Müller H L, Yamazaki M, Michigami T, Kageyama T, Schönau E, Schneider P, Ozono K
Department of Pediatrics, University Hospital, Würzburg, Germany.
J Clin Endocrinol Metab. 2000 Feb;85(2):743-7. doi: 10.1210/jcem.85.2.6373.
Hypophosphatasia is characterized by the hypomineralization of bone associated with the mutation of the tissue-nonspecific alkaline phosphatase (TNSALP) gene. Although the disease is usually autosomal recessive, an autosomal dominant form is also recognized. Approximately 50 mutations have been found in the TNSALP gene in patients with hypophosphatasia. However, the mutations identified to date do not seem to account for the dominantly inherited form of the disease. We have examined a German family in which the father and all 4 children were affected with hypophosphatasia, whereas the mother was healthy. The affected members of this family showed premature loss of deciduous teeth at or shortly before 2 yr of age and low levels of serum ALP with elevated levels of urinary phosphoethanolamine. DNA analysis by direct sequencing revealed a heterozygous missense mutation that caused the conversion of amino acid Asp to Val at position 361 (D361V) in the patients. Another substitution was detected in exon 12 (Val to Ala conversion at codon 505: V505A) in 1 allele of the mother and 3 children, indicating no association of the substitution with the disease. Reconstruction experiments demonstrated that the D361V mutant protein lost its enzymatic activity and that it inhibited the function of wild-type enzyme when coexpressed in COS-7 cells. On the other hand, the V505A mutant exhibited enzymatic activities equal to those of the wild-type ALP. It is likely that the mutant D361V protein forms dimers with the wild-type protein, and the protein-protein interaction contributes to the dominant effect of the mutant D361V. The mutation that causes D361V is the first one proven to be associated with the dominant form of hypophosphatasia.
低磷酸酯酶症的特征是骨矿化不足，与组织非特异性碱性磷酸酶（TNSALP）基因突变有关。虽然该疾病通常为常染色体隐性遗传，但也存在常染色体显性遗传形式。在低磷酸酯酶症患者的TNSALP基因中已发现约50种突变。然而，迄今为止所鉴定出的突变似乎并不能解释该疾病的显性遗传形式。我们研究了一个德国家庭，父亲和所有4个孩子都患有低磷酸酯酶症，而母亲健康。这个家庭的患病成员在2岁时或接近2岁时乳牙过早脱落，血清碱性磷酸酶水平低，尿磷酸乙醇胺水平升高。通过直接测序进行的DNA分析显示，患者存在杂合错义突变，导致第361位氨基酸天冬氨酸（Asp）转换为缬氨酸（Val，D361V）。在母亲和3个孩子的1个等位基因的第12外显子中检测到另一个替换（密码子505处缬氨酸转换为丙氨酸：V505A），表明该替换与疾病无关。重组实验表明，D361V突变蛋白失去了酶活性，并且当在COS-7细胞中共表达时，它会抑制野生型酶的功能。另一方面，V505A突变体表现出与野生型碱性磷酸酶相同的酶活性。突变的D361V蛋白可能与野生型蛋白形成二聚体，并且蛋白质-蛋白质相互作用导致了突变体D361V的显性效应。导致D361V的突变是首个被证实与低磷酸酯酶症显性形式相关的突变。
