gallus assembly (galGal4). BPCR fragment size was determined by three methods: 1) using the UCSC genome browser (http://genome.ucsc.edu/, 2006 Gallus gallus assembly (galGal3)), 2) sizing by gel electrophoresis, and 3) DNA sequencing.
Coloboma validation primer information for variants within exons, UTRs and splice sites. Sequence variants found within an exon, UTR or splice site of known genes within the Co.003 linked region (990 kb) were further assessed for linkage to the co mutation. (A) A total of 14 Co.003 SNPs were found within an exon, UTR, or splice site. (B) Six micro-indels were found within an exon, UTR, or splice site within the Co.003 linked region. (C) A total of 16 sequence gaps were found within an exon, UTR, or splice site within the 990 kb linked Co.003 region. AChromosomal location (bp) on GGA Z was assigned based on the November 2011 Gallus gallus assembly (galGal4; UCSC genome browser, http://genome.ucsc.edu). BVariant location was obtained through position assessment using the 2011 Gallus gallus assembly. CThe Primer Reference ID is the internal (Delany Laboratory, UC Davis) tag assigned to that particular variant. Note that cm = co. DPCR fragment size was determined by three methods: 1) using the UCSC genome browser (2006 Gallus gallus assembly (galGal3)), 2) sizing by gel electrophoresis, and 3) DNA sequencing. NCBI accession numbers have been assigned to variants (SNPs, indels) identified upon sequencing the capture array gaps. Below outlines the NCBI accession numbers associated with a particular UCD-Co.003 gap sequence variant. Ess475871115; ss475875322; ss475875324 Fss475871117 Gss550120101 Hss475871118–ss475871119 Iss550120102–ss550120103 Jss475871124 Kss475871125–ss475871128; ss475875337 Lss475871130–ss475871133 Mss475871152–ss475871167 Nss550120104–ss550120108 Oss475871168–ss475871169.
Putative Coloboma.003 translocation assessment: primer and amplicon information, predicted and actual results. AFor the particular Mauve alignment figures, please see . BThe Reference genome refers to the normal genome found in the UCSC Genome Browser (http://genome.ucsc.edu). The Translocation genome refers to the Mauve-predicted (putative) translocation and assembly of the coloboma genetic line. CFrom left to right: the first set of numbers/letters on the left hand size of the primer name refers to the Mauve alignment (e.g., 1–12-Z, Z-1–12, 1-only). Note that primers were designed for only three of the 6 alignments as three of the Mauve alignments were redundant. The “RefG” or “Co” refers to the sequence genome/information from which the primer was designed. The letters after the targeted genome (e.g., P-O, O-G, G-T, P-G, G-O, O-T, R-O, etc.) refers to the section of DNA in each Mauve assembly that primers were designed to flank. For example, under the 1–12-Z Mauve alignment, primers (1–12-Z:Co_P-G) were designed to span the “pink to green” blocks. Lastly, F and R refer to the forward and reverse primers, respectively. See  for Mauve alignments. DA dash (–) indicates that no amplification (i.e., no PCR product) is expected. The numbers found in the table refer to the size of the PCR amplicon expected (in basepairs). ENote that the control PCR product produced the proper size amplicon in all samples. Translocation absent indicates that PCR amplicons were visualized based upon the “translocation absent” predictions.
Chicken EST probes used in whole-mount in situ hybridization: Analysis of UCD-Co.003 candidate gene expression. AThe BBSRC ChickEST database (http://www.chick.manchester.ac.uk) was utilized to identify ESTs for each of the genes. The particular EST sequence can be found at the aforementioned website. ESTs were purchased from Source BioScience UK Limited geneservice (Cambridge, UK), through the BBSRC ChickEST database (http://www.lifesciences.sourcebioscience.com/). Clones were selected on carbenicillin plates (50 µg/mL) prior to growth in LB broth+ carbenicillin (50 µg/mL) and clone purification (using Qiagen’s Plasmid Purification Kit). ESTs were sequenced prior to use in ISH to confirm clone identity. BUCSC genome browser (http://genome.ucsc.edu/) was utilized to identify the location of each gene and the EST percent identity to chicken mRNAs previously identified. The coordinate location is based upon the November 2011 Gallus gallus assembly (galGal4). CEach EST clone was inserted and amplified in the pBluescript II KS+ vector, 3.0 kb (Stratagene). The estimated size of each EST was determined through standard restriction enzyme digest (NotI and EcoRI) and subsequent gel electrophoresis. NotI was used to cleave the vector for sense-strand RNA creation using T3 polymerase. Similarly, EcoRI was used, paired with T7 polymerase to generate anti-sense-strand RNA.
We thank Jackie Pisenti for her dedication to the breeding of the UCD-Coloboma.003 line. We thank D. Winston Bellott and David Page (Whitehead Institute) for the ∼1 Mb GGA Z RJF reference sequence utilized for capture array probe design as the assembly (May 2006, galGal3) contained large gaps on the sex chromosome. We thank Jerry Dodgson (Michigan State University) and Hsiao-Ching Liu (North Carolina State University) for helpful conversations. Finally, we thank the anonymous reviewers for helpful comments and suggestions.
Conceived and designed the experiments: EAR MED. Performed the experiments: EAR MED PBA. Analyzed the data: EAR MED PBA. Contributed reagents/materials/analysis tools: MED PBA. Wrote the paper: EAR MED.
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