Recurrent missense variant identified in two unrelated families with MPZL2-related hearing loss, expanding the variant spectrum associated with DFNB111

MPZL2-related hearing loss is a rare form of autosomal recessive hearing loss characterized by progressive, mild sloping to severe sensorineural hearing loss. Thirty-five previously reported patients had biallelic truncating variants in MPZL2, with the exception of one patient with a missense variant of uncertain significance and a truncating variant. Here, we describe the clinical characteristics and genotypes of five patients from four families with confirmed MPZL2-related hearing loss. A rare missense likely pathogenic variant [NM_005797.4(MPZL2):c.280C>T,p.(Arg94Trp)] located in exon 3 was confirmed to be in trans with a recurrent pathogenic truncating variant that segregated with hearing loss in three of the patients from two unrelated families. This is the first recurrent likely pathogenic missense variant identified in MPZL2. Apparently milder or later-onset hearing loss associated with rare missense variants in MPZL2 indicates that some missense variants in this gene may cause a milder phenotype than that resulting from homozygous or compound heterozygous truncating variants. This study, along with the identification of truncating loss of function and missense MPZL2 variants in several diverse populations, suggests that MPZL2-related hearing loss may be more common than previously appreciated and demonstrates the need for MPZL2 inclusion in hearing loss testing panels.

MPZL2-a common autosomal recessive deafness gene related to moderate sensorineural hearing loss in the Chinese population

BACKGROUND

Mutations in MPZL2, the characteristic genetic etiology of autosomal recessive deafness loci 111 (DFNB111), cause non-syndromic and moderate sensorineural hearing loss.

METHODS

In this study, we analyzed the phenotype and genotype of eight pedigrees consisting of 10 hearing loss patients with bi-allelic pathogenic or likely pathogenic variants in MPZL2. These patients were identified from a 3272 Chinese patient cohort who underwent genetic testing.

RESULTS

Apart from symmetrical and moderate sensorineural hearing loss, the MPZL2-related phenotype was characterized by progressive hearing loss with variation in the onset age (congenital defect to onset at the young adult stage). We determined that in the Chinese population, the genetic load of MPZL2 defects was 0.24% (8/3272) in patients diagnosed with hearing loss and 7.02% (8/114) in patients diagnosed with hereditary moderate sensorineural hearing loss caused by STRC, OTOA, OTOG, OTOGL, TECTA, MPZL2 and others. Three known MPZL2 variants (c.220C > T (p.Gln74*), c.68delC (p.Pro23Leufs*2), c.463delG (p.Ala155Leufs*10)) and a novel start loss variant (c.3G > T (p.Met1?)) were identified. MPZL2 c.220C > T was identified as the hotspot variant in the Chinese population and even in East Asia compared with c.72delA (p.Ile24Metfs*22) in European and West Asia through allele frequency.

CONCLUSIONS

We concluded that apart from moderate HL, progressive HL is another character of MPZL2-related HL. No specified variant was verified for the progression of HL, the penetrance and expressivity cannot be determined yet. A novel MPZL2 variant at the start codon was identified, enriching the variant spectrum of MPZL2. The hotspot variants of MPZL2 vary in different ethnicities. This study provides valuable data for the diagnosis, prognosis evaluation and genetic counseling of patients with moderate sensorineural hearing loss related to MPZL2.

MPZL2 variant analysis with whole exome sequencing in a cohort of Chinese hearing loss patients

BACKGROUND

Hearing loss is a genetically heterogeneous disease with more than 100 genes identified. Pathogenic variants in the MPZL2 gene cause autosomal recessive non-syndromic hearing loss. MPZL2 patients showed mild to moderate progressive hearing loss with onset age around 10 years old. To date, four pathogenic variants have been identified.

AIMS

To explore the clinical characteristics and variants of MPZL2-related hearing loss, and summarize the prevalence rate in overall hearing loss patients.

MATERIAL AND METHODS

To determine the prevalence of MPZL2-related hearing loss in the Chinese population, we analyzed MPZL2 variants of whole exome sequencing data derived from a cohort of 385 hearing loss patients.

RESULTS

Overall, homozygous MPZL2 variants were identified in 5 sporadic cases (diagnostic rate = 1.30%). A novel missense variant c.52C > T;p.Leu18Phe was identified in one other patient with compound heterozygous mutations in MPZL2, but the pathogenicity was uncertain according to the American College of Medical Genetics guidelines (2015). A patient homozygous for the c.220C > T,p.Gln74Ter variant showed congenital profound hearing loss at all frequencies, a phenotype different from previous reports.

CONCLUSIONS

Our results enriched the mutation and phenotype spectrum of MPZL2-related hearing loss. Comparisons between allele frequencies of MPZL2:c.220C > T;p.Gln74Ter and other common deafness variants suggested that MPZL2:c.220C > T;p.Gln74Ter should be included in the group of common deafness variants for prescreening.

A homozygous MPZL2 deletion is associated with non syndromic hearing loss in a moroccan family

Adhesion glycoproteins are implicated in the pathophysiology of hearing loss, the most frequent inherited sensory disorder, affecting 1 in 1000 new-borns. Exome sequencing of a consanguineous Moroccan patient with mild hearing loss identified for the first time in a North African family a single homozygous mutation c.72delA in MPZL2 gene, encoding the Myelin Protein Zero-Like 2, reported as causing deafness in two other populations. Variable tandem repeat genotyping of this family revealed that the c.72delA MPZL2 allele shared a common haplotype with Turkish and Dutch families. These results confirm the pathogenicity of this MPZL2 mutation in recessive mild to moderate non-syndromic deafness.

MPZL2, Encoding the Epithelial Junctional Protein Myelin Protein Zero-like 2, Is Essential for Hearing in Man and Mouse

In a Dutch consanguineous family with recessively inherited nonsyndromic hearing impairment (HI), homozygosity mapping combined with whole-exome sequencing revealed a MPZL2 homozygous truncating variant, c.72del (p.Ile24Metfs^∗22). By screening a cohort of phenotype-matched subjects and a cohort of HI subjects in whom WES had been performed previously, we identified two additional families with biallelic truncating variants of MPZL2. Affected individuals demonstrated symmetric, progressive, mild to moderate sensorineural HI. Onset of HI was in the first decade, and high-frequency hearing was more severely affected. There was no vestibular involvement. MPZL2 encodes myelin protein zero-like 2, an adhesion molecule that mediates epithelial cell-cell interactions in several (developing) tissues. Involvement of MPZL2 in hearing was confirmed by audiometric evaluation of Mpzl2-mutant mice. These displayed early-onset progressive sensorineural HI that was more pronounced in the high frequencies. Histological analysis of adult mutant mice demonstrated an altered organization of outer hair cells and supporting cells and degeneration of the organ of Corti. In addition, we observed mild degeneration of spiral ganglion neurons, and this degeneration was most pronounced at the cochlear base. Although MPZL2 is known to function in cell adhesion in several tissues, no phenotypes other than HI were found to be associated with MPZL2 defects. This indicates that MPZL2 has a unique function in the inner ear. The present study suggests that deleterious variants of Mplz2/MPZL2 affect adhesion of the inner-ear epithelium and result in loss of structural integrity of the organ of Corti and progressive degeneration of hair cells, supporting cells, and spiral ganglion neurons.