PURPOSE Galactosemia is caused by metabolic disturbances at various stages of galactose metabolism, including deficiencies in enzymes involved in the Leloir pathway (GALT, GALK1, and GALE). Nevertheless, the etiology of galactosemia has not been identified in a subset of patients. This study aimed to explore the causes of unexplained galactosemia. METHODS Trio-based exome sequencing and/or Sanger sequencing was performed in eight patients with unexplained congenital galactosemia. In vitro enzymatic assays and immunoblot assays were performed to confirm the pathogenicity of the variants. RESULTS The highest blood galactose levels observed in each patient were 17.3-41.9 mg/dl. Bilateral cataracts were observed in two patients. In all eight patients, we identified biallelic variants (p.Arg82*, p.Ile99Leufs*46, p.Gly142Arg, p.Arg267Gly, and p.Trp311*) in the GALM encoding galactose mutarotase, which catalyzes epimerization between β- and α-D-galactose in the first step of the Leloir pathway. GALM enzyme activities were undetectable in lymphoblastoid cell lines established from two patients. Immunoblot analysis showed the absence of the GALM protein in the patients' peripheral blood mononuclear cells. In vitro GALM expression and protein stability assays revealed altered stabilities of the variant GALM proteins. CONCLUSION Biallelic GALM pathogenic variants cause galactosemia, suggesting the existence of type IV galactosemia.

中文翻译：

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双等位基因 GALM 致病变异导致一种新型的半乳糖血症。

目的 半乳糖血症是由半乳糖代谢各个阶段的代谢紊乱引起的，包括参与 Leloir 途径（GALT、GALK1 和 GALE）的酶缺乏。然而，尚未在一部分患者中确定半乳糖血症的病因。本研究旨在探讨原因不明的半乳糖血症的原因。方法对 8 名原因不明的先天性半乳糖血症患者进行基于 Trio 的外显子组测序和/或 Sanger 测序。进行体外酶测定和免疫印迹测定以确认变体的致病性。结果在每位患者中观察到的最高血液半乳糖水平为 17.3-41.9 mg/dl。在两名患者中观察到双侧白内障。在所有 8 名患者中，我们发现了双等位基因变异（p.Arg82*、p.Ile99Leufs*46、p.Gly142Arg、p.Arg267Gly 和 p. Trp311*) 在 GALM 中编码半乳糖变旋酶，在 Leloir 途径的第一步中催化 β- 和 α-D-半乳糖之间的差向异构化。在从两名患者建立的淋巴母细胞系中检测不到 GALM 酶活性。免疫印迹分析显示患者的外周血单个核细胞中不存在 GALM 蛋白。体外 GALM 表达和蛋白质稳定性测定揭示了变体 GALM 蛋白质的稳定性改变。结论双等位基因GALM致病变异引起半乳糖血症，提示存在IV型半乳糖血症。免疫印迹分析显示患者的外周血单个核细胞中不存在 GALM 蛋白。体外 GALM 表达和蛋白质稳定性测定揭示了变体 GALM 蛋白质的稳定性改变。结论双等位基因GALM致病变异引起半乳糖血症，提示存在IV型半乳糖血症。免疫印迹分析显示患者的外周血单个核细胞中不存在 GALM 蛋白。体外 GALM 表达和蛋白质稳定性测定揭示了变体 GALM 蛋白质的稳定性改变。结论双等位基因GALM致病变异引起半乳糖血症，提示存在IV型半乳糖血症。