DM Families Research Articles

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.

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

Myotonic dystrophy type 1 (DM1) is the most complex and variable trinucleotide repeat disorder caused by an unstable CTG repeat expansion, reaching up to 4000 CTG in the most severe cases. The genetic and clinical variability of DM1 depend on the sex and age of the transmitting parent, but also on the CTG repeat number, presence of repeat interruptions and/or on the degree of somatic instability. Currently, it is difficult to simultaneously and accurately determine these contributing factors in DM1 patients due to the limitations of gold standard methods used in molecular diagnostics and research laboratories. Our study showed the efficiency of the latest PacBio long-read sequencing technology to sequence large CTG trinucleotides, detect multiple and single repeat interruptions and estimate the levels of somatic mosaicism in DM1 patients carrying complex CTG repeat expansions inaccessible to most methods. Using this innovative approach, we revealed the existence of de novo CCG interruptions associated with CTG stabilization/contraction across generations in a new DM1 family. We also demonstrated that our method is suitable to sequence the DM1 locus and measure somatic mosaicism in DM1 families carrying more than 1000 pure CTG repeats. Better characterization of expanded alleles in DM1 patients can significantly improve prognosis and genetic counseling, not only in DM1 but also for other tandem DNA repeat disorders.

A DM1 family with interruptions associated with atypical symptoms and late onset but not with a milder phenotype.

Carriage of interruptions in CTG repeats of the myotonic dystrophy protein kinase gene has been associated with a broad spectrum of myotonic dystrophy type 1 (DM1) phenotypes, mostly mild. However, the data available on interrupted DM1 patients and their phenotype are scarce. We studied 49 Spanish DM1 patients, whose clinical phenotype was evaluated in depth. Blood DNA was obtained and analyzed through triplet-primed polymerase chain reaction (PCR), long PCR-Southern blot, small pool PCR, AciI digestion, and sequencing. Five patients of our registry (10%), belonging to the same family, carried CCG interruptions at the 3'-end of the CTG expansion. Some of them presented atypical traits such as very late onset of symptoms ( > 50 years) and a severe axial and proximal weakness requiring walking assistance. They also showed classic DM1 symptoms including cardiac and respiratory dysfunction, which were severe in some of them. Sizes and interrupted allele patterns were determined, and we found a contraction and an expansion in two intergenerational transmissions. Our study contributes to the observation that DM1 patients carrying interruptions present with atypical clinical features that can make DM1 diagnosis difficult, with a later than expected age of onset and a previously unreported aging-related severe disease manifestation.

Survival patterns and cancer determinants in families with myotonic dystrophy type 1.

Research indicates that patients with myotonic dystrophy type 1 (DM1) are at increased risk of cancer and early death. Family data may provide insights given DM1 phenotypic heterogeneity, the broad range of non-muscular manifestations and the usual delays in the diagnosis of DM1. Family history data were collected from 397 genetically and/or clinically confirmed DM1 patients (respondents) enrolled in the US or UK myotonic dystrophy registries. Standardized mortality ratios were calculated for DM1 first-degree relatives (parents, siblings and offspring) by their reported DM1 status (affected, unaffected or unknown). For cancer-related analyses, mixed effects logistic regression models were used to evaluate factors associated with cancer development in DM1 families, including familial clustering. A total of 467 deaths and 337 cancers were reported amongst 1737 first-degree DM1 relatives. Mortality risk amongst relatives reported as DM1-unaffected was comparable to that of the general population [standardized mortality ratio (SMR)0.82, P=0.06], whilst significantly higher mortality risks were noted in DM1-affected relatives (SMR=2.47, P<0.0001) and in those whose DM1 status was unknown (SMR=1.60, P<0.0001). In cancer risk analyses, risk was higher amongst families in which the DM1 respondent had cancer (odds ratio 1.95, P=0.0001). Unknown DM1 status in the siblings (odds ratio2.59, P=0.004) was associated with higher cancer risk. There is an increased risk of death, and probably cancer, in relatives with DM1 and in those whose DM1 status is unknown. This suggests a need to perform a careful history and physical examination, supplemented by genetic testing, to identify family members at risk for DM1 and who might benefit from disease-specific clinical care and surveillance.

