Molecular genetic studies on Huntington disease

Abstract: Huntington disease (HD) is an autosomal dominant neurodegenerative disorder associatedwith an expanded trinucleotide repeat (CAG). Prior to the gene identification in 1993,linkage analysis was used for predictive testing for at-risk individuals. Linkage with G8,the first polymorphic marker linked to the HD gene, was found in two Swedish HDfamilies and altered risk estimates for the at-risk individuals was made possible (Paper I).In order to plan for a predictive testing program in Sweden, we interviewed 10 at-riskindividuals about their attitudes toward a predictive test, exploring their motives,concerns and anticipated emotional response to an unfavourable test result (Paper II). Predictive testing of individuals with a CAG repeat in the intermediate range (29-35 CAG) or lower affected range is problematic because they may receive a false negativeor false positive predictive test result if the CAG repeat is not accurately assessed. Inpaper VIII, the importance of assessing the adjacent CCG repeat as well as careful sizingof the CAG repeat with an accurate ladder is presented. Furthermore, all sporadic cases ofHD have arisen from an unaffected father with CAG repeat size in the intermediate range.Together with the predominance of paternal origin in persons with juvenile onset of HDsuggest that sperrn may display a high instability of the CAG repeat. Varying degrees ofCAG repeat instability was shown in sperm DNA from 20 HD patients (Paper III). Thisobserved mosaicism was correlated to intergenerational CAG changes in their families.Single sperm analysis on intermediate alleles showed that CAG size has significantinfluence on the instability of the CAG repeat (Paper IV). However, changes in the 12basepair sequence adjacent to the CAG repeat were the major contributor of instability.The mutational frequency of intermediate alleles of 35 CAG was higher than previouslyestimated. The data showed a 6-10% risk to offspring to inherit a CAG repeat in theaffected range which is important information for genetic counselling. It has previously been suggested that HD originates from a single ancientmutation. The Swedish population is relatively homogenous and is therefore suitable forhaplotype studies. Two intragenic polymorphic markers were analysed in 22 SwedishHD families. Three different haplotypes were found and the majority of these familiesshared the same haplotype. This suggests that there is more than one origin of the HDmutation in Sweden (Paper V). To further analyse intragenic markers in differentpopulations, a novel microsatellite within the HD gene was analysed on HD and controlchromosomes in the European and the Japanese populations. Linkage disequilibrium wasshown but a number of different alleles that segregates on HD chromosomes suggest thatthere are multiple origins for the HD mutation (Paper VI). It has previously been shownthat the evolution of HD chromosomes can be explained be a multi-step model of smallchanges in CAG size until the size in the affected range is reached. To further explore thishypothesis, we performed studies to address what factors are associated with normalvariation of the CAG by analysing a glutamic acid polymorphism within the HD gene indifferent populations (Paper VII). The data suggests that this intragenic polymorphismshows significant variation in different populations and is associated with varying CAGlength on normal chromosomes. This study also provides additional evidence for geneticcontributions to demographic differencies in prevalence rates for HD.Key words: Huntington disease, PCR, predictive testingISBN 91-628-2139-3