Putative neuroprotective agents in Huntington's disease may have particular application before brain pathology becomes manifest clinically. If these agents were to be tested in clinical trials, a reliable marker of the burden and rate of progression of pathological change in the pre-clinical group would be needed. The present study investigates whether the Huntington's disease genotype is associated with regional differences in brain structure, particularly differences that could not be predicted from clinical or neuropsychological assessment. A secondary aim is to seek indirect evidence of pathological progression in the form of changes in local tissue volume with age, specific to the Huntington's disease genotype. Formal motor examination, neuropsychological assessment, and T(1)-weighted cerebral MRI were performed in 34 subjects who had undergone predictive genetic testing for Huntington's disease. Clinical and cognitive testing were performed blinded to gene status. A linear discriminant analysis revealed the combination of test scores (the 'optimal clinical score') which best differentiated 18 subjects carrying the Huntington's disease gene mutation (the 'gene-positive' group). Voxel-based morphometry (VBM) was used to identify regions of significant main effect of Huntington's disease gene status on grey and white matter volume and regions of significant interaction of gene status with age. In the gene-positive group, there was significant reduction in grey matter volume in the left striatum, bilateral insula, dorsal midbrain and bilateral intra-parietal sulcus relative to 'gene-negative' controls. There was a significant reduction of periventricular white matter volume with age bilaterally in the gene-positive relative to the gene-negative group. Changes remained significant when controlled for differences in optimal clinical score between subjects. This study provides evidence of distributed grey matter pathology and progressive white matter atrophy with age before clinical onset of Huntington's disease. This suggests that VBM may be useful in monitoring cross-sectional and longitudinal changes in brain structure in pre-clinical Huntington's disease and for determining the efficacy of neuroprotective agents.

Huntington disease (HD) is caused by a CAG repeat expansion in the huntingtin (HTT) gene. Although the length of this repeat is inversely correlated with age of onset (AOO), it does not fully explain the variability in AOO. We assessed the sequence downstream of the CAG repeat in HTT [reference: (CAG)n-CAA-CAG], since variants within this region have been previously described, but no study of AOO has been performed. These analyses identified a variant that results in complete loss of interrupting (LOI) adenine nucleotides in this region [(CAG)n-CAG-CAG]. Analysis of multiple HD pedigrees showed that this LOI variant is associated with dramatically earlier AOO (average of 25 years) despite the same polyglutamine length as in individuals with the interrupting penultimate CAA codon. This LOI allele is particularly frequent in persons with reduced penetrance alleles who manifest with HD and increases the likelihood of presenting clinically with HD with a CAG of 36-39 repeats. Further, we show that the LOI variant is associated with increased somatic repeat instability, highlighting this as a significant driver of this effect. These findings indicate that the number of uninterrupted CAG repeats, which is lengthened by the LOI, is the most significant contributor to AOO of HD and is more significant than polyglutamine length, which is not altered in these individuals. In addition, we identified another variant in this region, where the CAA-CAG sequence is duplicated, which was associated with later AOO. Identification of these cis-acting modifiers have potentially important implications for genetic counselling in HD-affected families.

We have directly compared intergenerational stability of intermediate alleles (IAs) derived from new mutation families (IANM) for Huntington disease (HD) with IAs in the general population (IAGP) which occur in approximately 1 in 50 persons. Analysis of meiotic events in blood and sperm reveals that IANM are significantly more unstable than IAGP despite similar size. However, for both IANM and IAGP CAG changes were small and risks for inheriting an expansion into the HD affected range were low. Sequence analysis reveals that the CAG tract is generally interrupted by a penultimate CAA in IAGP, IANM and alleles in the affected range. In one new mutation family, however, two A-->G mutations result in a pure CAG tract which is associated with very marked instability. These mutations alter the predicted DNA hairpin structure with a predicted increase in the likelihood of large expansion, supporting the model that hairpin loop formation plays an important role in trinucleotide instability.

