CLCN7, a gene shared by autosomal recessive and autosomal dominant osteopetrosis

Pathological Functions of Lysosomal Ion Channels in the Central Nervous System

Lysosomes are highly dynamic organelles that maintain cellular homeostasis and regulate fundamental cellular processes by integrating multiple metabolic pathways. Lysosomal ion channels such as TRPML1-3, TPC1/2, ClC6/7, CLN7, and TMEM175 mediate the flux of Ca2+, Cl-, Na+, H+, and K+ across lysosomal membranes in response to osmotic stimulus, nutrient-dependent signals, and cellular stresses. These ion channels serve as the crucial transducers of cell signals and are essential for the regulation of lysosomal biogenesis, motility, membrane contact site formation, and lysosomal homeostasis. In terms of pathophysiology, genetic variations in these channel genes have been associated with the development of lysosomal storage diseases, neurodegenerative diseases, inflammation, and cancer. This review aims to discuss the current understanding of the role of these ion channels in the central nervous system and to assess their potential as drug targets.

Osteopetrosis in the pediatric patient: what the radiologist needs to know

Osteopetrosis describes several types of rare sclerosing bone dysplasias of varying clinical and radiographic severity. The classic autosomal dominant subtype emerges most often in adolescence but can present from infancy through adulthood. The autosomal recessive osteopetrosis, or "malignant infantile osteopetrosis," presents in infancy with a grimmer prognosis, though the autosomal dominant forms (often mislabeled as "benign") actually can have life-threatening consequences as well. Often osteopetrosis is detected due to skeletal findings on radiographs performed to evaluate injury or as an incidental finding during evaluation for illness. Given the varied phenotypic severity and presentations at different ages, radiologists play an integral role in the care of these patients both in diagnosis and in clinical evaluation and monitoring. A deeper understanding of the underlying genetic basis of the disease can aid in the radiologist in diagnosis and in anticipation of unique complications. An overview of current clinical management is also discussed.

The PDE4 Inhibitors Roflumilast and Rolipram Rescue ADO2 Osteoclast Resorption Dysfunction

Autosomal Dominant Osteopetrosis type II (ADO2) is a rare bone disease of impaired osteoclastic bone resorption caused by heterozygous missense mutations in the chloride channel 7 (CLCN7). Adenylate cyclase, which catalyzes the formation of cAMP, is critical for lysosomal acidification in osteoclasts. We found reduced cAMP levels in ADO2 osteoclasts compared to wild-type (WT) osteoclasts, leading us to examine whether regulating cAMP would improve ADO2 osteoclast activity. Although forskolin, a known activator of adenylate cyclase and cAMP levels, negatively affected osteoclast number, it led to an overall increase in ADO2 and WT osteoclast resorption activity in vitro. Next, we examined cAMP hydrolysis by the phosphodiesterase 4 (PDE4) proteins in ADO2 versus WT osteoclasts. QPCR analysis revealed higher expression of the three major PDE4 subtypes (4a, 4b, 4d) in ADO2 osteoclasts compared in WT, consistent with reduced cAMP levels in ADO2 osteoclasts. In addition, we found that the PDE4 antagonists, rolipram and roflumilast, stimulated ADO2 and WT osteoclast formation in a dose-dependent manner. Importantly, roflumilast and rolipram displayed a concentration-dependent increase in osteoclast resorption activity which was greater in ADO2 than WT osteoclasts. Moreover, treatment with roflumilast rescued cAMP levels in ADO2 OCLs. The key findings from our studies demonstrate that osteoclasts from ADO2 mice exhibit reduced cAMP levels and PDE4 inhibition rescues cAMP levels and ADO2 osteoclast activity dysfunction in vitro. The mechanism of action of PDE4 inhibitors and their ability to reduce the high bone mass of ADO2 mice in vivo are currently under investigation. Importantly, these studies advance the understanding of the mechanisms underlying the ADO2 osteoclast dysfunction which is critical for the development of therapeutic approaches to treat clinically affected ADO2 patients.

Craniofacial disorders and dysplasias: Molecular, clinical, and management perspectives

There is a wide spectrum of craniofacial bone disorders and dysplasias because embryological development of the craniofacial region is complex. Classification of craniofacial bone disorders and dysplasias is also complex because they exhibit complex clinical, pathological, and molecular heterogeneity. Most craniofacial disorders and dysplasias are rare but they present an array of phenotypes that functionally impact the orofacial complex. Management of craniofacial disorders is a multidisciplinary approach that involves the collaborative efforts of multiple professionals. This review provides an overview of the complexity of craniofacial disorders and dysplasias from molecular, clinical, and management perspectives.

