Robust Detection of Somatic Mosaicism and Repeat Interruptions by Long-Read Targeted Sequencing in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 testing, 2024 revision: A technical standard of the American College of Medical Genetics and Genomics (ACMG)

Myotonic dystrophy type 1 (DM1) is a form of muscular dystrophy causing progressive muscle loss and weakness. Although clinical features can manifest at any age, it is the most common form of muscular dystrophy with onset in adulthood. DM1 is an autosomal dominant condition, resulting from an unstable CTG expansion in the 3'-untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. The age of onset and the severity of the phenotype are roughly correlated with the size of the CTG expansion. Multiple methodologies can be used to diagnose affected individuals with DM1, including polymerase chain reaction, Southern blot, and triplet repeat-primed polymerase chain reaction. Recently, triplet repeat interruptions have been described, which may affect clinical outcomes of a fully-variable allele in DMPK. This document supersedes the Technical Standards and Guidelines for Myotonic Dystrophy originally published in 2009 and reaffirmed in 2015. It is designed for genetic testing professionals who are already familiar with the disease and the methods of analysis.

Sequence composition changes in short tandem repeats: heterogeneity, detection, mechanisms and clinical implications

Short tandem repeats (STRs) are a class of repetitive elements, composed of tandem arrays of 1-6 base pair sequence motifs, that comprise a substantial fraction of the human genome. STR expansions can cause a wide range of neurological and neuromuscular conditions, known as repeat expansion disorders, whose age of onset, severity, penetrance and/or clinical phenotype are influenced by the length of the repeats and their sequence composition. The presence of non-canonical motifs, depending on the type, frequency and position within the repeat tract, can alter clinical outcomes by modifying somatic and intergenerational repeat stability, gene expression and mutant transcript-mediated and/or protein-mediated toxicities. Here, we review the diverse structural conformations of repeat expansions, technological advances for the characterization of changes in sequence composition, their clinical correlations and the impact on disease mechanisms.

FMR1 Protein Expression Correlates with Intelligence Quotient in Both Peripheral Blood Mononuclear Cells and Fibroblasts from Individuals with an FMR1 Mutation

Fragile X syndrome (FXS) is the most common heritable form of intellectual disability and is caused by CGG repeat expansions exceeding 200 (full mutation). Such expansions lead to hypermethylation and transcriptional silencing of the fragile X messenger ribonucleoprotein 1 (FMR1) gene. As a consequence, little or no FMR1 protein (FMRP) is produced; absence of the protein, which normally is responsible for neuronal development and maintenance, causes the syndrome. Previous studies have demonstrated the causal relationship between FMRP levels and cognitive abilities in peripheral blood mononuclear cells (PBMCs) and dermal fibroblast cell lines of patients with FXS. However, it is arguable whether PBMCs or fibroblasts would be the preferred surrogate for measuring molecular markers, particularly FMRP, to represent the cognitive impairment, a core symptom of FXS. To address this concern, CGG repeats, methylation status, FMR1 mRNA, and FMRP levels were measured in both PBMCs and fibroblasts derived from 66 individuals. The findings indicated a strong association between FMR1 mRNA expression levels and CGG repeat numbers in PBMCs of premutation males after correcting for methylation status. Moreover, FMRP expression levels from both PBMCs and fibroblasts of male participants with a hypermethylated full mutation and with mosaicism demonstrated significant association between the intelligence quotient levels and FMRP levels, suggesting that PBMCs may be preferable for FXS clinical studies, because of their greater accessibility.

