Analysis of spinocerebellar ataxia types 1, 2, 3, and 6, dentatorubral-pallidoluysian atrophy, and Friedreich's ataxia genes in spinocerebellar ataxia patients in the UK.

Accurate clinical diagnosis of the spinocerebellar ataxias (SCAs) can be difficult because of overlap in phenotype with other disorders and variation in clinical manifestations. Six SCA loci have been mapped and four disease causing genes identified, in addition to the causative gene for Friedreich's ataxia (FA). All of the identified mutations are expansions of trinucleotide repeat tracts. The SCA2 and SCA6 genes were published recently. The extent of the normal CAG size ranges at these loci and the relative frequencies of the known causes of SCA in the UK are not known. This study first investigated the normal size ranges of the SCA2 and SCA6 loci by genotyping control populations of West African and South African subjects, since African populations generally show the greatest allelic diversity. We found one allele larger than the previously determined normal range for SCA2, and our results at the SCA6 locus agreed with the previously reported normal range. The second component of the study assessed the relative frequencies of the SCA1, 2, 3, and 6, DRPLA, and FA trinucleotide repeat mutations in 146 patients presenting with SCA-like symptoms referred to genetic diagnostic laboratories in the UK. We detected mutations in 14% of patients referred with a diagnosis of autosomal dominant SCA, and in 15% of patients referred with spinocerebellar ataxia where we did not have sufficient family history data available to allow categorisation as familial or sporadic cases. Friedreich's ataxia accounted for 3% of the latter category of cases in our sample, but the most common causes of SCA were SCA2 and SCA6.

Phenotypic characterization of individuals with 30-40 CAG repeats in the Huntington disease (HD) gene reveals HD cases with 36 repeats and apparently normal elderly individuals with 36-39 repeats.

Abnormal CAG expansions in the IT-15 gene are associated with Huntington disease (HD). In the diagnostic setting it is necessary to define the limits of the CAG size ranges on normal and HD-associated chromosomes. Most large analyses that defined the limits of the normal and pathological size ranges employed PCR assays, which included the CAG repeats and a CCG repeat tract that was thought to be invariant. Many of these experiments found an overlap between the normal and disease size ranges. Subsequent findings that the CCG repeats vary by 8 trinucleotide lengths suggested that the limits of the normal and disease size ranges should be reevaluated with assays that exclude the CCG polymorphism. Since patients with between 30 and 40 repeats are rare, a consortium was assembled to collect such individuals. All 178 samples were reanalyzed in Cambridge by using assays specific for the CAG repeats. We have optimized methods for reliable sizing of CAG repeats and show cases that demonstrate the dangers of using PCR assays that include both the CAG and CCG polymorphisms. Seven HD patients had 36 repeats, which confirms that this allele is associated with disease. Individuals without apparent symptoms or signs of HD were found at 36 repeats (aged 74, 78, 79, and 87 years), 37 repeats (aged 69 years), 38 repeats (aged 69 and 90 years), and 39 repeats (aged 67, 90, and 95 years). The detailed case histories of an exceptional case from this series will be presented: a 95-year-old man with 39 repeats who did not have classical features of HD. The apparently healthy survival into old age of some individuals with 36-39 repeats suggests that the HD mutation may not always be fully penetrant.

Molecular re-investigation of patients with Huntington's disease in Wessex reveals a family with dentatorubral and pallidoluysian atrophy.

Dentatorubral and pallidolysian atrophy (DRPLA), a neurological disorder thought to be rare in European populations, is caused by a triplet repeat expansion in the B37 gene on chromosome 12. This disorder can phenotypically mimic Huntington's disease (HD) which is also caused by a repeat expansion. We have analysed 139 affected individuals for the HD triplet repeat expansion and found 132 patients had one normal and one expanded allele. Two patients had an expansion on both chromosomes and five patients had two normal-size alleles. Of these five patients, two were considered to be atypical Two patients who were father and daughter were found to have an expansion of the DRPLA triplet repeat. This therefore constitutes the second such family described in the United Kingdom.

Cystic fibrosis mutation analysis: report from 22 U.K. regional genetics laboratories.

We have collated the results of cystic fibrosis (CF) mutation analysis conducted in 22 laboratories in the United Kingdom. A total of 9,807 CF chromosomes have been analysed, demonstrating 56 different mutations so far observed and accounting for 86% of CF genes in the native Caucasian population of the United Kingdom. delta F508 is the most common at 75.3% of CF mutations (range 56.5-83.7%), followed by G551D (3.08%; range 0.71-7.60%), G542X (1.68%; range 0.85-3.66%), 621 + 1 (G > T) (0.93%; range 0.41-3.16%), 1717-1(G > A) (0.57%; range 0.17-1.14%), 1898 + 1)(G > A) (0.46%), R117H (0.46%), N1303K (0.46%), and R553X (0.46%). The data show a clear geographical variation in the distribution of some of the mutations, most notably a marked regional variation in the distribution of 621 + 1 (G > T) and 1989 + 1(G > A), which are both apparently more frequent in Wales. R560T and R117H appear to be more frequent in Ireland and Scotland, and G551D more frequent in Scotland. In summary, these data illustrate that the mutations present within a particular population need to be defined in order to provide meaningful carrier screening and testing for rare mutations in affected individuals. Furthermore, it is apparent that the ethnic origin of a patient, even within a small country such as the United Kingdom, should be taken into account.