Molecular genetic studies on Huntington disease

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder associated with 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 HD families 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-risk individuals 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 negative or false positive predictive test result if the CAG repeat is not accurately assessed. In paper VIII, the importance of assessing the adjacent CCG repeat as well as careful sizing of the CAG repeat with an accurate ladder is presented. Furthermore, all sporadic cases of HD 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 HD suggest that sperm may display a high instability of the CAG repeat. Varying degrees of CAG repeat instability was shown in sperm DNA from 20 HD patients (Paper III). This observed mosaicism was correlated to intergenerational CAG changes in their families. Single sperm analysis on intermediate alleles showed that CAG size has significant influence on the instability of the CAG repeat (Paper IV).

However, changes in the 12 base pair sequence adjacent to the CAG repeat were the major contributor of instability. The mutational frequency of intermediate alleles of 35 CAG was higher than previously estimated. The data showed a 6-10% risk to offspring to inherit a CAG repeat in the affected range which is important information for genetic counselling. It has previously been suggested that HD originates from a single ancient mutation. The Swedish population is relatively homogenous and is therefore suitable for haplotype studies. Two intragenic polymorphic markers were analysed in 22 Swedish HD families. Three different haplotypes were found and the majority of these families shared the same haplotype. This suggests that there is more than one origin of the HD mutation in Sweden (Paper V).

To further analyse intragenic markers in different populations, a novel microsatellite within the HD gene was analysed on HD and control chromosomes in the European and the Japanese populations. Linkage disequilibrium was shown but a number of different alleles that segregates on HD chromosomes suggest that there are multiple origins for the HD mutation (Paper VI). It has previously been shown that the evolution of HD chromosomes can be explained be a multi-step model of small changes in CAG size until the size in the affected range is reached. To further explore this hypothesis, we performed studies to address what factors are associated with normal variation of the CAG by analysing a glutamic acid polymorphism within the HD gene indifferent populations (Paper VII).

The data suggests that this intragenic polymorphism shows significant variation in different populations and is associated with varying CAG length on normal chromosomes. This study also provides additional evidence for genetic contributions to demographic differences in prevalence rates for HD.