Long read sequencing on its way to the routine diagnostics of genetic diseases

The clinical application of technological progress in the identification of DNA alterations has always led to improvements of diagnostic yields in genetic medicine. At chromosome side, from cytogenetic techniques evaluating number and gross structural defects to genomic microarrays detecting cryptic copy number variants, and at molecular level, from Sanger method studying the nucleotide sequence of single genes to the high-throughput next-generation sequencing (NGS) technologies, resolution and sensitivity progressively increased expanding considerably the range of detectable DNA anomalies and alongside of Mendelian disorders with known genetic causes. However, particular genomic regions (i.e., repetitive and GC-rich sequences) are inefficiently analyzed by standard genetic tests, still relying on laborious, time-consuming and low-sensitive approaches (i.e., southern-blot for repeat expansion or long-PCR for genes with highly homologous pseudogenes), accounting for at least part of the patients with undiagnosed genetic disorders. Third generation sequencing, generating long reads with improved mappability, is more suitable for the detection of structural alterations and defects in hardly accessible genomic regions. Although recently implemented and not yet clinically available, long read sequencing (LRS) technologies have already shown their potential in genetic medicine research that might greatly impact on diagnostic yield and reporting times, through their translation to clinical settings. The main investigated LRS application concerns the identification of structural variants and repeat expansions, probably because techniques for their detection have not evolved as rapidly as those dedicated to single nucleotide variants (SNV) identification: gold standard analyses are karyotyping and microarrays for balanced and unbalanced chromosome rearrangements, respectively, and southern blot and repeat-primed PCR for the amplification and sizing of expanded alleles, impaired by limited resolution and sensitivity that have not been significantly improved by the advent of NGS. Nevertheless, more recently, with the increased accuracy provided by the latest product releases, LRS has been tested also for SNV detection, especially in genes with highly homologous pseudogenes and for haplotype reconstruction to assess the parental origin of alleles with de novo pathogenic variants. We provide a review of relevant recent scientific papers exploring LRS potential in the diagnosis of genetic diseases and its potential future applications in routine genetic testing.

Single-Molecule Poly(A) Tail Sequencing (SM-PATseq) Using the PacBio Platform

The polyadenylation of the 3' ends of messenger RNAs is an important regulator of stability and translation. We developed the single-molecule poly(A) tail sequencing method, SM-PATseq, to assay tail lengths of the whole transcriptome at nucleotide resolution using long-read sequencing. This method generates cDNA using an oligo-dT 3' splint adaptor ligation to prime first-strand cDNA synthesis, followed by random hexamer priming for second-strand synthesis. By directly sequencing the cDNA on long-read platforms, we can resolve tail lengths at nucleotide resolution, identify non-A bases within the tail, and quantify transcript abundance analogous to traditional RNAseq methods. Here, we discuss the method for generating, sequencing, and primary analysis of poly(A) tail data from total RNA using the Pacific Biosciences Sequel platform.

Long-read sequencing improves diagnostic rate in neuromuscular disorders

Massive parallel sequencing methods, such as exome, genome, and targeted DNA sequencing, have aided molecular diagnosis of genetic diseases in the last 20 years. However, short-read sequencing methods still have several limitations, such inaccurate genome assembly, the inability to detect large structural variants, and variants located in hard-to-sequence regions like highly repetitive areas. The recently emerged PacBio single-molecule real-time (SMRT) and Oxford nanopore technology (ONT) long-read sequencing (LRS) methods have been shown to overcome most of these technical issues, leading to an increase in diagnostic rate. LRS methods are contributing to the detection of repeat expansions in novel disease-causing genes (e.g., ABCD3, NOTCH2NLC and RILPL1 causing an Oculopharyngodistal myopathy or PLIN4 causing a Myopathy with rimmed ubiquitin-positive autophagic vacuolation), of structural variants (e.g., in DMD), and of single nucleotide variants in repetitive regions (TTN and NEB). Moreover, these methods have simplified the characterization of the D4Z4 repeats in DUX4, facilitating the diagnosis of Facioscapulohumeral muscular dystrophy (FSHD). We review recent studies that have used either ONT or PacBio SMRT sequencing methods and discuss different types of variants that have been detected using these approaches in individuals with neuromuscular disorders.

Long-Read DNA Sequencing: Recent Advances and Remaining Challenges

DNA sequencing has revolutionized medicine over recent decades. However, analysis of large structural variation and repetitive DNA, a hallmark of human genomes, has been limited by short-read technology, with read lengths of 100-300 bp. Long-read sequencing (LRS) permits routine sequencing of human DNA fragments tens to hundreds of kilobase pairs in size, using both real-time sequencing by synthesis and nanopore-based direct electronic sequencing. LRS permits analysis of large structural variation and haplotypic phasing in human genomes and has enabled the discovery and characterization of rare pathogenic structural variants and repeat expansions. It has also recently enabled the assembly of a complete, gapless human genome that includes previously intractable regions, such as highly repetitive centromeres and homologous acrocentric short arms. With the addition of protocols for targeted enrichment, direct epigenetic DNA modification detection, and long-range chromatin profiling, LRS promises to launch a new era of understanding of genetic diversity and pathogenic mutations in human populations.

