The lower incidence of myotonic dystrophy in Ashkenazic Jews compared to North African Jews is associated with a significantly lower number of CTG trinucleotide repeats.

Myotonic dystrophy (DM) is associated with an increased number of CTG repeats in the 3' untranslated region of the myotonin gene. Because DM has been observed more frequently in North African Jews than in Ashkenazic Jews in Israel, a study of the CTG repeat polymorphism was undertaken in these 2 groups. Alleles from 70 unrelated North African subjects and 70 unrelated Ashkenazic subjects were studied by PCR analysis of the trinucleotide repeat in the DM gene to determine the ethnic distribution of the number of CTG repeats. The alleles ranged in length from 5 to 26 repeats in the North Africans and 5 to 23 in the Ashkenazim. As has been seen in other populations, none of the chromosomes had a 9-repeat length. North African Jews were found to have significantly more repeats in the normal range than Ashkenazim (for over 14 repeats: 34/140 compared to 7/140; p < 0.0001). It is suggested that this more frequent occurrence of a large number of CTG repeats in the normal range may represent a greater predisposition to DM.