Molecular genetics and genetic testing in myotonic dystrophy type 1

Abstract

Myotonic dystrophy type 1 (DM1) is the most common adult onset muscular dystrophy, presenting as a multisystemic disorder with extremely variable clinical manifestation, from asymptomatic adults to severely affected neonates. A striking anticipation and parental-gender effect upon transmission are distinguishing genetic features in DM1 pedigrees. It is an autosomal dominant hereditary disease associated with an unstable expansion of CTG repeats in the 3'-UTR of the DMPK gene, with the number of repeats ranging from 50 to several thousand. The number of CTG repeats broadly correlates with both the age-at-onset and overall severity of the disease. Expanded DM1 alleles are characterized by a remarkable expansion-biased and gender-specific germline instability, and tissue-specific, expansion-biased, age-dependent, and individual-specific somatic instability. Mutational dynamics in male and female germline account for observed anticipation and parental-gender effect in DM1 pedigrees, while mutational dynamics in somatic tissues contribute toward the tissue-specificity and progressive nature of the disease. Genetic test is routinely used in diagnostic procedure for DM1 for symptomatic, asymptomatic, and prenatal testing, accompanied with appropriate genetic counseling and, as recommended, without predictive information about the disease course. We review molecular genetics of DM1 with focus on those issues important for genetic testing and counseling.

Figure 1

Flow diagram of a genetic test on myotonic dystrophy type 1 (DM1). A two-step procedure is used in DM1 genetic testing. The first step is PCR followed by fragment length analysis, which identifies and sizes alleles within normal range. The second step employs one of the techniques which differentiates between individuals who are homozygous for an allele within normal range and DM1 individuals carrying one allele within normal range and one unamplifiable expanded allele. The most widely used technique in the second step is the triplet-repeat primed PCR (TP-PCR), which utilizes locus-specific PCR primers in combination with a primer designed across the repeated sequence, and provides no size estimation, but rather a simple “present”/“absent” result for an expanded allele. After the fragment length analysis step, products of different sizes are visible as a continuous ladder with a 3-base-pair periodicity. In the presence of a DM1-expanded allele, a continuous ladder exceeds the normal size range. The lower part of the flow diagram shows optional methods used to confirm the obtained result of the two-step diagnostic procedure for DM1, employed when some samples show inconclusive findings. Applied together, PCR, TP-PCR, and Southern blotting methods provide high sensitivity and specificity, and diagnostic laboratories should have a facility to use more than only one methodological approach (usually TP-PCR and one of the Southern blot methods).