In Cis Effect of DMPK Expanded Alleles in Myotonic Dystrophy Type 1 Patients Carrying Variant Repeats at 5' and 3' Ends of the CTG Array

Myotonic dystrophy type 1 (DM1) is an autosomal dominant multisystemic disease caused by a CTG repeat expansion in the 3'-untranslated region (UTR) of DMPK gene. DM1 alleles containing non-CTG variant repeats (VRs) have been described, with uncertain molecular and clinical consequences. The expanded trinucleotide array is flanked by two CpG islands, and the presence of VRs could confer an additional level of epigenetic variability. This study aims to investigate the association between VR-containing DMPK alleles, parental inheritance and methylation pattern of the DM1 locus. The DM1 mutation has been characterized in 20 patients using a combination of SR-PCR, TP-PCR, modified TP-PCR and LR-PCR. Non-CTG motifs have been confirmed by Sanger sequencing. The methylation pattern of the DM1 locus was determined by bisulfite pyrosequencing. We characterized 7 patients with VRs within the CTG tract at 5' end and 13 patients carrying non-CTG sequences at 3' end of the DM1 expansion. DMPK alleles with VRs at 5' end or 3' end were invariably unmethylated upstream of the CTG expansion. Interestingly, DM1 patients with VRs at the 3' end showed higher methylation levels in the downstream island of the CTG repeat tract, preferentially when the disease allele was maternally inherited. Our results suggest a potential correlation between VRs, parental origin of the mutation and methylation pattern of the DMPK expanded alleles. A differential CpG methylation status could play a role in the phenotypic variability of DM1 patients, representing a potentially useful diagnostic tool.

Applications of long-read sequencing to Mendelian genetics

Advances in clinical genetic testing, including the introduction of exome sequencing, have uncovered the molecular etiology for many rare and previously unsolved genetic disorders, yet more than half of individuals with a suspected genetic disorder remain unsolved after complete clinical evaluation. A precise genetic diagnosis may guide clinical treatment plans, allow families to make informed care decisions, and permit individuals to participate in N-of-1 trials; thus, there is high interest in developing new tools and techniques to increase the solve rate. Long-read sequencing (LRS) is a promising technology for both increasing the solve rate and decreasing the amount of time required to make a precise genetic diagnosis. Here, we summarize current LRS technologies, give examples of how they have been used to evaluate complex genetic variation and identify missing variants, and discuss future clinical applications of LRS. As costs continue to decrease, LRS will find additional utility in the clinical space fundamentally changing how pathological variants are discovered and eventually acting as a single-data source that can be interrogated multiple times for clinical service.

Repeat Detector: versatile sizing of expanded tandem repeats and identification of interrupted alleles from targeted DNA sequencing

Targeted DNA sequencing approaches will improve how the size of short tandem repeats is measured for diagnostic tests and preclinical studies. The expansion of these sequences causes dozens of disorders, with longer tracts generally leading to a more severe disease. Interrupted alleles are sometimes present within repeats and can alter disease manifestation. Determining repeat size mosaicism and identifying interruptions in targeted sequencing datasets remains a major challenge. This is in part because standard alignment tools are ill-suited for repetitive and unstable sequences. To address this, we have developed Repeat Detector (RD), a deterministic profile weighting algorithm for counting repeats in targeted sequencing data. We tested RD using blood-derived DNA samples from Huntington's disease and Fuchs endothelial corneal dystrophy patients sequenced using either Illumina MiSeq or Pacific Biosciences single-molecule, real-time sequencing platforms. RD was highly accurate in determining repeat sizes of 609 blood-derived samples from Huntington's disease individuals and did not require prior knowledge of the flanking sequences. Furthermore, RD can be used to identify alleles with interruptions and provide a measure of repeat instability within an individual. RD is therefore highly versatile and may find applications in the diagnosis of expanded repeat disorders and in the development of novel therapies.

