Repeat Interruptions Modify Age at Onset in Myotonic Dystrophy Type 1 by Stabilizing DMPK Expansions in Somatic Cells

Abstract

CTG expansions in DMPK gene, causing myotonic dystrophy type 1 (DM1), are characterized by pronounced somatic instability. A large proportion of variability of somatic instability is explained by expansion size and patient’s age at sampling, while individual-specific differences are attributed to additional factors. The age at onset is extremely variable in DM1, and inversely correlates with the expansion size and individual-specific differences in somatic instability. Three to five percent of DM1 patients carry repeat interruptions and some appear with later age at onset than expected for corresponding expansion size. Herein, we characterized somatic instability of interrupted DMPK expansions and the effect on age at onset in our previously described patients. Repeat-primed PCR showed stable structures of different types and patterns of repeat interruptions in blood cells over time and buccal cells. Single-molecule small-pool PCR quantification of somatic instability and mathematical modeling showed that interrupted expansions were characterized by lower level of somatic instability accompanied by slower progression over time. Mathematical modeling demonstrated that individual-specific differences in somatic instability had greater influence on age at onset in patients with interrupted expansions. Therefore, repeat interruptions have clinical importance for disease course in DM1 patients due to stabilizing effect on DMPK expansions in somatic cells.

Variant repeats within the DMPK CTG expansion protect function in myotonic dystrophy type 1

“…[30][31][32][33] Variant repeat-mediated suppression of somatic instability in DM1 has also been observed and strongly associated with milder symptoms and later age at onset. 13,[18][19][20][21]34 Patients with DM1 experience a slow progression of muscle weakness and atrophy, initially involving the distal muscles of the extremities and later affecting the proximal musculature. Patients can develop dysphagia and respiratory muscle weakness, with an increased risk for weight loss and aspiration.…”

“…18,19 The attenuation of symptoms is hypothesized to result, at least in part, from a stabilizing effect of RI that reduces expansion-biased instability in somatic cells. 13,20,21 In this context, we set out to compare motor, neurocognitive, and behavioral outcome measures of participants with adult-onset DM1 with RIs matched to participants with DM1 with PRs, as well as comparison of both groups to participants unaffected by DM1.…”

On the wrong DNA track: Molecular mechanisms of repeat-mediated genome instability

“…In other words, a loss of AGG interruption likely predisposes CGG repeat tracts to large-scale expansions (149,152). Generally, the stabilizing role of interruptions is common for expandable repeats and has been reported for GAA (153,154), CAG (38,128,129,(155)(156)(157)(158)(159)(160)(161)(162), CCTG (54,58,(163)(164)(165), and other repeats in somatic tissues, intergenerational transmission, and experimental systems.…”

Stable Longitudinal Methylation Levels at the CpG Sites Flanking the CTG Repeat of DMPK in Patients with Myotonic Dystrophy Type 1

“…Research over the last decade has shown that the variability in clinical phenotype and age of onset can be correlated to sequence interruptions in the CTG repeat [13,[16][17][18], as well as DNA methylation of the area surrounding the CTG repeat [19,20]. The hypermethylation of the DM1 locus was, at first, thought to only be present in patients with congenital onset DM1 (CDM1) [21]; however, studies have shown hypermethylation to also be present in patients with non-congenital DM1 [4,19,22] and particularly in patients with repeat interruptions [4,20].…”

“…Barbè et al [19] reported the hypermethylation of both upstream and downstream CpG sites, mainly in patients with maternal inheritance of the disease allele and only in patients with more than 600 CTG repeats, while Legarè et al [20] observed variability in the methylation of downstream, but not upstream, CpG sites. While repeat interruptions have been associated with later onset [16,17], lower levels of somatic instability [17,18], and a milder phenotype [18], the association of methylation with clinical features has been more unclear. Hypermethylation has been correlated with earlier disease onset [4,19], muscular and respiratory features [20], and longer CTG repeats [4,19,20] but also with lower levels of somatic instability [20].…”