Fuchs' Endothelial Corneal Dystrophy in Patients With Myotonic Dystrophy, Type 1.

PurposeRNA toxicity from CTG trinucleotide repeat (TNR) expansion within noncoding DNA of the transcription factor 4 (TCF4) and DM1 protein kinase (DMPK) genes has been described in Fuchs' endothelial corneal dystrophy (FECD) and myotonic dystrophy, type 1 (DM1), respectively. We prospectively evaluated DM1 patients and their families for phenotypic FECD and report the analysis of CTG expansion in the TCF4 gene and DMPK expression in corneal endothelium.MethodsFECD grade was evaluated by slit lamp biomicroscopy in 26 participants from 14 families with DM1. CTG TNR length in TCF4 and DMPK was determined by a combination of Gene Scan and Southern blotting of peripheral blood leukocyte DNA.ResultsFECD grade was 2 or higher in 5 (36%) of 14 probands, significantly greater than the general population (5%) (P < 0.001). FECD segregated with DM1; six of eight members of the largest family had both FECD and DM1, while the other two family members had neither disease. All DNA samples from 24 subjects, including four FECD-affected probands, were bi-allelic for nonexpanded TNR length in TCF4 (<40 repeats). Considering a 75% prevalence of TCF4 TNR expansion in FECD, the probability of four FECD probands lacking TNR expansion was 0.4%. Neither severity of DM1 nor DMPK TNR length predicted the presence of FECD in DM1 patients.ConclusionsFECD was common in DM1 families, and the diseases cosegregated. TCF4 TNR expansion was lacking in DM1 families. These findings support a hypothesis that DMPK TNR expansion contributes to clinical FECD.

Incidence of amplification failure in DMPK allele due to allelic dropout event in a diagnostic laboratory

BackgroundMyotonic dystrophy type 1 (DM1) is caused by an expanded CTG repeat in the non-coding 3’ UTR of the DMPK gene. PCR and Southern Blot Analysis (SBA) of long-range PCR represent the routine molecular testing most widely used for DM1 diagnosis.However, in these conventional methods artifacts such as allele dropout (ADO) represent a risk of misdiagnosis for DM1. Subjects, who show a single product by conventional methods, require a complementary technique such as triplet repeat primed PCR (TP-PCR). ObjectTo estimate and minimize the incidence of allele dropout event in our diagnostic molecular laboratory by the use of new kit TP-PCR-based. MethodsWe retrospectively studied 190 DMPK alleles, on blood samples from to ninety-five subjects, previously genotyped by traditional methods to validate a new assay. The pedigree of a DM1 family was used to expand our analysis. ResultsTP-PCR assay correctly identified all 95/95 (100%) subjects and these results were in agreement with the other molecular laboratory. By conventional methods the amplification failure due to allele dropout in our cohort was in 12/190 (6.3%) DMPK alleles analyzed. When these 12 alleles were detected and solved by new assay, we found that the 2.6% was caused by primer sequence-dependent and the remaining 3.6% by polymerase-hindering secondary structures. ConclusionsAllele dropout could be considered as a potentially important problem in DM1 diagnosis that may lead to the attribution of a wrong genotype with long-term consequences for both proband and family.

Unusual association of a unique CAG interruption in 5' of DM1 CTG repeats with intergenerational contractions and low somatic mosaicism.

Myotonic dystrophy type 1 (DM1) is a dominant multisystemic disorder associated with high variability of symptoms and anticipation. DM1 is caused by an unstable CTG repeat expansion that usually increases in successive generations and tissues. DM1 family pedigrees have shown that ∼90% and 10% of transmissions result in expansions and contractions of the CTG repeat, respectively. To date, the mechanisms of CTG repeat contraction remain poorly documented in DM1. In this report, we identified two new DM1 families with apparent contractions and no worsening of DM1 symptoms in two and three successive maternal transmissions. A new and unique CAG interruption was found in 5' of the CTG expansion in one family, whereas multiple 5' CCG interruptions were detected in the second family. We showed that these interruptions are associated with maternal intergenerational contractions and low somatic mosaicism in blood. By specific triplet-prime PCR, we observed that CTG repeat changes (contractions/expansions) occur preferentially in 3' of the interruptions for both families.

