ABSTRACT
Myotonic Dystrophy type I [DM1] the most common form of muscular dystrophy in adults affecting 1/800 individuals is a dominantly inherited disorder with a multi-systemic pattern affecting skeletal muscle, heart, eye, endocrine and central nervous system. DM1 is associated with the expansion and instability of a trinucleotide [CTG] repeat in the 3 untranslated region of the myotonic dystrophy protein kinase [DMPK] gene located on choromosome 19q13.3. The normal copy number is 5-37 CTG repeat whereas it is expanded in DM1 patients and the expansion size broadly correlates with the severity of the symptoms. The aim of this study was to determine the clinical and genetic characteristic of DM1 in Iranian patients for genotype-phenotype correlation methods. Clinical assessment was based on the muscular disability rating scale [MDRS] and a sum of symptoms score [SSS]. Molecular analysis [PCR and Southern blot] was used to clarify uncertain clinical diagnoses and confirm the clinical findings. Forty six patients from twenty five DM1 families were reviewed. In all the DM1 patients, the wide clinical symptoms confirmed the reported phenotypic vaiability of disorder. The range of CTG expansion of the mutated allele was 97-833 CTG repeats and an inverse correlation between age of onest and repeat length was observed. A clear relation between the size of the CTG repeat and the clinical disease score [MDRS] was found but not with SSS. No correlation was seen between the endocrine dysfunction and the expansion size in DM1 patients
ABSTRACT
Myotonic dystrophy type 1 [DM1] is due to an unstable expansion of CTG repeat in the DMPK gene [19q13.3]. The CTG repeat is highly polymorphic [5 to 37] in healthy individuals. According to the hypothesis that expanded [CTG]n alleles originated from larger normal alleles, there may exist a correlation between the prevalence of DM1 and the frequency of large size normal alleles. Strong linkage disequilibrium between different length alleles and the three biallelic markers, Alu, Hinf1 and Taq1, has been reported. Therefore, we decided to determine the distribution of normal alleles, the frequency of larger normal alleles and analysis of the three biallelic markers, in healthy Iranian controls. Polymerase chain reaction was conducted on two hundred unrelated healthy individuals from different ethnic groups living in Iran to determine the size of the alleles. Markers were analyzed by PCR/RFLP on 174 chromosomes from other control healthy individuals. Our data reveals that 23.7% of alleles had 5 CTG repeats and 7.2% of alleles had >18 CTG repeats. The analysis of haplotypes revealed that 75% of CTG5 and 80% of CTG>18 had the [+++] haplotype. In this study, we find that the frequency of alleles with CTG>18 is similar to that of Western Europe and Japan