Lysosomal degradation of GMPPB is associated with limb‐girdle muscular dystrophy type 2T

Abstract

Objective GDP‐mannose pyrophosphorylase B (GMPPB) related phenotype spectrum ranges widely from congenital myasthenic syndrome (CMS), limb‐girdle muscular dystrophy type 2T (LGMD 2T) to severe congenital muscle‐eye‐brain syndrome. Our study investigates the clinicopathologic features of a patient with novel GMPPB mutations and explores the pathogenetic mechanism. Methods The patient was a 22‐year‐old woman with chronic proximal limb weakness for 9 years without cognitive deterioration. Weakness became worse after fatigue. Elevated serum creatine kinase and decrements on repetitive nerve stimulation test were recorded. MRI showed fatty infiltration in muscles of lower limbs and shoulder girdle on T1 sequence. Open muscle biopsy and genetic analysis were performed. Results Muscle biopsy showed myogenic changes. Two missense mutations in GMPPB gene (c.803T>C and c.1060G>A) were identified in the patient. Western blotting and immunostaining showed GMPPB and α‐dystroglycan deficiency in the patient's muscle. In vitro, mutant GMPPB forming cytoplasmic aggregates completely colocalized with microtubule‐associated protein 1 light chain 3‐II (LC3‐II), a classical marker of autophagosome. Degradation of GMPPB was accompanied by an upregulation of LC3‐II, which could be restored by lysosomal inhibitor leupeptin. Interpretation We identified two novel GMPPB mutations causing overlap phenotype between LGMD 2T and CMS. We provided the initial evidence that mutant GMPPB colocalizes with autophagosome at subcellular level. GMPPB mutants degraded by autophagy‐lysosome pathway is associated with LGMD 2T. This study shed the light into the enzyme replacement which could become one of the therapeutic targets in the future study.

GMPPB-congenital disorders of glycosylation associate with decreased enzymatic activity of GMPPB

“…So far, more than 50 different GMPPB mutations have been reported. The clinical spectrum of GMPPB-CDG spans from CMS, LGMD to CMD with or without brain abnormalities [ 10 , 13 , 14 , 18 , 22 ]. However, the mechanism underlying this broad phenotypic spectrum remains unknown.…”

“…GMPPB is comprised of two functional domains: N-terminal catalytic domain (residues 1–250) and a left-handed parallel beta-helix domain (residues 251–360) [ 18 ]. Whereas disease-causing mutations are found through the entire GMPPB protein, mutations within its catalytic domain impair activity more severely than those in the LbH domain.…”

“…How do GMPPB mutations affect its functions? Previous studies suggested that some mutants, such as D334N, P22S, P32L, R287Q, V330I, V254M, R287W, I268T, and G354S, altered subcellular localization of GMPPB [ 10 , 13 , 18 ]. And V111G and G214S mutations identified in our patient also altered subcellular localization of GMPPB (Supplementary Fig.…”

“…Furthermore, some mutations, including V254M,. R287W, I268T, and G354S, impaired GMPPB protein level [ 10 , 18 ]. Interestingly, reduced protein levels of I268T and G354S could be rescued by lysosomal inhibitors, indicating the involvement of autophagy-lysosomal pathway in destabilization of GMPPB [ 18 ].…”

“…Most of the variants are missense, and the remaining mutations include nonsense, frame shift, and splicing. Interestingly, GMPPB mutations-associated phenotypic spectrum ranges from LGMD, to congenital myasthenic syndrome (CMS), to severe CMD with eye and brain symptoms [10][11][12][13][14][15][16][17][18]. However, there is no clear genotype-phenotype correlation.…”