Molecular genetic and clinical characterization of myotonic dystrophy type 1 patients carrying variant repeats within DMPK expansions

Abstract

Myotonic dystrophy type 1 (DM1) is caused by a highly unstable expansion of CTG repeats in the DMPK gene. Its huge phenotypic variability cannot be explained solely by the repeat number. Recently, variant repeats within the DMPK expansions have emerged as potential disease modifiers. The frequency of variant expanded alleles was estimated in 242 DM1 patients from 174 Serbian families using repeat-primed PCR (RP-PCR). The patterns of variant repeats were determined by direct sequencing of RP-PCR or PCR products. PCR-based southern blot was performed to get insight into the intergenerational mutational dynamics of variant expanded alleles. All patients carrying variant repeats were clinically re-examined. Variant repeats were observed in eight patients from five families (2.9%). They were detected only at the 3' end of DMPK expansions. CCG variant repeats were present in seven patients, either as a part of regular runs of CCGCTG hexamer, individual repeats, or CCG blocks. Analyses of three intergenerational transmissions revealed a considerable stability or likely a contraction of variant expanded alleles. Intriguingly, a decrease in age at onset accompanied these transmissions. Overall, patients were characterized by a milder phenotype and/or some atypical symptoms that could be rather clinically suggestive of myotonic dystrophy type 2. In addition, the first case of de novo CTC variant repeat was observed. Variant repeats might explain a part of the phenotypic variability in a small percent of DM1 patients and likely display a stabilizing effect on the meiotic instability of DMPK expanded alleles.

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

“…The prevalence of interrupted alleles among our patients was ~10%, and 3% among the studied DM1 families. This is in overall agreement with previous studies in which the prevalence in families ranged from 3% to 5% (Botta et al, ; Braida et al, ; Musova et al, ; Pešović et al, ). The type of interruption present in our cohort was CCG, which is currently the most frequently reported variant repeat.…”

“…In the rest of intergenerational transmissions reported, and in the case of our patients P2 and P4, anticipation could not be assessed since patients in the next generation are still asymptomatic. The explanation for these findings is not apparent, since anticipation is not expected in these families; indeed, interruptions are thought to be related to a stabilization or even contraction of the pathological expansion (Braida et al, ; Cumming et al, ; Musova et al, ; Pešović et al, ; Tomé et al, ). However, anticipation was found in our studied intergenerational transmission, with this finding being also reported in other interrupted DM1 patients based on reported age of onset (Table ).…”

“…We detected the contraction of the expansion between patients P2 and P4, but expansion between patients P3 and P5. Previous studies suggest that CTG expansion containing variant repeat patterns display more frequently stable, or even contracted, DMPK alleles instead of further expanded DMPK alleles (Cumming et al, ; Musova et al, ; Pešović et al, ; Tomé et al, ). However, some studies have also found the expansion of the interrupted alleles from one generation to the other (Braida et al, ; Cumming et al, ; Pešović et al, ).…”

“…In our family, no congenital, childhood or juvenile cases of DM1 were observed. Among the interrupted families reported in the literature (Table ), at least three juvenile DM1 cases (age < 18 years) have been described (Braida et al, ; Pešović et al, ), but no congenital or childhood case. Thus, the absence of infantile DM1 seems also to be a distinctive trait for interrupted expansions.…”

Mutational mechanism for DAB1 (ATTTC) n insertion in SCA37: ATTTT repeat lengthening and nucleotide substitution

“…Interestingly, this mutational mechanism has previously been suggested in the origin of pure repetitive stretches of (CCG) n in the middle of the expanded (CTG) n , in DM1. The CCG interruptions may appear individually, in runs of CCGCTG or in a continuously block (Braida et al., ; Cumming et al., ; Pešović et al., ). The CCG blocks may be as large as 40 repeat units (Pešović et al., ).…”