Lysosomal storage, neurodegeneration, and albinism due to effects of a de novo CLCN7 mutation on lysosomal acidification

Authors

• E-R Nicoli
• MR Weston
• ME Hackbarth
• AJ Becerril
• A Larson
• JD Burke
• H Dorward
• Y Huang
• DR Adams
• PM Zerfas
• TC Markello
• C Toro
• G Elliot
• L Garett
• W Zheng
• CJ Tifft
• WA Gahl
• DL Day‐Salvatore
• JA Mindell
• MCV Malicdan

Abstract

Lysosomes are cellular organelles containing enzymes that degrade macromolecules for salvage of small molecules. For optimal function, lysosomes rely on the maintenance of an acidic lumenal pH. While active accumulation of protons is driven primarily by V-ATPase, luminal acidification also requires a neutralizing ion movement, maintained by specific channels in the lysosomal membrane. Defects in the latter can lead to physiological disruptions, including neurodegeneration and lysosomal storage diseases. CLCN7 is a member of the CLC gene family encoding Cl- channels and Cl-/H+ exchangers. ClC-7 provides the primary route for Cl- passage through the lysosomal membrane, but its role in lysosomal acidification has been controversial. Inactivating mutations in CLCN7 and its beta-subunit, Ostm1, cause both autosomal dominant and recessive forms of osteopetrosis and neurodegeneration. Missense mutations of CLCN7 have led to milder forms of osteopetrosis. No gain-of-function CLCN7 mutation has been reported. Here, through next generation sequencing, we identify a novel de novo CLCN7 Y715C mutation in two children of different ethnicities. These patients presented with delayed myelination and development, lysosomal storage, and hypopigmentation. However, the patients do not present with osteopetrosis. To characterize this mutation, we have measured the outward currents in mutant Xenopus oocytes, which showed an increased current, and measured lysosomal pH in patients' cultured fibroblasts, which demonstrated a 0.2-unit reduction. Patient fibroblasts also exhibited greatly enlarged cytoplasmic vacuoles, a finding recapitulated by overexpression of Y715C in control fibroblasts, reflecting the dominant, gain-of-function nature of the mutation. Furthermore, we created a knock-in Clcn7 Y713C/+ mouse (Y715C in human corresponds to Y713C in mouse) that unequivocally supports the pathogenicity of CLCN7 Y715C in the patients. This murine model has exhibited hypopigmentation, hepatosplenomegaly with storage, and enlarged vacuoles in cultured fibroblasts. Treatment with the alkalinizing drug, chloroquine, normalized the lysosomal pH, reduced the number of large cytoplasmic vacuoles in patients' fibroblasts, and showed hints of clinical improvement in one of the patients. Our results support that Y715C is a novel gain-of- function CLCN7 mutation associated with lysosomal hyperacidity and that the ClC-7 antiporter plays a critical role in maintaining lysosomal pH.

Scientific Focus Area: Genetics and Genomics

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