Objectives

The overarching goal of this study is to overcome obstacles in the design and conduct of clinical trials for newly developed therapies in DM1.

The primary objectives of the study are:

1. To characterize the phenotypic heterogeneity and disease progression of DM1 in a large cohort;
2. To identify baseline characteristics that predict subsequent progression;
3. To further validate RNA biomarkers of disease severity; and
4. To build and harmonize a network of centers and investigators trained in standardized outcome assessments and study procedures for the DM1 population, thereby facilitating future multicenter clinical trials.

Design

The END-DM1 study is a prospective, multi-site observational study with a follow-up period of 24 months. The study is registered at clinicaltrials.gov (NCT03981575). Recruitment started in October 2018, was impacted by the Covid pandemic, and is currently ongoing. Participant recruitment is expected to be completed by Q3 2025, with final data collection concluding in 2027. Baseline analyses will be conducted and published during the study, while complete longitudinal results are anticipated from 2027 onwards.

Participant timeline

The SPIRIT schedule (Fig 1) summarizes the time points for enrolment and all study procedures, including an additional month 3 visit for 60 participants in the muscle biopsy subgroup. Visits may be carried out over 2 days as long as the second day occurs within 7 days. End of Study visits will be allowed out of window to allow for more complete data collection. Missed visits can be made up out of window.

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Fig 1. SPIRIT schedule of enrolment and assessments for the END-DM1 study.

https://doi.org/10.1371/journal.pone.0331163.g001

Study setting

Approximately 700 patients will be recruited from 22 centers around the world. The participating centers are part of the Myotonic Dystrophy Clinical Research Network (DMCRN), an international consortium of investigators who are focused on DM1.

Ethics approval and consent to participate

For each participating center the protocol and informed consent forms were approved by an ethics committee and relevant regulatory authorities. The END-DM1 study was reviewed by WIRB-Copernicus Group Institutional Review Board at the study level and at the site level for the majority of sites in the United States (reference number 20204399, initial approval on Dec 13^th 2017). Each site performed a local context review to adjust the informed consent to address any local considerations. Sites outside of the US had protocol reviewed by their separate institution human subject committees. For the study site in Milan, Italy, the ethics review committee Comitato Etico Milano Area 3 reviewed and approved the study (reference number ID 4797). For the study site in Nijmegen, Netherlands, the ethics review committee CMO Arnhem-Nijmegen Region reviewed and approved the study (reference number NL.71565.091.20). For the study site in Auckland, New Zealand, the Central Health and Disability Ethics Committee reviewed and approved the study (reference number 2024 AM 11251). For the study site in Sherbrooke, Canada, the ethics review committee Le Comité d’éthique de la recherche du Centre intégré universitaire de santé et de services sociaux du Saguenay-Lac-Saint-Jean reviewed and approved the study (reference number 2022−055). For the study site in London, United Kingdom, the ethics review committee London – South East Research Ethics Committee reviewed and approved the study (reference number 282941). For the study site in Munich, Germany, the ethics review committee Ethik- Kommission bei der Medizinischen Fakultät der Ludwig- Maximilians Universität München reviewed and approved the study (reference number 20−0822). For the study site in Osaka, Japan, the ethics review committee Osaka University Research Ethics Committee reviewed and approved the study (reference number 23469(T1)-2).

The study is being conducted under the Declaration of Helsinki and Good Clinical Practice guidelines, which include the protection of data and patient privacy. All participants are required to provide written informed consent before participating.

Population

Patients that fulfill the following criteria are eligible to participate in the study:

Inclusion criteria

• A clinical diagnosis of DM1 based on research criteria, [10] or genetic confirmation of DM1 diagnosis
• Adult patients ages 18 to 70 years old (inclusive)
• Competent to provide informed consent

Exclusion criteria

• Symptomatic renal or liver disease, uncontrolled diabetes or thyroid disorder, or active malignancy other than skin cancer
• Current alcohol or substance abuse
• Concurrent enrollment in clinical trial for DM1, or participation in trial within 6 months of entry
• Concurrent pregnancy or planned pregnancy during the course of the study
• Concurrent medical condition that would, in the opinion of the investigator or clinical evaluator compromise performance on study measures
• Use of mexiletine or other anti-myotonia agents within 72 hours of baseline visit
• Note: non-ambulatory participants are not excluded but are limited to <15% of enrollment

At least half of the 60 patients undergoing the tibialis anterior (TA) muscle biopsy will need to have moderate or severe weakness of ankle dorsiflexion, defined as MRC score ≤ 4 + , to mimic expected populations in clinical trials, and drive robust comparison of muscle weakness with splicing defects. For this subgroup there are additional exclusion criteria, namely:

• CTG repeat expansion size less than 80 repeats, unless there are clear cut signs of limb weakness and muscle wasting
• Conditions or medication that increase the risk of bleeding, including anticoagulant use, history of bleeding or a known platelet count < 50 × 10⁹/L
• Advanced wasting of TA muscle that, in the opinion of the investigator, makes it unlikely that needle biopsy can obtain an adequate muscle sample
• Previous muscle biopsy of either TA, in order to provide muscle tissue samples of non-biopsied muscles

Procedures and assessments

Prior to enrollment, written informed consent will be obtained from every participant. Study visits will be performed at baseline and months 12 and 24. A subset of 60 participants has an extra visit 3 months after baseline and undergoes a muscle biopsy at baseline and the 3 month visit in addition to the standard set of assessments. An overview of assessments at each visit is provided in Fig 1. Study visits can take place over 2 days, provided that day 2 is within 7 days of day 1. Assessments will be performed in a fixed order by trained physical therapists serving as clinical evaluators.