Molecular and clinical spectrum of type 1 myotonic dystrophy

Myotonic dystrophy type 1 is a chronic, slowly progressing, inherited multisystemic autosomal-dominant disease, caused by expansion of CTG repeats in DMPK gene. The aim of present study was to analyze the molecular expansion in DM1 families and in normal individuals and to establish its relationships with disease. Clinically suspected 30 subjects (from 29 families) of DM1, 75 family members and 400 normal individuals were included in the study. Molecular diagnosis of CTG repeat expansion was performed by Myotonic Dystrophy Short PCR (MDSP) and Triplet primed-PCR (TP-PCR) and followed fragment analysis on ABI-310 Genetic Analyzer. SPSS version 16 and Pearson correlation coefficient were used for statistical analysis. All 30 patients had CTG repeat expansion pattern, correlated well with clinical phenotypes of DM1. Among 75, nine family members were permutated, while sixty six were normal. The allele frequency to repeat no. 5, 9–13 constituted approximate 80% of all 15 distributed allele (3–29) in normal individuals of DM1 families. Among, 400 normal individuals, the allele frequency to repeat number 5, 9–12, and 14–15 constituted approximate 85% out of 23 distributed allele (3–29). The expanded CTG repeat significantly correlated with age at onset, myotonia, wasting, weakness, hypersomnia, abnormal nerve conduction, clinical score and muscular disability rating scale. MDSP and TP-PCR could be successfully used for the identification of repeat expansion in normal individuals and in DM1 families. The finding is helpful not only for laboratory physicians performing tests and making molecular diagnosis, but also for clinicians diagnosing and counselling patients with variable onset and systemic involvement.

Identification and characterization of 5' CCG interruptions in complex DMPK expanded alleles.

Myotonic dystrophy type 1 is a multisystemic autosomal dominant disorder caused by the expansion of (CTG) n triplets in the 3'UTR of the DMPK gene, on chromosome 19q13.3. In the last years, few DM1 patients with different patterns of CCG/CTC interruptions at the 3' end of the DMPK expanded tract have been described. However, the role of these interruptions in DM1 pathogenesis is still unclear. To study the frequency, stability and the structure of DMPK variant expanded alleles in the Italian population, we have re-evaluated 254 Italian DM1 patients using triplet-primed PCR (TP-PCR), at both the 3' and 5' ends of the CTG expansion. In addition, three DM1 families were also investigated in order to analyze the intergenerational stability of the interrupted DMPK alleles. Fourteen DM1 patients showed a TP-PCR electrophoretic profile indicating CCG/CTC interruptions within the CTG expansion. Interestingly, interruptions have been detected and, for the first time, sequenced at the 5' end of the CTG array. Analysis of five intergenerational transmissions revealed a substantial intrafamilial stability of the DM1 mutation among relatives. Our results support the hypothesis that CCG/CTC interruptions within the DMPK expanded alleles have a stabilizing effect on the mutational dynamics and can modulate the severity of symptoms in DM1 patients.

Parent-reported multi-national study of the impact of congenital and childhood onset myotonic dystrophy.

The frequency and impact of symptoms experienced by patients with congenital, childhood, and juvenile-onset myotonic dystrophy (CDM/ChDM/JDM) is not documented. This report identifies symptomatic areas with the greatest disease burden in an international population of patients with early-onset myotonic dystrophy type-1 (DM1). We distributed surveys to parents of patients with CDM/ChDM/JDM. Patients with CDM/ChDM/JDM were members of the US National Registry of DM1 Patients and Family Members, the Canadian Neuromuscular Disease Registry, or the Swedish Health System. Surveys inquired about 325 symptoms and 20 themes associated with CDM/ChDM/JDM. Parents identified the importance of each symptom and theme to their affected child. The prevalence of each symptom and theme were compared across subgroups of patients. The statistical analysis was performed using Fisher's exact and Kruskal-Wallis tests. One hundred and fifty parents returned surveys. The most frequently reported symptomatic themes in children were issues involving communication (81.7%) and problems with hands or fingers (79.6%). Problems with communication and fatigue were the issues that were reported to have the greatest impact on childrens' lives, while 24.1% of children reported cardiac disorders and 15.8% had problems with anesthesia. A range of symptoms contribute to the burden of disease faced by children with DM1. Many of these symptoms are under-recognized.

