Tanaka A, Ohno K, Sandhoff K, Maire I, Kolodny E H, Brown A, Suzuki K
Biological Sciences Research Center, University of North Carolina School of Medicine, Chapel Hill 27599-7250.
Am J Hum Genet. 1990 Feb;46(2):329-39.
A single nucleotide transition within exon 5 of the beta-hexosaminidase alpha chain gene was identified in a Puerto Rican patient with GM2-gangliosidosis B1 variant as the mutation responsible for the unusual enzymological characteristics of this variant (G533----A; Arg178----His) (the DN-allele). A total of seven patients with enzymological characteristics of B1 variant have since been studied. They were Puerto Rican (DN), Italian, French, Spanish, two patients of mixed ethnic origin (English/Italian/Hungarian and English/French/Azores), and a Czechoslovakian. In confirmation of our earlier finding based on screening with allele-specific probes, all patients except the one from Czechoslovakia carried the same DN-allele. A new point mutation found in this patient changed the same codon affected in the DN-allele (C532----T; Arg178----Cys). An asymptomatic Japanese individual included as a control also carried one allele with the DN-mutation. Site-directed mutagenesis and expression studies in COS I cells demonstrated that either of the two point mutations abolishes the catalytic activity of the alpha subunit. The Spanish patient was homozygous for the DN-allele, but others were all compound heterozygotes. The Puerto Rican patient was a compound heterozygote with the DN-mutation in one allele and with the four-base insertion in exon 11, one of the two mutations found in the classical Ashkenazi Jewish Tay-Sachs disease, in the other allele. Abnormalities of the other allele were not identified in all other compound heterozygous patients. In these patients, the level of mRNA derived from the other allele was variable, ranging from being undetectable to being much lower than normal. This series of studies uncovered a new B1 variant mutation, confirmed our preliminary finding that the DN-allele has a surprisingly wide geographic and ethnic distribution, and pointed out the highly complex nature of the molecular genetics of this rare disorder. They also support our working hypothesis that mutations responsible for the unique enzymological characteristics of the B1 variant should be located in or near exon 5 of the gene and that this region of the enzyme protein is critical for its catalytic function.
在一名患有GM2神经节苷脂贮积症B1变异型的波多黎各患者中，发现β-己糖胺酶α链基因第5外显子内有一个单核苷酸转换，该突变导致了此变异型异常的酶学特征（G533→A；Arg178→His）（DN等位基因）。此后，共研究了7名具有B1变异型酶学特征的患者。他们分别是波多黎各人（DN）、意大利人、法国人、西班牙人、两名混血患者（英/意/匈混血和英/法/亚速尔群岛混血）以及一名捷克斯洛伐克人。正如我们之前基于等位基因特异性探针筛查得出的结果，除捷克斯洛伐克的那名患者外，所有患者均携带相同的DN等位基因。在该捷克斯洛伐克患者中发现了一个新的点突变，改变了与DN等位基因中受影响相同的密码子（C532→T；Arg178→Cys）。作为对照纳入研究的一名无症状日本个体也携带一个带有DN突变的等位基因。在COS I细胞中进行的定点诱变和表达研究表明，这两个点突变中的任何一个都会消除α亚基的催化活性。西班牙患者为DN等位基因纯合子，但其他患者均为复合杂合子。那名波多黎各患者是复合杂合子，一个等位基因带有DN突变，另一个等位基因带有外显子11的四碱基插入，这是在经典的阿什肯纳兹犹太型泰-萨克斯病中发现的两种突变之一。在所有其他复合杂合子患者中未鉴定出另一个等位基因的异常情况。在这些患者中，来自另一个等位基因的mRNA水平各不相同，从检测不到到远低于正常水平。这一系列研究发现了一种新的B1变异型突变，证实了我们的初步发现，即DN等位基因具有惊人广泛的地理和种族分布，并指出了这种罕见疾病分子遗传学的高度复杂性。这些研究还支持了我们的工作假设，即导致B1变异型独特酶学特征的突变应位于该基因第5外显子内或其附近，并且该酶蛋白的这一区域对其催化功能至关重要。
