Somatic expansion of the Huntington's disease CAG repeat in the brain is associated with an earlier age of disease onset

Abstract

The age of onset of Huntington's disease (HD) is determined primarily by the length of the HD CAG repeat mutation, but is also influenced by other modifying factors. Delineating these modifiers is a critical step towards developing validated therapeutic targets in HD patients. The HD CAG repeat is somatically unstable, undergoing progressive length increases over time, particularly in brain regions that are the targets of neurodegeneration. Here, we have explored the hypothesis that somatic instability of the HD CAG repeat is itself a modifier of disease. Using small-pool PCR, we quantified somatic instability in the cortex region of the brain from a cohort of HD individuals exhibiting phenotypic extremes of young and old disease onset as predicted by the length of their constitutive HD CAG repeat lengths. After accounting for constitutive repeat length, somatic instability was found to be a significant predictor of onset age, with larger repeat length gains associated with earlier disease onset. These data are consistent with the hypothesis that somatic HD CAG repeat length expansions in target tissues contribute to the HD pathogenic process, and support pursuing factors that modify somatic instability as viable therapeutic targets.

Figure 1.

Small-pool PCR analyses of the HD CAG repeat amplified from human HD cortex. The graphs show examples from four individual cortex samples, two exhibiting extreme young onset (a and c) and two exhibiting extreme old onset (b and d) as predicted from the sizes of their constitutive mutant HD CAG repeats, determined in cerebellar DNA. (a) and (b) are matched for mutant HD CAG repeat length and (c) and (d) are matched for mutant HD CAG repeat length. The inset boxes indicate constitutive HD CAG repeat sizes (mutant and normal alleles) and ages of onset (AO). Number of amplified alleles from single molecule inputs: a = 136 mutant, 106 normal; b = 103 mutant, 104 normal; c = 122 mutant, 90 normal; d = 115 mutant, 136 normal. (A) Frequency distributions of the HD CAG repeat sizes of the mutant and normal alleles. (B) Frequency distributions of repeat size changes of the mutant HD CAG repeat relative to the modal mutant HD CAG repeat size determined in the small-pool PCR analysis. Blue bars: contractions (repeat size change <1); grey bars: unchanged alleles (repeat size change = 0); red bars: all expanded alleles (repeat size change ≥1); pink bars: alleles that are expanded beyond specific thresholds (repeat size change ≥5, ≥10 etc.).

Figure 2.

Residual age of onset versus skewness. Shown are the residual onset age and skewness for each of the 24 extreme young onset individuals (filled circles, residuals less than −0.5) and 24 extreme old onset individuals (open circles, residuals >0.5). Skewness values were calculated from the distribution of mutant HD CAG repeat lengths in cortex, determined using SP-PCR.