Mapping and analysis of Caenorhabditis elegans transcription factor sequence specificities.

Caenorhabditis elegans is a powerful model for studying gene regulation, as it has a compact genome and a wealth of genomic tools. However, identification of regulatory elements has been limited, as DNA-binding motifs are known for only 71 of the estimated 763 sequence-specific transcription factors (TFs). To address this problem, we performed protein binding microarray experiments on representatives of canonical TF families in C. elegans, obtaining motifs for 129 TFs. Additionally, we predict motifs for many TFs that have DNA-binding domains similar to those already characterized, increasing coverage of binding specificities to 292 C. elegans TFs (∼40%). These data highlight the diversification of binding motifs for the nuclear hormone receptor and C2H2 zinc finger families and reveal unexpected diversity of motifs for T-box and DM families. Motif enrichment in promoters of functionally related genes is consistent with known biology and also identifies putative regulatory roles for unstudied TFs.

Resveratrol enhances splicing of insulin receptor exon 11 in myotonic dystrophy type 1 fibroblasts.

Myotonic dystrophy type 1 (DM1) is characterized by splicing abnormalities caused by CUG expansion of the DMPK gene transcript. Splicing of exon 11 of the insulin receptor (IR) gene is deregulated to suppress exon 11 inclusion into mRNA in DM1. Consequently, the exon 11-deleted IR isoform that is less sensitive to insulin is predominantly produced, leading to glucose intolerance in DM1. Upregulation of exon 11 retaining full-length IR mRNA is a potential way to recover insulin sensitivity in DM1. We examined candidate chemicals for their ability to enhance inclusion of exon 11 of the IR gene in cultured cells by reverse transcription-PCR amplification of a fragment extending from exons 10 to 12 of IR mRNA. We revealed that resveratrol (RES) enhanced the percentage of exon 11-containing IR mRNA among the total IR mRNA in HeLa cells. The RES-mediated enhancement of exon 11 inclusion was cell-specific and highest in fibroblasts. We tested RES on four fibroblast samples from three generations of one DM1 family. In each sample, RES treatment significantly upregulated the percentage of exon 11-containing IR mRNA to levels higher than that of the control, irrespective of the length of the sample's CTG repeat expansion. A natural compound, RES, was shown for the first time to upregulate the full-length IR mRNA in fibroblasts from DM1 cases. Our results provide the justification of RES as a leading compound to improve glucose tolerance in DM1.

Parental age effects, but no evidence for an intrauterine effect in the transmission of myotonic dystrophy type 1.

Myotonic dystrophy type 1 (DM1) is caused by the expansion of an unstable CTG repeat (g.17294_17296(45_1000)) with more repeats associated with increased disease severity and reduced age at onset. Expanded disease-associated alleles are highly unstable in both the germline and soma. Germline instability is expansion biased, providing a molecular explanation for anticipation. Somatic instability is expansion biased, size- and age-dependent, features that have compromised genotype-phenotype correlations and intergenerational studies. We corrected these confounding factors by estimating the progenitor allele length in 54 father-offspring and 52 mother-offspring pairs in Costa Rican DM1 families. Not surprisingly, we found major parental allele length effects on the size of the allele transmitted, the magnitude of the intergenerational length change, the age at onset in the next generation and the degree of anticipation in both male and female transmissions. We also detected, for the first time, an age-of-parent effect for both male and female transmission. Interestingly, we found no evidence for an intrauterine effect in the transmission of congenital DM1, suggesting previous reports may have been an artefact of age-dependent somatic instability and sampling bias. These data provide new insights into the germline dynamics of the CTG repeat and opportunities for providing additional advice and more accurate risk assessments to prospective parents in DM1 families.

Illuminating CNS and cognitive issues in myotonic dystrophy: Workshop report

A workshop entitled Illuminating CNS and Cognitive Issues in Myotonic Dystrophy, sponsored by the Marigold Foundation, was held in Toronto in May, 2012. Participants included 24 clinicians, scientists and patient representatives from six countries and from diverse fields of myotonic dystrophy type 1 (DM1) research. The purpose was to promote a more accurate understanding of DM1 pathology [1], to highlight specific areas and focus efforts on understanding CNS dysfunction as a critical area of investigation for DM1 research, with the overarching aim of reducing the heavy burden that CNS dysfunction places on DM1 patients and families. DM CNS dysfunction and cognitive impairment is a challenging area of study due to the disease’s complex pathophysiology and the diversity and variability of the clinical presentation, coupled with our limited understanding of brain function. Cognitive function in myotonic dystrophy type 2 (DM2) has similarities, though reportedly less acute than in DM1. A discussion-based forum explored numerous topics summarized below, and yielded a consensus list of action items and research areas needing development. Coordinated CNS research in DM is needed and this report serves as a starting point on this path.

