Molecular genetic analysis of familial amyloidotic polyneuropathy

家族性淀粉样变性多发性神经病(FAP)是一种常染色体显性遗传性疾病，其特征是细胞外淀粉样纤维沉积，并明显累及周围神经和自主神经。虽然该疾病的致病基因已被鉴定为转甲状腺素(TTR)基因，并在分子水平上得到了很好的鉴定，但许多问题，如淀粉样蛋白沉积的机制，仍有待阐明。我们以前通过导入含有自身启动子(0.6-hTTR30)和金属硫蛋白启动子(MT-hTTR30)的人突变型TTR基因获得了两个转基因小鼠系。在这两个株系中，人突变TTR的血清总浓度是相同的。然而，MT-hTTR30系的淀粉样蛋白沉积开始于6个月龄，比0.6-hTTR30系早9个月。这些结果表明，由人突变TTR组成的同源四聚体的浓度而不是人突变TTR分子的总量对淀粉样蛋白沉积是重要的。为了分析人血清淀粉样蛋白P成分(SAP)的作用，SAP是淀粉样纤维的次要成分，我们还培育了SAP转基因小鼠。有趣的是，人SAP的血清水平与整合的拷贝数大致成正比，并在三条线上高于人血清。将这些小鼠与MT-hTTR30转基因小鼠交配，获得同时携带两种基因的小鼠。在这些小鼠中，淀粉样蛋白沉积的开始并没有加速，这表明人类SAP不参与淀粉样蛋白沉积的启动。奇怪的是，在FAP患者中常见的周围神经组织中没有淀粉样蛋白沉积。这些结果表明，突变型TTR分子在脊索丛中的产生或高水平表达是淀粉样蛋白在这些组织中沉积所必需的。

Familial amyloidotic polyneuropathy (FAP) is an autosomal dominant disorder characterized by extracellular deposition of amyloid fibrils and by prominent peripheral and autonomic nerve involvement. Although the gene responsible for this disease has been identified as the transthyretin (TTR) gene and well characterized at molecular level, many questions, such as the mechanism of amyloid deposition, remain to be elucidated. We formerly produced two lines of transgenic mice by introducing the human mutant TTR gene containing its own promoter (0.6-hTTR30) or metallothionein promoter (MT-hTTR30). In these two lines the total serum concentrations of human mutant TTR were the same. However, the onset of amyloid deposition in MT-hTTR30 lines was 6 months of age and was 9 months earlier than that in the 0.6-hTTR30 lines. These results suggest that the concentration of homo-tetramers composed of human mutant TTR but not the total amount of human mutant TTR molecule is important for amyloid deposition.To analyze the role of the human serum amyloid P component (SAP) which is a minor component of amyloid fibrils, we also produced SAP transgenic mice. Interestingly, the serum levels of human SAP were roughly proportional to the number of copies integrated and were higher than that of the human serum in three lines. These mice were mated with MT-hTTR30 transgenic to obtain mice carrying both genes. In these mice the onset of amyloid deposition was not accelerated suggesting that human SAP is not involved in the initiation of amyloid deposition.Strangely enough, there were no amyloid deposits in peripheral nervous tissues where amyloid deposition is commonly observed in FAP patients. These results suggest that the production of mutant TTR molecule in the chorid plexus or high level expression is required for the amyloid deposition in these tissues.