ATTCT and ATTCC repeat expansions in the ATXN10 gene affect disease penetrance of spinocerebellar ataxia type 10

Spinocerebellar ataxia type 10 (SCA10) is an autosomal-dominant disorder caused by an expanded pentanucleotide repeat in the ATXN10 gene. This repeat expansion, when fully penetrant, has a size of 850-4,500 repeats. It has been shown that the repeat composition can be a modifier of disease, e.g., seizures. Here, we describe a Mexican kindred in which we identified both pure (ATTCT)n and mixed (ATTCT)n-(ATTCC)n expansions in the same family. We used amplification-free targeted sequencing and optical genome mapping to decipher the composition of these repeat expansions. We found a considerable degree of mosaicism of the repeat expansion. This mosaicism was confirmed in skin fibroblasts from individuals with ATXN10 expansions with RNAScope in situ hybridization. All affected family members with the mixed ATXN10 repeat expansion showed typical clinical signs of spinocerebellar ataxia and epilepsy. In contrast, individuals with the pure ATXN10 expansion present with Parkinson's disease or are unaffected, even in individuals more than 20 years older than the average age at onset for SCA10. Our findings suggest that the pure (ATTCT)n expansion is non-pathogenic, while repeat interruptions, e.g., (ATTCC)n, are necessary to cause SCA10. This mechanism has been recently described for several other repeat expansions including SCA31 (BEAN1), SCA37 (DAB1), and three loci for benign adult familial myoclonic epilepsy BAFME (SAMD12, TNRC6A, RAPGEF2). Therefore, long-read sequencing and optical genome mapping of the entire genomic structure of repeat expansions are critical for clinical practice and genetic counseling, as variations in the repeat can affect disease penetrance, symptoms, and disease trajectory.

Identification of a CCG-Enriched Expanded Allele in Patients with Myotonic Dystrophy Type 1 Using Amplification-Free Long-Read Sequencing

Myotonic dystrophy type 1 (DM1) exhibits highly heterogeneous clinical manifestations caused by an unstable CTG repeat expansion reaching up to 4000 CTG. The clinical variability depends on CTG repeat number, CNG repeat interruptions, and somatic mosaicism. Currently, none of these factors are simultaneously and accurately determined due to the limitations of gold standard methods used in clinical and research laboratories. An amplicon method for targeting the DMPK locus using single-molecule real-time sequencing was recently developed to accurately analyze expanded alleles. However, amplicon-based sequencing still depends on PCR, and the inherent bias toward preferential amplification of smaller repeats can be problematic in DM1. Thus, an amplification-free long-read sequencing method was developed by using CRISPR/Cas9 technology in DM1. This method was used to sequence the DMPK locus in patients with CTG repeat expansion ranging from 130 to >1000 CTG. We showed that elimination of PCR amplification improves the accuracy of measurement of inherited repeat number and somatic repeat variations, two key factors in DM1 severity and age at onset. For the first time, an expansion composed of >85% CCG repeats was identified by using this innovative method in a DM1 family with an atypical clinical profile. No-amplification targeted sequencing represents a promising method that can overcome research and diagnosis shortcomings, with translational implications for clinical and genetic counseling in DM1.

Characterization of full-length CNBP expanded alleles in myotonic dystrophy type 2 patients by Cas9-mediated enrichment and nanopore sequencing

Myotonic dystrophy type 2 (DM2) is caused by CCTG repeat expansions in the CNBP gene, comprising 75 to >11,000 units and featuring extensive mosaicism, making it challenging to sequence fully expanded alleles. To overcome these limitations, we used PCR-free Cas9-mediated nanopore sequencing to characterize CNBP repeat expansions at the single-nucleotide level in nine DM2 patients. The length of normal and expanded alleles can be assessed precisely using this strategy, agreeing with traditional methods, and revealing the degree of mosaicism. We also sequenced an entire ~50 kbp expansion, which has not been achieved previously for DM2 or any other repeat-expansion disorders. Our approach precisely counted the repeats and identified the repeat pattern for both short interrupted and uninterrupted alleles. Interestingly, in the expanded alleles, only two DM2 samples featured the expected pure CCTG repeat pattern, while the other seven presented also TCTG blocks at the 3′ end, which have not been reported before in DM2 patients, but confirmed hereby with orthogonal methods. The demonstrated approach simultaneously determines repeat length, structure/motif, and the extent of somatic mosaicism, promising to improve the molecular diagnosis of DM2 and achieve more accurate genotype–phenotype correlations for the better stratification of DM2 patients in clinical trials.