Report of an unusual family of myotonic dystrophy with possible extended approach for genetic counselling: a case report

Myotonic Dystrophy Type 1 is caused by an expanded number of CTG repeats in the 3′ UTR of a DMPK gene and has an incidence of ~1 in 7500 adults. It varies in normal population from 5-34 repeats to > 40 repeats in affected individuals. We report the CTG repeat pattern of a DM1 family in which three patients with their asymptomatic or normal family members. The intention of this case report is the diagnosis of a DM1 family and the establishment of TP-PCR methodology and genetic counseling approaches in early screening of affected patient and their family members. Clinical and biochemical testing involve in the initial prediction and diagnosis of the disease which is confirmed by molecular testing. Thus, TP-PCR could be successfully used for the identification of repeat size in DM1 and enable us to prediction of those myotonic cases where presymptomatic and prenatal diagnosis is required.

INTERGENERATIONAL CONTRACTION OF THE CTG REPEATS IN 2 FAMILIES WITH MYOTONIC DYSTROPHY TYPE 1

Myotonic dystrophy type 1 (DM1) is an autosomal dominant multisystemic disorder caused by the expansion of an unstable CTG trinucleotide repeat at chromosome region 19q13.3.1 The number of CTG repeats is unstable in the abnormal range and usually increases in size in successive generations, in association with anticipation.2,3 A decrease in the CTG repeat size during transmission from parents to child can also occur in about 6.4% of transmissions, most frequently during paternal transmissions (10%).4 In the French-Canadian DM1 population, intergenerational contractions occur in about 7.4% of transmissions, all in cases of paternal transmission. We report here 2 French-Canadian DM1 families with paternal transmission, both originating from the Saguenay-Lac St-Jean, in which all affected children display CTG contractions. Although it was already reported that intergenerational contractions could be observed in several sibs in a same family, it was not noted whether this occurred in all affected sibs.4 Overall, these observations support the existence of a paternal factor that prevents CTG expansion. ### Case reports. The pedigree of the 2 families and Southern blot analysis are shown in the figure. Figure Family tree and Southern blot Pedigree of family (A, B). (C, D) Southern blot analysis. The DNA (3–5 μg) was …

Prevalence of allergic symptoms among children with diabetes mellitus type 1 of different socioeconomic status

The aim of the study was to assess the possible associations between allergies and type 1 diabetes mellitus (DM1), stratified by social class. We studied 127 children with DM1 with a median age of 10.8 yr and 150 controls of comparable age and sex distribution. The parents completed questionnaires on their education and occupation and on their children's history of allergic symptoms, breast-feeding, viral infections, and measles-mumps-rubella (MMR) vaccination. Lower family's social class was more frequently encountered among the DM1 families than in the controls (OR = 0.56, 95% CI: 0.35-0.92). The occurrence of any allergic symptoms among children with DM1 (35.45%) was not significantly different from the controls (38.78%), neither in the total group (OR = 0.87, 95% CI: 0.52-1.45) nor in the stratified analysis by social class. Similar findings were observed regarding the different types of allergic symptoms. In the univariate analysis, breast-feeding, the experience of viral infections, and MMR vaccination were found to be protective of DM1 presentation in both upper and lower social classes. In the multiple logistic regression analysis, the experience of more than 2 infections/yr (OR = 0.12, 95% CI: 0.04-0.34), the origin from middle and upper social classes (OR = 0.42, 95% CI: 0.22-0.80) and breast-feeding (OR = 0.58, 95% CI: 0.31-1.07) were protective of DM1 occurrence. In children with DM1, the presence of allergic symptoms was not associated with the development of DM1. Among the environmental factors, the origin from middle or upper social classes, breast-feeding, the experience of viral infections, and MMR vaccination were found to have a protective effect on DM1 presentation.

