Clustering of the human CLCA gene family on the short arm of chromosome 1 (1p22-31)

A newly identified missense mutation in CLCA2 is associated with autosomal dominant cardiac conduction block

BACKGROUND

Progressive cardiac conduction defect (PCCD), also known as Lenegre-Lev disease, is one of the most common heart conduction abnormalities. Previous studies have screened for known mutation sites that cause heart block in a 68-person family with a history of PCCD, revealed no mutations.

OBJECTIVE

To screen pathogenic genes of the PCCD family and to study the function of the gene mutations related to heart block diseases.

METHODS

Whole exome sequencing (WES) was performed on two PCCD patients and one non-PCCD family member to find the related pathogenic gene. After family co-segregation and preliminary functional analysis, we identified the mutant gene CLCA2. To study the function of this gene, we constructed mutant-gene mice using CRISPR-Cas9 technology, and electrocardiogram monitoring was performed after genotype verification.

RESULTS

The CLCA2 c.G1725T mutation was identified and co-segregated with the phenotype. The analysis showed that the CLCA2 c.G1725T mutation is harmful and mainly affects protein glycosylation. Immunofluorescence staining revealed that CLCA2 was highly expressed in the sinoatrial node (SAN) tissues. Electrocardiogram monitoring of the mice revealed that CLCA2 point mutations induced mild conduction block and ectopic pacemakers.

CONCLUSION

Our findings indicate that a novel heterozygous missense mutation c.G1725T of the CLCA2 gene may be associated with heart block disease and the mutation in this gene may lead to sinus node lesions and conduction blocking.

Quantitative microsatellite analysis to delineate the commonly deleted region 1p22.3 in mantle cell lymphomas

The molecular pathogenesis of mantle cell lymphomas (MCL), a subset of B-cell non-Hodgkin's lymphomas with a poor prognosis, is still poorly understood. In addition to the characteristic primary genetic alteration t(11;14)(q13;q32), several further genetic changes are present in most cases. One of the most frequent genomic imbalances is the deletion of 1p22.1-p31.1 observed in nearly one-third of MCL cases. This might indicate the presence of tumor suppressor gene(s) in this critical region of deletion. Quantitative microsatellite analysis (QuMA) is a real-time PCR-based method to detect DNA copy number changes. Since QuMA has the resolving power to detect subtle genomic alterations, including homozygous deletions, this may help to identify candidate tumor suppressor genes from deleted regions. To gain more insight into the molecular pathogenesis of MCL, QuMA was performed on genomic DNA from 57 MCL cases. Eight microsatellite loci mapping to the chromosomal region 1p22.3 were analyzed. Losses were observed in 51 of the 57 ( approximately 89.5%) samples. Two cases showed a homozygous deletion at the locus containing the gene SH3GLB1, which plays a key role in Bax-mediated apoptosis. Two hotspots with copy number losses were detected at chromosomal localizations 85.4 and 86.6 Mb encompassing BCL10 and CLCA2. Both the genes seem to be attractive candidates to study tumor suppressor function in MCL.

Association of the hCLCA1 gene with childhood and adult asthma

Asthma is caused by bronchial inflammation. This inflammation involves mucus overproduction and hypersecretion. Recently, a mouse model of asthma showed that gob-5 is involved in the pathogenesis of asthma. The gob-5 gene is involved in mucus secretion and its expression is upregulated upon antigen attack in sensitized mice. The observation suggests that human homologue of gob-5, hCLCA1 (human calcium-dependent chloride channel-1), may be involved in human disease. We screened for single-nucleotide polymorphisms (SNPs) in hCLCA1 in the Japanese population. We identified eight SNPs, and performed association studies using 384 child patients with asthma, 480 adult patients with asthma, and 672 controls. In haplotype analysis, we found a different haplotype distribution pattern between controls and childhood asthma (P<0.0001) and between controls and adult asthma (P=0.0031). We identified a high-risk haplotype (CATCAAGT haplotype; P=0.0014) and a low-risk haplotype (TGCCAAGT haplotype; P=0.00010) in cases of childhood asthma. In diplotype analysis, patients who had the CATCAAGT haplotype showed a higher risk for childhood asthma than those who did not (P=0.0011). Individuals who had the TGCCAAGT haplotype showed a lower risk for childhood asthma than those who did not (P<0.0001). Our data suggested that variation of the hCLCA1 gene affects patients' susceptibility for asthma.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.