Novel variants in GALE cause syndromic macrothrombocytopenia by disrupting glycosylation and thrombopoiesis

Key Points

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  GALE is expressed during late-stage megakaryopoiesis, regulating the glycosylation and surface exposure of GPIbα and β1 integrin.

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  Novel GALE variants cause syndromic macrothrombocytopenia associated with impaired platelet formation and function.

Visual Abstract

Abstract

Glycosylation is recognized as a key process for proper megakaryopoiesis and platelet formation. The enzyme uridine diphosphate (UDP)-galactose-4-epimerase, encoded by GALE, is involved in galactose metabolism and protein glycosylation. Here, we studied 3 patients from 2 unrelated families who showed lifelong severe thrombocytopenia, bleeding diathesis, mental retardation, mitral valve prolapse, and jaundice. Whole-exome sequencing revealed 4 variants that affect GALE, 3 of those previously unreported (Pedigree A, p.Lys78ValfsX32 and p.Thr150Met; Pedigree B, p.Val128Met; and p.Leu223Pro). Platelet phenotype analysis showed giant and/or grey platelets, impaired platelet aggregation, and severely reduced alpha and dense granule secretion. Enzymatic activity of the UDP-galactose-4-epimerase enzyme was severely decreased in all patients. Immunoblotting of platelet lysates revealed reduced GALE protein levels, a significant decrease in N-acetyl-lactosamine (LacNAc), showing a hypoglycosylation pattern, reduced surface expression of gylcoprotein Ibα-IX-V (GPIbα-IX-V) complex and mature β1 integrin, and increased apoptosis. In vitro studies performed with patients-derived megakaryocytes showed normal ploidy and maturation but decreased proplatelet formation because of the impaired glycosylation of the GPIbα and β1 integrin, and reduced externalization to megakaryocyte and platelet membranes. Altered distribution of filamin A and actin and delocalization of the von Willebrand factor were also shown. Overall, this study expands our knowledge of GALE-related thrombocytopenia and emphasizes the critical role of GALE in the physiological glycosylation of key proteins involved in platelet production and function.

Abstract

Genetic mutations affecting glycosylation can contribute to abnormal platelet production and function. In this context, Marín-Quílez and colleagues characterize the functions of UDP-galactose-4-epimerase (GALE), identifying 3 new GALE mutations in patients with macrothrombocytopenia, and moderate-to-severe bleeding tendencies. The authors explain their phenotypic effects on platelet production, structure, and function through elegant mechanistic studies.