P-11

Molecular profile of Myotonic Dystrophy type 1 (DM1) in portuguese families

Márcia E. Oliveira^I; Nuno Maia^I; Isabel Marques^I; Rosário Santos^I,II

^IUnidade de Genética Molecular, Centro Genética Médica Doutor Jacinto Magalhães, Centro Hospitalar do Porto - EPE, Porto, Portugal; Unidade Multidisciplinar de Investigação Biomédica (UMIB), Instituto de Ciências Biomédicas Abel Salazar (ICBAS), Universidade do Porto, Porto, Portugal
^IIUCIBIO/REQUIMTE, Departamento de Ciências Biológicas, Laboratório de Bioquímica, Faculdade de Farmácia, Universidade do Porto, Porto, Portugal

marcia.oliveira@chporto.min-saude.pt

Myotonic Dystrophy type 1 (DM1), also known as Steinert disease, is the most common adult form of muscular dystrophy. It is an autosomal dominant disorder caused by the expansion of unstable [CTG] repeats in the 3’ untranslated region (3’UTR) of the gene of myotonic dystrophy protein kinase (DMPK), located at 19q13.3. The expanded mRNA products are toxic to cells, affecting normal splicing of other proteins in different tissues.

Myotonia is the principal manifestation of DM1, but other organ systems are also affected (e.g. ocular, cardiac and respiratory). Clinically, DM1 may be phenotipically classified into four main subtypes: i) mild, ii) classical or adult-onset, iii) juvenile and iv) congenital. The age of onset and severity are variable and directly associated with the number of expanded [CTG] repeats: larger expansions usually result in earlier onset and a more severe phenotype. Due to the instability of the expanded [CTG] repeats during transmission, it is common to observe in affected families a decreasing in age at onset and an increasing degree of severity in successive generations (anticipation).

In this work we present the molecular profile of DM1 in affected families studied at our diagnostic service, on a national basis, since the implementation of the molecular genetic testing for this disorder in 1997.

All the cases were tested for the presence of expanded pathogenic alleles. Depending on the size of the expanded alleles, a combination of three different methods was used to determine the number of [CTG] repeats in 3’UTR of the DMPK gene: 1) conventional PCR amplification of the [CTG]- repeat region; 2) Triplet repeat-Primed (TP)-PCR and 3) Southern blotting (SB) technique. Larger expansions (about [CTG]_>100 repeats) are only detected by the last two methods; moreover, the exact size of these larger expanded repeats can only be assessed by SB. In order to establish the most accurate molecular diagnosis, the SB technique is therefore essential, particularly in the prenatal diagnostic setting.

We also present both clinical and genotype heterogeneity in the DM1 families, thus demonstrating some of the characteristics associated with this triplet repeat disorder: somatic mosaicism, anticipation, influence of gender of the transmitting parent and occurrence of inter-generational size reduction in the expanded repeats.

The aim of the present work is to establish the mutation profile in patients and their families, where detailed molecular characterization is fundamental, not only to confirm the clinical diagnosis and to establish genotype-phenotype correlations, but also for trial-readiness given the emergent mutation-based therapies for DM1.