A study on the beta-Cell glucose sensors in insulin secretion : The relationship between dysfunction of glucose sensors and development of diabetes mellitus

遗传因素和环境因素在非胰岛素依赖型糖尿病（NIDDM）的发病机制中起重要作用。近年来的研究表明，单基因突变可能在糖尿病的发病机制中起一定的致病作用。在日本NIDDM患者中，葡萄糖诱导的胰岛素分泌在发病早期受损。在这里，我们报道了NIDDM受试者中葡萄糖传感器候选基因的突变。首先，我们检查了GLUT2基因的突变，发现核苷酸替换Phe^<479> (TTT->TTC)。虽然这是一个沉默突变，但该等位基因的纯合子仅在NIDDM受试者中发现，并且该等位基因在NIDDM受试者中的等位基因频率显著高于对照组。其次，研究电压依赖性钙通道基因（CACNL1A2）的突变。外显子1的PCR-SSCP程序显示NIDDM患者的ATG三核苷酸重复从7到8个变化，导致在氨基端增加了蛋氨酸。在正常对照中没有发现这种变化。第三，我们利用PCR-SSCP技术研究了胃抑制肽受体（GIPR）基因的整个编码区。我们发现了两个缺失突变，Glu^< 198>->Cys （Gly198Cys）位于第7外显子，Glu^<354>->Gln （Glu354Gln）位于第12外显子。对这两种基因突变的GIPR进行功能分析发现，Gly198Cys基因突变的GIPR在中国仓鼠卵巢细胞中诱导cAMP反应的半最大刺激值明显高于野生型，而gl354gln基因突变的GIPR与野生型的差异不显著。gl354gln的等位基因频率在NIDDM患者中与正常对照无明显差异，而Gly198Cys的纯合子仅在NIDDM患者中检测到。这里描述的突变可能是NIDDM发病机制的一部分，进一步的研究将揭示其余的机制。

Genetic factors play important roles in pathogenesis of non-insulin-dependent diabetes mellitus (NIDDM) as well as environmental factors. Recent studies showed that single gene mutation might have a causative effect in pathogenesis of diabetes mellitus. In Japanese patients with NIDDM,the insulin secretion induced by glucose is impaired in early stage of pathogenesis. Here, we report the mutations of candidate genes for glucose sensor in NIDDM subjects.First, we examined the mutations of the GLUT2 gene and found a nucleotide substitution Phe^<479> (TTT->TTC). Although it was a silent mutaion, homozygote of this allele was found only in NIDDM subjects, and allelic frequency of this allele was significantly higher in NIDDM subjects than in controls.Second, the mutations of voltage-dependent calcium channel gene (CACNL1A2) was studied. The PCR-SSCP procedure of exon 1 revealed a change from 7 to 8 ATG trinucleotide repeats in a patient with NIDDM,resulting in an addition of methionine at the amino-terminus. This change was not found in normal controls.Third, we investigated the entire coding region of the gastric inhibitory poltpeptide receptor (GIPR) gene by PCR-SSCP.We identified two missence mutations, Gly^<198>->Cys (Gly198Cys) in exon 7 and Glu^<354>->Gln (Glu354Gln) in exon 12. Functional analysis of the GIPR with either of these mutations revealed that half-maximal stimulation value of GIP-induced cAMP response in Chinese hamster ovary cells expressing the GIPR with Gly198Cys was considerably higher than that of the wild type, whereas that of the GIPR with Glu354Gln was not significantly different from that of wild type. While allelic frequency of Glu354Gln in NIDDM subjects was not different from that in normal control, homozygotes of the Gly198Cys was detected only in NIDDM subjects.Mutations described here might be part of mechanisms which contribute to the pathogenesis of NIDDM,and further studies will reveal the rest.