Xpa deficiency reduces CAG trinucleotide repeat instability in neuronal tissues in a mouse model of SCA1

Abstract

Expansion of trinucleotide repeats (TNRs) is responsible for a number of human neurodegenerative disorders. The molecular mechanisms that underlie TNR instability in humans are not clear. Based on results from model systems, several mechanisms for instability have been proposed, all of which focus on the ability of TNRs to form alternative structures during normal DNA transactions, including replication, DNA repair and transcription. These abnormal structures are thought to trigger changes in TNR length. We have previously shown that transcription-induced TNR instability in cultured human cells depends on several genes known to be involved in transcription-coupled nucleotide excision repair (NER). We hypothesized that NER normally functions to destabilize expanded TNRs. To test this hypothesis, we bred an Xpa null allele, which eliminates NER, into the TNR mouse model for spinocerebellar ataxia type 1 (SCA1), which carries an expanded CAG repeat tract at the endogenous mouse Sca1 locus. We find that Xpa deficiency does not substantially affect TNR instability in either the male or female germline; however, it dramatically reduces CAG repeat instability in neuronal tissues-striatum, hippocampus and cerebral cortex-but does not alter CAG instability in kidney or liver. The tissue-specific effect of Xpa deficiency represents a novel finding; it suggests that tissue-to-tissue variation in CAG repeat instability arises, in part, by different underlying mechanisms. These results validate our original findings in cultured human cells and suggest that transcription may induce NER-dependent TNR instability in neuronal tissues in humans.

Figure 1.

Intergenerational changes in CAG tract length in progeny of Xpa^+/− SCA1 and Xpa^−/− SCA1 mice. Percent transmission is the number of alleles with a given repeat length divided by the total number of alleles (n) multiplied by 100%. Change in repeat length is defined as the number of repeats in a progeny mouse minus the number of repeats in its donor parent.

Figure 2.

Somatic instability of CAG repeats at the Sca1 locus in the striatum and kidney from 45–50-week-old Xpa^+/− SCA1 and Xpa^−/− SCA1 mice. Thirty small-pool PCR reactions were prepared for each striatum and kidney sample from two different Xpa^+/− SCA1 and Xpa^−/− SCA1 mice, and the products were separated by gel electrophoresis and visualized by hybridization with a radioactive probe. Representative sections of the resulting phosphorimages are shown. Two-headed arrows indicate the length of the CAG repeat tract in tail DNA from the analyzed mice, which was included as a control, but is not shown here.

Figure 3.

Somatic instability of CAG repeats at the Sca1 locus in the striatum and kidney from 30-week-old Xpa^+/− SCA1 and Xpa^−/− SCA1 mice. Small-pool PCR reactions were prepared for each tissue sample from two different Xpa^+/− SCA1 and Xpa^−/− SCA1 mice and analyzed and displayed as described in Figure 2.

Figure 4.

Somatic instability of CAG repeats at the Sca1 locus in the hippocampus, cerebral cortex and liver from 45–50-week-old Xpa^+/− SCA1 and Xpa^−/− SCA1 mice. Small-pool PCR reactions were prepared for each tissue sample from two different Xpa^+/− SCA1 and Xpa^−/− SCA1 mice and analyzed and displayed as described in Figure 2.