New mutations for Huntington disease (HD) arise from intermediate alleles (IAs) with between 29 and 35 CAG repeats that expand on transmission through the paternal germline to 36 CAGs or greater. Using single sperm analysis, we have assessed CAG mutation frequencies for four IAs in families with sporadic HD (IANM) and IAs ascertained from the general population (IAGP) by analyzing 1161 single sperm from three persons. We show that IANM are more unstable than IAGP with identical size and sequence. Furthermore, comparison of different sized IAs and IAs with different sequences between the CAG and the adjacent CCG tracts indicates that DNA sequence is a major influence on CAG stability. These studies provide estimates of the likelihood of expansion of IANM and IAGP to > or = 36 CAG repeats for these individuals. For an IA with a CAG of 35 in this family with sporadic HD, the likelihood for siblings to inherit a recurrent mutation > or = 36 CAG is approximately 10%. For IAGP of a similar size, the risk of inheriting an expanded allele of > or = 36 CAG through the paternal germline is approximately 6%. These risk estimates are higher than previously reported and provide additional information for counselling in these families. Further studies on persons with IAs will be needed to determine whether these results can be generalized to other families.

These data provide novel insights into the origins of new mutations for HD. The CAG size-specific risk estimates inform clinical practice and provide accurate risk information for persons who receive an IA predictive test result.

A qualitative study, based on family systems theory, was undertaken in order to gain a better understanding of the impact of predictive testing and of living with the risk or reality of Huntington disease (HD), on couple relationships. Semi-structured interviews were conducted with 14 couples; in 9 couples the at-risk partner had undergone testing, and of these, 4 were already affected with HD. At-risk partners in the remaining five couples had not been tested. Interview transcripts were analyzed to obtain a range of themes, which reflect the salient experiences of these couples in relation to HD. Most couples reported that receiving a predictive test result had little or no adverse effect on their relationship. However for two couples who separated after the at-risk partner received a non-carrier result, emotional factors associated with years of living with the HD risk, rather than the result itself, were regarded as having caused irreparable damage to the relationship. For two couples who have remained together since the diagnosis of one partner, loyalty was identified as the main factor contributing to the continuance of the relationship. The separations of the other two couples in which one partner was diagnosed were attributed to emotional distancing, and to the obsessive behavior of the affected partner. The findings of this study highlight both the individuality and the complexity of psychological effects on the intimate relationships of couples who live with the risk or reality of HD, and provide important insights for professionals offering support to these couples.

This study summarizes 10-years' experience of predictive and pre-natal testing and pre-implantation genetic diagnosis (PGD) for Huntington disease (HD) in Australia. Results are presented from 2036 direct mutation predictive tests conducted between January 1994 and December 2003. Thirty-eight per cent of results (776/2036) were positive, 56% (1140/2036) were negative, and 6% (120/2036)) were in the mutable normal (27-35 CAG repeats) or in the reduced penetrance (36-39 CAG repeats) ranges. Ninety-four per cent (1908/2036) and 6% (128/2036) of those tested had prior genetic risks of 50% and 25%, respectively. Twenty-seven per cent (34/128) of those at 25% risk had their genetic status changed to positive, thus revealing the positive status of their at-risk parent. During this period, 63 pre-natal tests were also conducted, and 13 children were born following PGD for HD. Social workers specializing in predictive testing counselling over this 10-year period across Australia identified and summarized particularly challenging counselling issues. These included the interpretation of mutable normal and reduced penetrance range test results, potential conflicts of interest between family members regarding testing decisions, unanticipated consequences of both predictive and pre-natal testing decisions, the importance of following protocols for predictive testing to facilitate long-term adjustment to results, and the potential for genetic discrimination. The identified issues highlight the importance of the protocols for predictive testing and indicate that extension of the international guidelines published in 1994 may be timely.

A retrospective study examined both pre- and post-result reproductive decision making for 281 people at risk for Huntington's disease aged 18-45 years who had undergone predictive testing in one centre in Australia between 1990 and 2002. Forty-eight per cent of subjects had one or more pre-result pregnancies, and of these, three had prenatal linkage testing. One high-risk (50%) pregnancy was terminated. Four couples chose an alternative reproductive option. Following testing, data were available for 231 subjects, and no significant difference was found between mutation carriers and non-carriers in the occurrence of post-result pregnancies. This contrasts with the finding of a recent European study, although the outcome of the present study may have been influenced by loss of follow-up data for 50 subjects. Five carriers (17%) had a total of six prenatal tests. Four showed a carrier result and these pregnancies were terminated. Two carriers utilized an alternative reproductive option (donor insemination and pre-implantation genetic diagnosis). The results of this study confirm previous findings of a low uptake of prenatal testing and alternative reproductive options by people at risk for Huntington's disease undergoing predictive testing.