Impaired Autophagic Clearance with a Gain-of-Function Variant of the Lysosomal Cl-/H+ Exchanger ClC-7

ClC-7 is a ubiquitously expressed voltage-gated Cl-/H+ exchanger that critically contributes to lysosomal ion homeostasis. Together with its β-subunit Ostm1, ClC-7 localizes to lysosomes and to the ruffled border of osteoclasts, where it supports the acidification of the resorption lacuna. Loss of ClC-7 or Ostm1 leads to osteopetrosis accompanied by accumulation of storage material in lysosomes and neurodegeneration. Interestingly, not all osteopetrosis-causing CLCN7 mutations from patients are associated with a loss of ion transport. Some rather result in an acceleration of voltage-dependent ClC-7 activation. Recently, a gain-of-function variant, ClC-7Y715C, that yields larger ion currents upon heterologous expression, was identified in two patients with neurodegeneration, organomegaly and albinism. However, neither the patients nor a mouse model that carried the equivalent mutation developed osteopetrosis, although expression of ClC-7Y715C induced the formation of enlarged intracellular vacuoles. Here, we investigated how, in transfected cells with mutant ClC-7, the substitution of this tyrosine impinged on the morphology and function of lysosomes. Combinations of the tyrosine mutation with mutations that either uncouple Cl- from H+ counter-transport or strongly diminish overall ion currents were used to show that increased ClC-7 Cl-/H+ exchange activity is required for the formation of enlarged vacuoles by membrane fusion. Degradation of endocytosed material was reduced in these compartments and resulted in an accumulation of lysosomal storage material. In cells expressing the ClC-7 gain-of-function mutant, autophagic clearance was largely impaired, resulting in a build-up of autophagic material.

Bone Material Properties in Bone Diseases Affecting Children

PURPOSE OF REVIEW

Metabolic and genetic bone disorders affect not only bone mass but often also the bone material, including degree of mineralization, matrix organization, and lacunar porosity. The quality of juvenile bone is moreover highly influenced by skeletal growth. This review aims to provide a compact summary of the present knowledge on the complex interplay between bone modeling and remodeling during skeletal growth and to alert the reader to the complexity of bone tissue characteristics in children with bone disorders.

RECENT FINDINGS

We describe cellular events together with the characteristics of the different tissues and organic matrix organization (cartilage, woven and lamellar bone) occurring during linear growth. Subsequently, we present typical alterations thereof in disorders leading to over-mineralized bone matrix compared to those associated with low or normal mineral content based on bone biopsy studies. Growth spurts or growth retardation might amplify or mask disease-related alterations in bone material, which makes the interpretation of bone tissue findings in children complex and challenging.

The molecular spectrum of Turkish osteopetrosis and related osteoclast disorders with natural history, including a candidate gene, CCDC120

BACKGROUND

Osteopetrosis and related osteoclastic disorders are a heterogeneous group of inherited diseases characterized by increased bone density. The aim of this study is to investigate the molecular spectrum and natural history of the clinical and radiological features of these disorders.

METHODS

28 patients from 20 families were enrolled in the study; 20 of them were followed for a period of 1-16 years. Targeted gene analysis and whole-exome sequencing (WES) were performed.

RESULTS

Biallelic mutations in CLCN7 and TCIRG1 were detected in three families each, in TNFRSF11A and CA2 in two families each, and in SNX10 in one family in the osteopetrosis group. A heterozygous variant in CLCN7 was also found in one family. In the osteopetrosis and related osteoclast disorders group, three different variants in CTSK were detected in five families with pycnodysostosis and a SLC29A3 variant causing dysosteosclerosis was detected in one family. In autosomal recessive osteopetrosis (ARO), a malignant infantile form, four patients died during follow-up, two of whom had undergone hematopoietic stem cell transplantation. Interestingly, all patients had osteopetrorickets of the long bone metaphyses in infancy, typical skeletal features such as Erlenmeyer flask deformity and bone-in-bone appearance that developed toward the end of early childhood. Two siblings with a biallelic missense mutation in CLCN7 and one patient with the compound heterozygous novel splicing variants in intron 15 and 17 in TCIRG1 corresponded to the intermediate form of ARO (IARO); there was intrafamilial clinical heterogeneity in the family with the CLCN7 variant. One of two patients with IARO and distal tubular acidosis was found to have a large deletion in CA2. In one family, two siblings with a heterozygous mutation in CLCN7 were affected, whereas the father with the same mutation was asymptomatic. In WES analysis of three brothers from a family without mutations in osteopetrosis genes, a hemizygous missense variant in CCDC120, a novel gene, was found to be associated with high bone mass.

CONCLUSION

This study extended the natural history of the different types of osteopetrosis and also introduced a candidate gene, CCDC120, potentially causing osteopetrosis.