Background: Myotonic dystrophy is caused by unstable (CTG)(n) repeat expansion. On normal chromosomes, this repeat is highly polymorphic, with a copy number ranging from 4 to 38. Myotonic dystrophy is considered more prevalent in Western European and Japanese populations but less prevalent, rare, or even absent in others. It has been proposed that the expanded (CTG)(n) alleles originated from the group of the large normal alleles.
Objective: To determine whether there is a lower prevalence of the large alleles in the Kuwaiti population.
Design and participants: We determined the size distribution of the CTG repeats by means of polymerase chain reaction in blood DNA derived from 185 healthy Kuwaiti individuals representing the 5 Kuwaiti provinces.
Results: We found a total of 17 (CTG)(n) alleles, with a range of 5 to 37 repeats. The (CTG)(5) allele was the most frequent single allele (100/370 [27.0%]), whereas the (CTG)(10-13) was the most frequent class of alleles (161/370 [43.5%]). Using 18 repeats as the cutoff point, chi(2) analysis showed a statistically significant lower frequency of greater than 18 alleles in the Kuwaiti population compared with the European population (chi(2) = 12.7; P<.001).
Conclusions: These data may explain the rare occurrence of myotonic dystrophy in the Kuwaiti population. Further study of healthy families within the high-normal repeat range is in progress to investigate the possible instability of the (CTG)(>18) alleles in our area.