No interventions are planned in this observational natural history study; the schedule outlines enrolment and assessment time points only. “X” indicates that the procedure is performed at the given time point. The baseline visit is at month 0, with follow-up visits at months 12 and 24 for all participants, and an additional visit at month 3 for 60 participants in the muscle biopsy subgroup.

At baseline, demographic and social status information such as sex, race, ethnicity, education, and employment status will be collected. A medical history and physical examination will be performed and vital signs will be recorded. The age at onset of various symptoms will be documented, as well as common comorbidities.

Forced vital capacity (in sitting and supine position) and electrocardiograms will be performed to assess respiratory and cardiac involvement. Measures of muscle strength and timed function will be performed to assess impact on skeletal muscle. Patient reported outcome measures will be used to assess overall impact across many organ systems and extend of neurologic impairment, as described below.

Biological samples

At each visit, blood samples will be collected for genetic testing and for biomarker discovery purposes. Samples may be used to extract plasma, DNA, RNA, metabolites or proteins. Urine samples will be collected at each visit for biochemical analysis.

A subgroup of 60 patients will undergo muscle biopsies from the tibialis anterior muscle at the baseline and three-month visits. The initial biopsy will be from the left leg, the second biopsy from the right leg. The tissue will be obtained through a needle biopsy as described previously. [26] A total of 2–4 aspirations from the same site will be taken. The samples will be snap frozen in liquid nitrogen in a single vial and used for analyses of RNA, and potentially DNA, proteins, or histological analysis.

Participants in the study will provide their consent for muscle tissue, blood, and urine to be placed in a BioBank. Materials in the BioBank that remain after completion of this study may be used for future research by investigators within or outside this study group.

Multi-site infrastructure

Virginia Commonwealth University will serve as the overall coordinating study, responsible for participant recruitment, project and data management, and statistical core. Study oversight will be the responsibility of the Executive committee, which is composed of the study co-investigators, the lead physical therapist (PT) clinical evaluator, and study biostatistician.

An in-person formal site initiation meeting took place prior to the start of the study, where focus was directed towards training investigators, study coordinators, clinical evaluators (CE) and other relevant site staff. To help ensure proper and consistent test administration across evaluators/sites, all evaluators undergo training during the investigator meeting on the standardized administration of clinical outcome assessments.

To minimize inter-center variability, standardized operating procedures are implemented for all assessments, and equipment such as QMT or IOPI devices is calibrated across sites. Centralized review is applied for selected measures (e.g., VHOT), and predefined data quality checks are integrated into the study database to ensure consistency and reliability.

A yearly meeting will be organized throughout the conduct of the study to review the protocol, recruitment and challenges with achieving study goals, and refresher training on clinical assessments. Additionally, regular conference calls will be scheduled with participating sites.

Statistical analyses and power/sample size calculation

The change in outcome measures from baseline to 12 and 24 months will analyzed using longitudinal models to account for within-subject correlations and repeated measures over time. Specifically, for continuous outcomes, linear mixed-effects models will be applied (including random intercepts for participants and site, and random slopes where appropriate), modeling time as a categorical factor to capture potential non-linear trajectories and allowing covariate adjustment (e.g., age, sex, baseline severity, and year of enrollment). For non-normal outcomes, generalized mixed-effects models with suitable link functions will be used. Paired t-tests may be retained for descriptive, secondary summaries.

All statistical significance will be set at alpha of 0.05. Responsiveness will be assessed by determining the effect size and standardized response mean.

The responsiveness-to-change index will be assessed by dividing the absolute change scores in the DM1 group with stratification at the subgroup level (e.g., age, gender). Go-no go decision point for each outcome measure will be set for a responsive index of at least 0.5. This decision will be reviewed by the study investigators and advisory panel periodically during the project period and adjusted as necessary.

Further statistical analyses will be mainly focused on demonstrating clinically relevant changes in clinical outcomes. For determining the ‘minimally clinically importance difference (MCID)’ both distributional and anchor-based methods will be used.

The distributional method chosen is that of the standard error of measurement (SEM). The SEM is the variability in the results from a specific clinical outcome measures that is attributed to instrument unreliability, in which a change smaller than the calculated SEM is likely to be due to measurement error rather than a true change. [27]

For the anchor-based method, clinical outcomes will be compared among the categories of the ‘Domain Delta’ questionnaire (e.g.,: unchanged, a little better, a lot better, etc.) using the Jonckheere-Terpsta test.

Interim analyses will be performed to identify domains that may need additional support or prioritization as the study progresses. To account for potential temporal variability in data collected over the long study period (2018–2027), year of enrollment will be included as a covariate in relevant models, and sensitivity analyses stratified by enrollment period will be performed to evaluate possible time-related effects on outcome measures or biomarkers.

Given the observational and multicenter nature of the study, potential selection bias due to underrepresentation of more severely affected patients will be explored in collaboration with biostatistical experts during the analytic phase. Appropriate statistical approaches will be considered to evaluate and, if possible, adjust for such effects.

Missing data will be addressed under the missing-at-random (MAR) assumption inherent to mixed-effects models, and sensitivity analyses using multiple imputation or pattern-mixture models may be conducted to assess robustness.

The sample size for this study was designed to capture a clinically meaningful change in the clinical outcome assessments. In order to estimate a sample size based on the continuous variable of change in time in the 10-meter walk/run, the following assumptions were made to capture precision: a mean change of 0.277 with a standard deviation of 1.00. With an alpha of 0.05 and a two-sided two samples t-test achieving 93.5% power, a total sample size of 700 subjects will be needed to compare two subgroups of 315 individuals each in the mild DM1 and the more severely affected DM1 groups. Separating these groups will be key to identifying those individuals most suited for clinical trials. This sample size accounts for a 10% drop out rate, as was identified in prior studies. This sample size would also allow for computation of sensitivity and specificity of the primary endpoints to discriminate these two subgroups.