Alternate Galactose Pathways in GALT-Deficient Mice

由于半乳糖-1-磷酸尿苷转移酶（GALT）缺乏而引起的人半乳糖血症是一种神秘的疾病。半乳糖限制饮食虽然减轻了新生儿的半乳糖毒性，但并不能预防后来的并发症、认知障碍、卵巢功能衰竭和神经失调。饮食疗法的无效要求人们努力了解疾病的病理生化基础，以便开发新的治疗策略。受影响患者临床研究的局限性促使构建了“敲除”小鼠，其中部分GALT基因已被删除，从而消除了GALT酶活性。然而，这些动物没有发展出人类的表型，即使喂食半乳糖也没有显示出半乳糖毒性的证据。这表明，缺乏高尔特是必要的，但不足以产生疾病。显然，除GALT基因突变外，其他因素也起着重要作用。GALT敲除小鼠提供了一个有价值的体内试管来确定为什么这些动物不发展人类表型的代谢解释。本提案的目的是检查两种可能性：第一，代谢物半乳糖醇的形成不足，半乳糖醇与半乳糖-1-磷酸一起是产生人类表型所必需的；第二，有一个强大的替代途径来处理半乳糖。这些将通过以下方式进行研究：1)通过基因操作构建表达人醛糖还原酶的转基因小鼠，当与galt缺乏的动物杂交时，将形成高水平的半乳糖醇和半乳糖- 1-磷酸，并具有人类表型；2)利用同位素半乳糖和复杂的分析技术对代谢途径进行有力的研究。通过理解为什么GALT敲除小鼠不表现出人类半乳糖血症表型，将获得对人类状况的深刻理解。

Human galactosemia due to deficiency of galactose-1- phosphate uridyltransferase (GALT) is an enigmatic disease. A galactose restricted diet although alleviating neonatal galactose toxicity does not prevent later complications, cognitive impairment, ovarian failure and neurologic ataxia. The inefficacy of diet therapy has mandated a vigorous effort to understand the pathobiochemical basis of the disease in order to develop new therapeutic strategies. The limitations of clinical studies of affected patients prompted the construction of a "knock-out" mouse where a portion of the GALT gene has been deleted thereby eliminating GALT enzyme activity. These animals, however, do not develop the human phenotype and show no evidence of galactose toxicity even when fed galactose. This suggests that the absence of GALT is necessary but not sufficient to produce disease. It appears obvious that factors other than the GALT gene mutation play an important role. The GALT knock-out mouse provides a valuable in vivo test tube to determine the metabolic explanation why these animals do not develop the human phenotype. The aim of this proposal is to examine two possibilities: first, that there is insufficient formation of the metabolite, galactitol, which together with galactose-1-phosphate is necessary to produce the human phenotype; and, second, that there is a robust alternate pathway for galactose disposal. These will be studied by: 1) genetic manipulation to construct a transgenic mouse which expresses human aldose reductase and, when bred with the GALT-deficient animal, will form high levels of galactitol as well as galactose- 1-phosphate and a human phenotype; and, 2) vigorous investigation of metabolic pathways employing isotopic galactose and sophisticated analytic techniques. Great insight into understanding the human condition will be gained by discerning why the GALT knock-out mouse does not exhibit the human galactosemic phenotype.