Autosomal recessive nonsyndromic hearing impairment in two Finnish families due to the population enriched CABP2 c.637+1G>T variant.

BACKGROUND: The genetic architecture of hearing impairment in Finland is largely unknown. Here, we investigated two Finnish families with autosomal recessive nonsyndromic symmetrical moderate-to-severe hearing impairment. METHODS: Exome and custom capture next-generation sequencing were used to detect the underlying cause of hearing impairment. RESULTS: In both Finnish families, we identified a homozygous pathogenic splice site variant c.637+1G>T in CAPB2 that is known to cause autosomal recessive nonsyndromic hearing impairment. Four CABP2 variants have been reported to underlie autosomal recessive nonsyndromic hearing impairment in eight families from Iran, Turkey, Pakistan, Italy, and Denmark. Of these variants, the pathogenic splice site variant c.637+1G>T is the most prevalent. The c.637+1G>T variant is enriched in the Finnish population, which has undergone multiple bottlenecks that can lead to the higher frequency of certain variants including those involved in disease. CONCLUSION: We report two Finnish families with hearing impairment due to the CABP2 splice site variant c.637+1G>T.

First reported CABP2-related non-syndromic hearing loss in Northern Europe.

BACKGROUND: CABP2-related non-syndromic hearing loss have only been reported in a few families worldwide (Iran, Turkey, Pakistan and Italy). The hearing loss was in these cases described as prelingual, symmetrical, and moderate to severe. METHODS: Following DNA isolation, exome sequencing was performed in 123 genes related to non-syndromic hearing loss. Variant verification and carrier testing were performed by direct sequencing. RESULTS: We report the first Northern European individual with CABP2-related hearing loss: an 8-year-old Danish Caucasian boy with non-syndromic, prelingual, and sensorineural hearing loss, who is homozygous for the splice site variant CABP2: c. 637+1G>T previously found in three Iranian families and in one Pakistani family. Both parents are of Danish Caucasian origin with no known history of consanguinity. This is in contrast to the four reported Middle Eastern families, who all were consanguineous. However, loss of heterozygosity in a 3.2 Mb area on chromosome 11 including CABP2 was observed, suggesting a common parental ancestor. CONCLUSION: We report the first case of CABP2-related autosomal recessive hearing loss in Northern Europe. The index is of Danish Caucasian origin and found to be homozygous for the splice site variant c.637+1G>T.

Homozygous mutations in Pakistani consanguineous families with prelingual nonsyndromic hearing loss.

Autosomal recessive nonsyndromic hearing loss (DFNB) is relatively frequent in Pakistan, which is thought to be mainly due to relatively frequent consanguinity. DFNB genes vary widely in their kinds and functions making molecular diagnosis difficult. This study determined the genetic causes in five Pakistani DFNB families with prelingual onset. The familial genetic analysis identified four pathogenic or likely pathogenic homozygous mutations by whole exome sequencing: two splicing donor site mutations of c.787+1G>A in ESRRB (DFNB35) and c.637+1G>T in CABP2 (DFNB93) and two missense mutations of c.7814A>G (p.Asn2605Ser) in CDH23 (DFNB12) and c.242G>A (p.Arg81His) in TMIE (DFNB6). The ESRRB and TMIE mutations were novel, and the TMIE mutation was observed in two families. The two missense mutations were located at well conserved sites and in silico analysis predicted their pathogenicity. This study identified four homozygous mutations as the underlying cause of DFNB including two novel mutations. This study will be helpful for the exact molecular diagnosis and treatment of deafness patients.

A Novel Pathogenic Variant in the CABP2 Gene Causes Severe Nonsyndromic Hearing Loss in a Consanguineous Iranian Family.

BACKGROUND AND OBJECTIVES: Hereditary hearing loss (HL) can originate from mutations in one of many genes involved in the complex process of hearing. CABP2 mutations have been reported to cause moderate HL. Here, we report the whole exome sequencing (WES) of a proband presenting with prelingual, severe HL in an Iranian family. METHODS: A comprehensive family history was obtained, and clinical evaluations and pedigree analysis were performed in the family with 2 affected members. After excluding mutations in the GJB2 gene and 7 other most common autosomal recessive nonsyndromic HL (ARNSHL) genes via Sanger sequencing and genetic linkage analysis in the family, WES was utilized to find the possible etiology of the disease. RESULTS: WES results showed a novel rare variant (c.311G>A) in the CABP2gene.This missense variant in the exon 4 of the CABP2gene meets the criteria of being pathogenic according to the American College of Medical Genetics and Genomics (ACMG) interpretation guidelines. CONCLUSIONS: Up to now, 3 mutations have been reported for the CABP2gene to cause moderate ARNSHL in different populations. Our results show that CABP2variantsalso cause severe ARNSHL, adding CABP2to the growing list of genes that exhibit phenotypic heterogeneity. Expanding our understanding of the mutational spectrum of HL genes is an important step in providing the correct clinical molecular interpretation and diagnosis for patients.