Intergenerational Influence of Gender and the DM1 Phenotype of the Transmitting Parent in Korean Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is the most common autosomal-dominant disorder caused by the CTG repeat expansion of the DMPK, and it has been categorized into three phenotypes: mild, classic, and congenital DM1. Here, we reviewed the intergenerational influence of gender and phenotype of the transmitting parent on the occurrence of Korean DM1. A total of 44 parent–child pairs matched for the gender of the transmitting parent and the affected child and 29 parent–child pairs matched for the gender and DM1 phenotype of the transmitting parent were reviewed. The CTG repeat size of the DMPK in the affected child was found to be significantly greater when transmitted by a female parent to a female child (DM1-FF) (median, 1309 repeats; range, 400–2083) than when transmitted by a male parent to a male child (650; 160–1030; p = 0.038 and 0.048 using the Tukey HSD and the Bonferroni test) or by a male parent to a female child (480; 94–1140; p = 0.003). The difference in the CTG repeat size of the DMPK between the transmitting parent and the affected child was also lower when transmitted from a male parent with classic DM1 (−235; −280 to 0) compared to when it was transmitted from a female parent with mild DM1 (866; 612–905; p = 0.015 and 0.019) or from a female parent with classic DM1 (DM1-FC) (605; 10–1393; p = 0.005). This study highlights that gender and the DM1 phenotype of the transmitting parent had an impact on the CTG repeat size of the DMPK in the affected child, with greater increases being inherited from the DM1-FF or DM1-FC situations in Korean DM1.

High Resolution Analysis of DMPK Hypermethylation and Repeat Interruptions in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a multisystemic neuromuscular disorder caused by the expansion of a CTG repeat in the 3′-UTR of DMPK, which is transcribed to a toxic gain-of-function RNA that affects splicing of a range of genes. The expanded repeat is unstable in both germline and somatic cells. The variable age at disease onset and severity of symptoms have been linked to the inherited CTG repeat length, non-CTG interruptions, and methylation levels flanking the repeat. In general, the genetic biomarkers are investigated separately with specific methods, making it tedious to obtain an overall characterisation of the repeat for a given individual. In the present study, we employed Oxford nanopore sequencing in a pilot study to simultaneously determine the repeat lengths, investigate the presence and nature of repeat interruptions, and quantify methylation levels in the regions flanking the CTG-repeats in four patients with DM1. We determined the repeat lengths, and in three patients, we observed interruptions which were not detected using repeat-primed PCR. Interruptions may thus be more common than previously anticipated and should be investigated in larger cohorts. Allele-specific analyses enabled characterisation of aberrant methylation levels specific to the expanded allele, which greatly increased the sensitivity and resolved cases where the methylation levels were ambiguous.

Uncovering Essential Tremor Genetics: The Promise of Long-Read Sequencing

Long-read sequencing (LRS) technologies have been recently introduced to overcome intrinsic limitations of widely-used next-generation sequencing (NGS) technologies, namely the sequencing limited to short-read fragments (150–300 base pairs). Since its introduction, LRS has permitted many successes in unraveling hidden mutational mechanisms. One area in clinical neurology in need of rethinking as it applies to genetic mechanisms is essential tremor (ET). This disorder, among the most common in neurology, is a syndrome often exhibiting an autosomal dominant pattern of inheritance whose large phenotypic spectrum suggest a multitude of genetic etiologies. Exome sequencing has revealed the genetic etiology only in rare ET families (FUS, SORT1, SCN4A, NOS3, KCNS2, HAPLN4/BRAL2, and USP46). We hypothesize that a reason for this shortcoming may be non-classical genetic mechanism(s) underpinning ET, among them trinucleotide, tetranucleotide, or pentanucleotide repeat disorders. In support of this hypothesis, trinucleotide (e.g., GGC repeats in NOTCH2NLC) and pentanucleotide repeat disorders (e.g., ATTTC repeats in STARD7) have been revealed as pathogenic in patients with a past history of what has come to be referred to as “ET plus,” bilateral hand tremor associated with epilepsy and/or leukoencephalopathy. A systematic review of LRS in neurodegenerative disorders showed that 10 of the 22 (45%) genetic etiologies ascertained by LRS include tremor in their phenotypic spectrum, suggesting that future clinical applications of LRS for tremor disorders may uncover genetic subtypes of familial ET that have eluded NGS, particularly those with associated leukoencephalopathy or family history of epilepsy. LRS provides a pathway for potentially uncovering novel genes and genetic mechanisms, helping narrow the large proportion of “idiopathic” ET.

