Background: Myotonic dystrophy type 1 (DM1; OMIM #160900) is an autosomal-dominant genetic disorder with multisystemic clinical features associated with a CTG expansion in the 3' untranslated region of the DMPK gene on chromosome 19q13.3. A long-PCR protocol to detect the DM1 expansion is rapid, sensitive, and accurate, but interpretative limitations can occur when the expansion size exceeds the PCR amplification range and in cases of somatic mosaicism.
Methods: To overcome these problems, we used RNA fluorescence in situ hybridization (RNA-FISH) to study cultured cells derived from chorionic villus samples (CVS) with the DM1 mutation. The RNA-FISH method is designed to detect the distinctive DM1 cellular phenotype, characterized by the presence of nuclei with focal ribonuclear inclusions (foci) containing the DMPK expanded transcripts. We analyzed 6 CVS from DM1-predicted pregnancies and 6 CVS from DM1-negative pregnancies.
Results: In 4 DM1-predicted fetuses with a CTG expansion >200 CTG, varying numbers of ribonuclear inclusions were clearly visible in all cells. One case with a somatic mosaicism for the DMPK mutation showed 15% of cells with no nuclear foci. No nuclear signals were detected in all controls examined (n = 6) and in 1 DM1-positive sample with a CTG expansion <100 copies.
Conclusion: Nuclear foci, and therefore the DM1 mutation they are caused by, can be detected efficiently on interphase nuclei of trophoblast cells with RNA-FISH when the CTG expansion is >200 copies.