Study of three intragenic polymorphisms in the Machado-Joseph disease gene(MJD1) in relation to genetic instability of the (CAG)(n) tract

Intergenerational instability is one of the most important features of the disease-associated trinucleotide expansions, leading to variation in size o f the repeat among and,within families, which manifests as variable age at onset and severity, and is probably the basis for the occurrence of anticip ation. Several factors are known to affect the degree of instability, namel y the type of repeated sequence, its initial size, the presence or absence of interruptions in the repetitive tract and the gender of the transmitting parent. A recent study demonstrated the effect of an intragenic polymorphi sm (C(987)GG/G(987)GG) in the Machado-Joseph disease causative gene, immedi ately downstream of the CAG repeat, on the intergenerational instability of the expanded repeat. Surprisingly, there was an effect not only of the spe cific allele in cis to the disease chromosome, but also of the allele on th e normal chromosome, suggesting the existence of an interaction between the normal and expanded alleles that affects the fidelity of replication of th e (CAG)(n) tract. This effect could be a direct effect of the polymorphism studied or, alternatively, this polymorphism could be in disequilibrium wit h some other flanking sequence which affects the instability of the repetit ive (CAG)(n) tract. In order to confirm the previous results in a different population and to distinguish between a direct and indirect effect of the CGG/GGG polymorphism, we typed 70 parent-progeny pairs for which the variat ion in the (CAG)(n) length in the MJD1 gene was known, for three intragenic polymorphisms: C(987)GG/G(987)GG and two additional, newly described ones, TA (A) under bar(1118)/TA (C) under bar(1118) and (A) under bar(669)TG/(G) under bar(669)TG. We also typed a control population of 125 individuals fo r the (A) under bar(669)TG/(G) under bar(669)TG, (C) under bar(987)GG/(G) u nder bar(987)GG and TA (A) under bar(1118)/TA (C) under bar(1118) polymorph isms, in an attempt to identify any association between haplotype and (CAG) (n) length in normal chromosomes, suggestive of an instability-predisposing effect of the repeat-flanking sequences, which could have led to the origi n of the MJD mutation in the human population. We confirmed the effect of t he (C) under bar(987)GG/(G) under bar(987)GG polymorphism on intergeneratio nal instability when present in trans, Our results suggest that this effect is restricted to a small region of the gene, immediately downstream of the CAG repeat, which includes this particular nucleotide substitution and the stop codon of the MJD1 cDNA, and is not a more widespread chromosomal effe ct, The lack of a significant association of any specific intragenic haplot ype with larger CAG repeats in normal chromosomes, together with the absenc e of an effect of the intragenic haplotype in cis on the intergenerational instability of the expanded (CAG)(n) in MJD families does not indicate the existence of an instability-predisposing haplotype.