Repeat Instability in the 27-39 CAG Range of the HD Gene in the Venezuelan Kindreds: Counseling Implications

D. Brocklebank*, J. Gayán, J. M. Andresen, S. A. Roberts, A. B. Young, S. R. Snodgrass, J. B. Penney, M. A. Ramos-Arroyo, J. J. Cha, H. D. Rosas, S. M. Hersch, A. Feigin, S. S. Cherny, N. S. Wexler, D. E. Housman, L. R. Cardon

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


The instability of the CAG repeat size of the HD gene when transmitted intergenerationally has critical implications for genetic counseling practices. In particular, CAG repeats between 27 and 35 have been the subject of debate based on small samples. To address this issue, we analyzed allelic instability in the Venezuelan HD kindreds, the largest and most informative families ascertained for HD. We identified 647 transmissions. Our results indicate that repeats in the 27-35 CAG range are highly stable. Out of 69 transmitted alleles in this range, none expand into any penetrant ranges. Contrastingly, 14% of alleles transmitted from the incompletely penetrant range (36-39 CAGs) expand into the completely penetrant range, characterized by alleles with 40 or more CAG repeats. At least 12 of the 534 transmissions from the completely penetrant range contract into the incompletely penetrant range of 36-39 CAG repeats. In these kindreds, none of the individuals with 27-39 CAGs were symptomatic, even though they ranged in age from 11 to 82 years. We expect these findings to be helpful in updating genetic counseling practices.

Original languageEnglish
Pages (from-to)425-429
Number of pages5
JournalAmerican Journal of Medical Genetics, Part B: Neuropsychiatric Genetics
Issue number3
StatePublished - 5 Apr 2009
Externally publishedYes


  • Genetic counseling
  • Huntington's disease
  • Trinucleotide repeat


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