Familial aggregation of white matter lesions in myotonic dystrophy type 1

This study aimed to determine whether white matter lesions, previously described as a frequent feature in myotonic dystrophy type 1 (DM1), aggregate within DM1 families or are sporadic findings, and to explore the relationship between these lesions and clinical or genetic features. Brain MRI of 60 DM1 patients belonging to 22 families were evaluated and white matter lesions were rated according to a semiquantitative method. Presence and extent of lobar, temporal or periventricular lesions showed a significant association with the family history of lesions and the disease duration, and no association with the CTG repeat size. Furthermore, parent-offspring and sibling pairs showed a significant positive concordance for lesion severity. White matter lesions demonstrate familial aggregation in DM1 and no relationship with CTG repeat length. These findings suggest that other genetic causes and/or unknown environmental factors influence the occurrence and severity of lesions in patients carrying the DM1 genetic defect.

Prenatal diagnosis in myotonic dystrophy type 1. Thirteen years of experience: implications for reproductive counselling in DM1 families

To analyse the results obtained from prenatal diagnoses in myotonic dystrophy type 1 (DM1) performed in our hospitals during the last 13 years. Molecular analyses were conducted on chorionic villi or cultured amniotic fluid samples obtained for prenatal diagnosis of DM1. CTG expansion was analyzed by polymerase chain reaction (PCR) and Southern blot techniques. From 154 prenatal diagnoses performed in 13 years, 51% were found to be healthy and 49% affected. Considering the 75 carriers of the mutation, in 65.3% of the cases, the mother was the transmitting parent versus 36.5% of fathers. From these female transmissions, 31/49 foetuses had expansion in the neonatal form range, namely, congenital myotonic dystrophy (CMD). In our series, no significant deviation of the 50% expected frequency of transmission in autosomal dominant disorder was seen. We show that when the disease is transmitted by a male, the mean intergenerational variation is minimal (mean = 56 CTG, SD = 177 CTG). However, this does not occur in the affected mothers, where the mean intergenerational expansion is very high (mean = 948 CTG, SD = 815 CTG) and the difference is statistically significant (t-Student p < 0.0001). Our data have important implications for the genetic counselling of DM1 families.

Anatomy of a founder effect: myotonic dystrophy in Northeastern Quebec

Founder effects are largely responsible for changes in frequency profiles of genetic variants in local populations or isolates. They are often recognized by elevated incidence of certain hereditary disorders as observed in regions of Charlevoix and Saguenay-Lac-Saint-Jean (SLSJ) in Northeastern Quebec. Dominantly transmitted myotonic dystrophy (DM1) is highly prevalent in SLSJ where its carrier rate reaches 1/550, compared with 1/5,000 to 1/50,000 elsewhere. To shed light on the origin of DM1 in this region, we have screened 50 nuclear DM1 families from SLSJ and studied the genetic variation in a 2.05 Mb (2.9 cM) segment spanning the site of the expansion mutation. The markers analyzed included 22 biallelic SNPs and two microsatellites. Among 50 independent DM1 chromosomes, we distinguished ten DM1-associated haplotypes and grouped them into three haplotype families, A, B and C, based on the relevant extent of allele sharing between them. To test whether the data were consistent with a single entry of the mutation into SLSJ, we evaluated the age of the founder effect from the proportion of recombinant haplotypes. Taking the prevalent haplotype A1_21 (58%) as ancestral to all the disease-associated haplotypes in this study, the estimated age of the founder effect was 19 generations, long predating the colonization of Nouvelle-France. In contrast, considering A1_21 as ancestral to the haplotype family A only, yielded the estimated founder age of nine generations, consistent with the settlement of Charlevoix at the turn of 17th century and subsequent colonization of SLSJ. We conclude that it was the carrier of haplotype A (present day carrier rate of 1/730) that was a "driver" of the founder effect, while minor haplotypes B and C, with corresponding carrier rates of 1/3,000 and 1/10,000, respectively, contribute DM1 to the incidence level known in other populations. Other studies confirm that this might be a general scenario in which a major "driver" mutation/haplotype issued from a founder effect is found accompanied by distinct minor mutations/haplotypes occurring at background population frequencies.