Myotonic Dystrophies: A Genetic Overview

Myotonic dystrophies (DM) are the most common muscular dystrophies in adults, which can affect other non-skeletal muscle organs such as the heart, brain and gastrointestinal system. There are two genetically distinct types of myotonic dystrophy: myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2), both dominantly inherited with significant overlap in clinical manifestations. DM1 results from CTG repeat expansions in the 3'-untranslated region (3'UTR) of the DMPK (dystrophia myotonica protein kinase) gene on chromosome 19, while DM2 is caused by CCTG repeat expansions in intron 1 of the CNBP (cellular nucleic acid-binding protein) gene on chromosome 3. Recent advances in genetics and molecular biology, especially in the field of RNA biology, have allowed better understanding of the potential pathomechanisms involved in DM. In this review article, core clinical features and genetics of DM are presented followed by a discussion on the current postulated pathomechanisms and therapeutic approaches used in DM, including the ones currently in human clinical trial phase.

Molecular and Clinical Implications of Variant Repeats in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is one of the most variable monogenic diseases at phenotypic, genetic, and epigenetic level. The disease is multi-systemic with the age at onset ranging from birth to late age. The underlying mutation is an unstable expansion of CTG repeats in the DMPK gene, varying in size from 50 to >1000 repeats. Generally, large expansions are associated with an earlier age at onset. Additionally, the most severe, congenital DM1 form is typically associated with local DNA methylation. Genetic variability of DM1 mutation is further increased by its structural variations due to presence of other repeats (e.g., CCG, CTC, CAG). These variant repeats or repeat interruptions seem to confer an additional level of epigenetic variability since local DNA methylation is frequently associated with variant CCG repeats independently of the expansion size. The effect of repeat interruptions on DM1 molecular pathogenesis is not investigated enough. Studies on patients indicate their stabilizing effect on DMPK expansions because no congenital cases were described in patients with repeat interruptions, and the age at onset is frequently later than expected. Here, we review the clinical relevance of repeat interruptions in DM1 and genetic and epigenetic characteristics of interrupted DMPK expansions based on patient studies.

[Identification of new factors inducing CTG.CAG repeat contractions in Myotonic Dystrophy type 1]

Myotonic dystrophy type 1 (DM1) is a multisystemic neuromuscular disease caused by an abnormal CTG repeat expansion in the 3'UTR region of the DMPK gene. In patients, the CTG repeat size varies from fifty to thousands CTG and usually increases across generations (intergenerational instability) and over time in tissues (somatic instability). Larger expansions are associated with more severe symptoms and a decreasing age of onset. Thus, the larger expansions are often associated with the most severe clinical form of DM1 (congenital form). Our PhD project is to identify new genetic and chemical factors reducing the number of repeats and to better understand the mechanisms underlying instability. To this end, genetic and pharmacological screenings are carried out in a HEK293 cell model allowing the rapid detection of expansions (increase in CTG repeat number) and contractions (decrease in CTG repeat number). The effects of different genes and chemical factors, selected during the screening, on the dynamics of the CTG repeat instability will be studied in a DM1 cell model. The results of our work will provide a better understanding of the mechanisms behind contractions. In addition, the identification of new pharmacological compounds promoting CTG contractions and thus reducing or even reversing the progression of disease will offer new therapeutic prospects for DM1 but also for other triplet repeat diseases.

DMPK hypermethylation in sperm cells of myotonic dystrophy type 1 patients

Myotonic dystrophy type 1 (DM1) is an autosomal dominant muscular dystrophy that results from a CTG expansion (50–4000 copies) in the 3′ UTR of the DMPK gene. The disease is classified into four or five somewhat overlapping forms, which incompletely correlate with expansion size in somatic cells of patients. With rare exception, it is affected mothers who transmit the congenital (CDM1) and most severe form of the disease. Why CDM1 is hardly ever transmitted by fathers remains unknown. One model to explain the almost exclusive transmission of CDM1 by affected mothers suggests a selection against hypermethylated large expansions in the germline of male patients. By assessing DNA methylation upstream to the CTG expansion in motile sperm cells of four DM1 patients, together with availability of human embryonic stem cell (hESCs) lines with paternally inherited hypermethylated expansions, we exclude the possibility that DMPK hypermethylation leads to selection against viable sperm cells (as indicated by motility) in DM1 patients.