Relationships of the 2642 deletion polymorphism (delta 2642) in the huntingtin gene with the CAG repeat expansion length and age at onset of the disease.

The deletion of 3bp at codon positions 2642-2645 (delta 2642) of the gene mutated in Huntington's disease (HD) was analysed on the normal (N) and HD chromosomes of 79 French families affected with HD, and previously typed for the (CAG)n repeats. delta 2642 Polymorphism has been found over-represented on HD chromosomes, the relative risk of HD with the deletion being at a value of 8.26. In this study, the presence of the deleted allele on HD chromosomes increases the (CAG)n number (47.93 +/- 1.80 versus 43.50 +/- 2.78) and decreases the age of onset (41.34 +/- 2.09 versus 36.90 +/- 2.41) in the patients with versus without delta 2642; so the deletion may add to the severity of the disease. Our studies of delta 2642 on N chromosomes confirm that the deletion event occurs on N chromosomes with a (CAG)n allele length at the upper end of the normal size range.