A novel, heritable, expanding CTG repeat in an intron of the SEF2-1 gene on chromosome 18q21.1.

There are currently 13 diseases known to be caused by unstable triplet repeat mutations; however, there are some instances (as with FRAXF and FRA16) when these mutations appear to be asymptomatic. In a search for polymorphic CTG repeats as candidate genes for bipolar disorder, we screened a genomic human chromosome 18-specific library and identified a 1.6 kb clone (7,6A) with a CTG24 repeat that maps to 18q21.1. The CTG repeat locus, termed CTG18.1, is located within an intron of human SEF2-1, a gene encoding a basic hellx-loop-hellx DNA binding protein involved in transcriptional regulation. The CTGn repeat is highly polymorphic and very enlarged alleles, consistent with expansions of up to CTG2100, were identified. PCR and Southern blot analysis in pedigrees ascertained for a Johns Hopkins University bipolar disorder linkage study and in CEPH reference pedigrees revealed a tripartite distribution of CTG18.1 alleles with stable alleles (CTG10-CTG37), moderately enlarged and unstable alleles (CTG53-CTG250), and very enlarged, unstable alleles (CTG800-CTG2100). Moderately enlarged alleles were not associated with an abnormal phenotype and have a combined enlarged allele frequency of 3% in the CEPH and bipolar populations. Very enlarged alleles, detectable only by Southern blot analysis of genomic digests, have thus far been found in only three individuals from our bipolar pedigrees, and to date, have not been found in any of the CEPH reference pedigrees. These enlarged alleles may arise, at least in part, via somatic mutation.

Anticipation and instability of IT-15 (CAG)n repeats in parent-offspring pairs with Huntington disease.

Huntington disease (HD) is an autosomal dominant degenerative disorder caused by an expanded and unstable trinucleotide repeat (CAG)n in a gene (IT-15) on chromosome 4. HD exhibits genetic anticipation--earlier onset in successive generations within a pedigree. From a population-based clinical sample, we ascertained parent-offspring pairs with expanded alleles, to examine the intergenerational behavior of the trinucleotide repeat and its relationship to anticipation. We find that the change in repeat length with paternal transmission is significantly correlated with the change in age at onset between the father and offspring. When expanded triplet repeats of affected parents are separated by median repeat length, we find that the longer paternal and maternal repeats are both more unstable on transmission. However, unlike in paternal transmission, in which longer expanded repeats display greater net expansion than do shorter expanded repeats, in maternal transmission there is no mean change in repeat length for either longer or shorter expanded repeats. We also confirmed the inverse relationship between repeat length and age at onset, the higher frequency of juvenile-onset cases arising from paternal transmission, anticipation as a phenomenon of paternal transmission, and greater expansion of the trinucleotide repeat with paternal transmission. Stepwise multiple regression indicates that, in addition to repeat length of offspring, age at onset of affected parent and sex of affected parent contribute significantly to the variance in age at onset of the offspring. Thus, in addition to triplet repeat length, other factors, which could act as environmental factors, genetic factors, or both, contribute to age at onset. Our data establish that further expansion of paternal repeats within the affected range provides a biological basis of anticipation in HD.

Expression of the mutant allele of IT-15 (the HD gene) in striatum and cortex of Huntington's disease patients.

Huntington's disease (HD) is an inherited neurodegenerative disorder expressed when a trinucleotide repeat in the gene IT-15 is expanded. The mechanism by which the expanded repeat causes the expression of the disease is unknown. Possible mechanisms include alterations in the amount of the mRNA, potentially resulting from changes in gene transcription or abnormal mRNA stability. In order to determine whether the expanded IT-15 allele is present in mRNA, we isolated total RNA from the cortex and striatum of patients and controls. To distinguish the two alleles of the IT-15 transcript in HD patients, we amplified across a region containing a dimorphic single triplet deletion observed on some chromosomes and found that the relative intensity of the two PCR bands amplified from genomic DNA and those amplified from first strand cDNA from brain tissue were essentially equal. In order to determine whether the exon containing the expanded CAG repeat is present in IT-15 mRNA from HD patients, we amplified across this region and demonstrated the presence of the expanded repeat in cDNA from both striatum and cortex. Based on this evidence, we suggest that the mechanism of disease expression does not occur during transcription or in the stability of the RNA, but rather occurs during translation or postranslationally.

Correlation between the onset age of Huntington's disease and length of the trinucleotide repeat in IT-15.

Huntington's disease (HD) is an autosomal dominant disorder with a variable age of onset that is influenced by the sex of the affected parent. The recent recognition that HD is caused by an expanded triplet repeat suggests the possibility that the onset age may be predicted by the length of the repeat. This hypothesis was tested by assaying the length of the repeat in 114 individuals who were clinically diagnosed with HD and had a known onset age. Every individual had an expanded allele. The range was from 36 to 82 repeats (mean = 48.4 +/- 9.51) and larger than the normal range (6 to 31). The size of the expanded allele was correlated with the age of onset (r = -0.65 p < .0001). Despite the highly significant correlation, the repeat size explains less than half of the variance in onset age. Furthermore, the age of onset cannot be predicted from the size of the triplet repeat, particularly if the number of repeats is in the smaller end of the expanded range. If the repeat is < or = 50 triplets, the amount of variation in the age of onset explained by the length of the triplet repeat is less than 10%. As an illustration, the onset age of individuals with 39 repeats ranges from 30 to 65 years old. The sex of the affected parent had no effect in our sample beyond the effect of the length of the repeat. Affected offspring of affected fathers had longer repeats and a larger variance in allele size than offspring of affected mothers, perhaps reflecting greater instability in paternal transmission.