Myotonic dystrophy type 1 (DM1) clinical subtypes and CTCF site methylation status flanking the CTG expansion are mutant allele length-dependent

Abstract

Myotonic dystrophy type 1 (DM1) is a complex disease with a wide spectrum of symptoms. The exact relationship between mutant CTG repeat expansion size and clinical outcome remains unclear. DM1 congenital patients (CDM) inherit the largest expanded alleles, which are associated with abnormal and increased DNA methylation flanking the CTG repeat. However, DNA methylation at the DMPK locus remains understudied. Its relationship to DM1 clinical subtypes, expansion size and age-at-onset is not yet completely understood. Using pyrosequencing-based methylation analysis on 225 blood DNA samples from Costa Rican DM1 patients, we determined that the size of the estimated progenitor allele length (ePAL) is not only a good discriminator between CDM and non-CDM cases (with an estimated threshold at 653 CTG repeats), but also for all DM1 clinical subtypes. Secondly, increased methylation at both CTCF sites upstream and downstream of the expansion was almost exclusively present in CDM cases. Thirdly, levels of abnormal methylation were associated with clinical subtype, age and ePAL, with strong correlations between these variables. Fourthly, both ePAL and the intergenerational expansion size were significantly associated with methylation status. Finally, methylation status was associated with ePAL and maternal inheritance, with almost exclusively maternal transmission of CDM. In conclusion, increased DNA methylation at the CTCF sites flanking the DM1 expansion could be linked to ePAL, and both increased methylation and the ePAL could be considered biomarkers for the CDM phenotype.

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

“…It was associated with large expansions in these patients and believed to be driven by expansion itself in a polarized manner, spanning only upstream regions [ 69 , 70 ]. However, more recent findings have suggested hypermethylation at both CTCF sites (upstream and downstream of expansion) is almost exclusive in congenital patients, thus being proposed as a clear biomarker for this DM1 form, in addition to expansion size [ 23 , 26 ]. Furthermore, increased methylation is anticipated to be the underlying mechanism for the parent-of-origin effect for the maternal-biased transmission of the congenital form [ 23 ].…”

“…Furthermore, increased methylation is anticipated to be the underlying mechanism for the parent-of-origin effect for the maternal-biased transmission of the congenital form [ 23 ]. However, rare paternal transmissions of hypermethylated expansions associated with congenital DM1 have been only recently observed [ 26 ]. Hypermethylation of the CpG sites surrounding DMPK expansion has been reported in non-congenital DM1 forms as well [ 23 , 26 , 27 , 28 ], although more rarely compared to congenital form.…”

“…However, rare paternal transmissions of hypermethylated expansions associated with congenital DM1 have been only recently observed [ 26 ]. Hypermethylation of the CpG sites surrounding DMPK expansion has been reported in non-congenital DM1 forms as well [ 23 , 26 , 27 , 28 ], although more rarely compared to congenital form. Moreover, the methylation levels in expanded DMPK locus were shown to be stable over time in blood cells [ 27 ].…”

“…Although the expansion size plays a major role in determining DM1 phenotype, studies on patients have constantly shown an unexplained proportion of clinical variability. The mean expansion size is increasing with the increased severity of DM1 clinical forms, but there are large overlaps in the number of repeats among different forms [ 9 , 20 , 26 ]. It should be emphasized that somatic mosaicism of DMPK expansion imposes a difficulty for genotype-phenotype correlations [ 18 ].…”

“…Nevertheless, even when age at sampling and variability in somatic mosaicism are taken into account as confounding factors, the expansion size explains 65% of the variability in age at disease onset [ 18 ]. Additionally, local DNA hypermethylation can be seen in DM1 patients with clinical forms other than congenital [ 23 , 26 , 27 , 28 ]. About a decade ago, an additional source of genetic and epigenetic variability of DM1 mutation emerged as a factor that in cis modifies the effect of the expansion size.…”