Genomic context drives SCA7 CAG repeat instability, while expressed SCA7 cDNAs are intergenerationally and somatically stable in transgenic mice
Genomic context drives SCA7 CAG repeat instability, while expressed SCA7 cDNAs are intergenerationally and somatically stable in transgenic mice
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DOI：
10.1093/hmg/ddg006
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发表时间：
2003-01-01
影响因子：
3.5
通讯作者：
La Spada, AR
中科院分区：
文献类型：
--
作者：
Libby, RT;Monckton, DG;La Spada, AR
脊髓小脑型共济失调7型(SCA7)是一种常染色体显性遗传性小脑性共济失调，由ataxin-7基因CAG重复扩增引起。在人类中，SCA7的特点是由于代际重复不稳定而具有明显的预期性，并倾向于扩展，因此被认为是最不稳定的多谷氨酰胺疾病。为了研究CAG/CTG重复序列不稳定性的分子基础及其病理学意义，我们建立了携带SCA7基因片段和13.5kb基因组片段的转基因小鼠品系，共92个CAG重复序列。基因片段转基因小鼠表现出明显的代际不稳定性，且存在明显的扩增偏向。然后我们继续产生更多的基因组片段转基因小鼠品系，并观察到3‘基因组区域的缺失显著地稳定了SCA7 CAG92重复序列的代际传播。这些结果表明，基因组片段上存在的顺式信息正在推动不稳定过程。由于SCA7基因组片段含有大量的复制相关基序，这些序列元件的存在可能会使SCA7 CAG重复区域更容易发生不稳定性。对基因组片段小鼠组织的小池和标准PCR分析显示，它们的大脑和肝脏出现了大量重复扩张，但在已被证明经历了神经退化的cdna转基因小鼠的任何组织中都没有发现这样的变化。由于SCA7cdna小鼠的大脑中没有大的体细胞重复扩张，我们的结果表明，至少在这些小鼠中，神经退化可以发生而没有明显的体细胞嵌合。
Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant cerebellar ataxia caused by a CAG repeat expansion in the ataxin-7 gene. In humans, SCA7 is characterized by marked anticipation due to intergenerational repeat instability with a bias toward expansion, and is thus regarded as the most unstable of the polyglutamine diseases. To study the molecular basis of CAG/CTG repeat instability and its pathological significance, we generated lines of transgenic mice carrying either a SCA7 cDNA construct or a 13.5 kb SCA7 genomic fragment with 92 CAG repeats. While the cDNA transgenic mice showed little intergenerational repeat instability, the genomic fragment transgenic mice displayed marked intergenerational instability with an obvious expansion bias. We then went on to generate additional lines of genomic fragment transgenic mice, and observed that deletion of the 3' genomic region significantly stabilized intergenerational transmission of the SCA7 CAG92 repeat. These results suggest that cis-information present on the genomic fragment is driving the instability process. As the SCA7 genomic fragment contains a large number of replication-associated motifs, the presence of such sequence elements may make the SCA7 CAG repeat region more susceptible to instability. Small-pool and standard PCR analysis of tissues from genomic fragment mice revealed large repeat expansions in their brains and livers, but no such changes were found in any tissues from cDNA transgenic mice that have been shown to undergo neurodegeneration. As large somatic repeat expansions are absent from the brains of SCA7 cDNA mice, our results indicate that neurodegeneration can occur without marked somatic mosaicism, at least in these mice.