Human Molecular Genetics, Vol 5, 207-213, Copyright © 1996 by Oxford University Press
P Limprasert, N Nouri, RA Heyman, C Nopparatana, M Kamonsilp, PL Deininger and BJ Keats
Machado-Joseph disease (MJD) is an autosomal dominant neurodegenerative
disorder associated with an unstable and expanded CAG repeat. We analyzed
this locus from various sources including MJD families, Acadian, African
American, Caucasian, Greenland Inuit and Thai populations. The range of the
CAG repeat size was 14-40 in the normal alleles while the MJD alleles
contained 73-78 repeats in our studies. We found 25 different alleles on
normal chromosomes with a heterozygosity of 0.86 in combined populations.
The most common alleles were 23 (22.9%) and 14 (25.5%) repeats. We also
examined 16 chimpanzees and various Old World monkeys: a pigtail macaque, a
mangabey and 12 rhesus macaques. The DNA sequences surrounding the CAG
repeat did not vary among species. The range of the number of the CAG
repeats is 13-14 in macaques, 16 in mangabey and 14-20 in chimpanzees.
Variant CAA or AAG triplets in the CAG repeat tracts were found in all 268
human, 28 monkey and 32 chimpanzee chromosomes. As reported in a previous
study [Kawaguchi et al. (1994) Nature Genet. 8, 221-228] the common variant
positions were the third (CAA), fourth (AAG) and sixth (CAA) positions.
However, we found three human chromosomes containing CAG at the sixth
position and the mangabey had AAG at the ninth position. In addition, we
found CAG at the fourth position and AAG at the sixth position in all
macaque chromosomes. The nucleotide following the CAG repeat tract was
usually G in all species studied. However, we sometimes found C at this
position in human and chimpanzee chromosomes. Interestingly, this variant C
was found in all expanded chromosomes and in 54.5% of chromosomes with
27-40 CAG repeats but it was not found in any chromosomes with less than 20
CAG repeats. We hypothesize that the variant C may be associated with CAG
repeat instability.
ARTICLES
Analysis of CAG repeat of the Machado-Joseph gene in human, chimpanzee and monkey populations: a variant nucleotide is associated with the number of CAG repeats
Department of Biometry and Genetics, Louisana State University Medical Center, New Orleans 70112, USA.
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