Elucidation of molecular mechanisms of hereditary neurologic diseases by positional cloning

我们已经应用了定位克隆策略作为一种策略来识别遗传神经退行性疾病的基因。在各种形式的脊髓小脑变性中，我们重点研究了Machado-Joseph病（MJD）、齿状小脑-苍白球萎缩（DRPLA）和低白蛋白血症相关的早发性共济失调（EOAHA）。我们利用微卫星多态性对MJD进行了系统的连锁分析。通过对90个微卫星标记的检测，我们发现MJD与D14S55和D14S48有紧密的连锁关系，最大的负荷评分为9.719。为了进一步缩小候选区域，需要进一步详细的连锁和连锁不平衡分析。鉴于不稳定的三核苷酸重复扩增是神经退行性疾病的常见机制，我们假设DRPLA是由类似的机制引起的，因为在DRPLA中也观察到一个突出的预期（连续几代的发病年龄加快），这是三核苷酸重复疾病的一个特征。通过寻找具有三核苷酸重复的基因，我们发现DRPLA是由位于12号染色体上的基因的三核苷酸重复扩增不稳定引起的。发病年龄与三核苷酸重复扩增程度密切相关，提示CAG重复扩增与DRPLA发病密切相关。通过详细的连锁分析，我们发现该基因位于第9号染色体上。虽然EOAHA基因位于弗里德赖希共济失调基因座附近，但对涉及弗里德赖希基因座的多个重组事件的观察表明，从遗传学角度来看，EOAHA是一种独特的疾病。

We have applied the strategy of positional cloning as a strategy to identify genes for hereditary neurodegenerative disorder. Among various forms of spinocerebellar degeneration, we have focused out effort on Machado-Joseph disease (MJD), dentatorubral-pallidoluysian atrophy (DRPLA), and early onset ataxia associated with hypoalbuminemia (EOAHA).We have initiated systematic linkage analyzes of MJD using microsatellite polymorphisms. After checking 90 microsatellite markers, we have found that the MJD is tightly linked to D14S55 and D14S48 with a maximum lod score of 9.719. To further narrow down the candidate region, further detailed linkage as well as linkage disequilibrium analysis will be required.With the background that unstable expansion of trinucleotide repeat is a common mechanism for neurodegenerative disorder, we have hypothesized that DRPLA is caused by the similar mechanism, because a prominent anticipation (accelerated ages of onset in successive generations), a characteristic feature for triplet repeat diseases, is observed in DRPLA as well. By searching for genes with trinucleotide repeat, we have discovered that DRPLA is caused by unstable expansion of trinucleotide repeat in the gene located on chromosome 12. Close correlation between ages of onset and the degree of expanded trinucleotide repeat suggests that the expansion of the CAG repeat is intimately involved in the pathogenesis of DRPLA.With detailed linkage analysis we have doscpvered that the gene is lcoated on chromosome 9. Although the gene for EOAHA is located near the locus for Friedreich's ataxia, observation of multiple recombination events involving the Friedreich's locussuggests that EOAHA is a distinct disease from a genetic point of view.