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

Abstract

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.

Keywords: Deiters cells; MPZL2; cochlea; deafness; hair cells; hearing impairment; human; mouse.

Figure 1

Pedigrees, VNTR Genotypes and Segregation of Variants of MPZL2 (A) Genotypes of VNTR markers and segregation of identified truncating variants of MPZL2 in family W05-682. Besides MPZL2, TECTA (DFNB21) is also located within the homozygous region shared by the affected individuals. Pathogenic variants in the coding and intronic regions of TECTA were excluded. (B) Pedigrees and segregation analyses of two additional families with deleterious variants in MPZL2. Index cases are indicated by arrows. Double lines indicate consanguinity (for extended pedigrees, see Figure S1).

Figure 2

Audiometric Characterization of Families Affected by Pathogenic MPZL2 Variants (A) Air-conduction thresholds of the better-hearing ear of all affected individuals. The better-hearing ear was determined from calculations of the mean of the pure-tone thresholds for 0.5, 1, 2, and 4 kHz of the last audiogram. First-visit and last-visit audiograms are depicted. Subject II:1 of family W16-0451 was not able to participate in the clinical evaluation; only retrospective data for this subject were used for analysis. R, right ear; L, left ear. (B) ARTA (age-related typical audiogram) constructed by cross-sectional linear-regression analysis of last-visit audiograms of all affected individuals (n = 8).

Figure 3

Hearing Phenotype of Mpzl2-Mutant (Mpzl2^ko/ko) Mice (A) Representative click-evoked ABR recordings at decreasing intensities (dB SPL) in 4-, 8-, and 12-week-old wild-type and Mpzl2-mutant mice. Waves are labeled I–IV and reflect the evoked activity of the auditory nerve (I) and ascending points of the auditory pathway in the midbrain (II–IV). As the stimulus level is reduced, amplitudes of ABR waves decrease and latencies of waves increase. The lowest intensity at which the ABR-wave profile is higher than the background-noise signal is the threshold (bold line; WT, blue; Mpzl2-mutant mice, green). (B) ABR thresholds, in response to click and 8–40 kHz tone bursts, of the different genotypes and age groups. (C and E) ABR wave I amplitude/intensity (C) and latency/intensity (E) curves in response to click stimuli of increasing intensities were determined in 4- and 12-week-old wild-type and Mpzl2-mutant mice. (D and F) Peak amplitude of ABR waves I, II, and IV (D) and interpeak latencies I-II, II-IV, and I-IV (F) in response to a 70 dB SPL click stimulus in 4- and 12-week-old wild-type and Mpzl2-mutant mice. Data are shown as means ± SEM. Statistical significance: ^∗p < 0.05; ^∗∗p < 0.01.

Figure 4

Cochlear Morphology of 12-Week-Old Wild-Type and Mpzl2-Mutant (Mpzl2^ko/ko) Mice Microphotographs show representative midmodiolar cross sections of the cochlea from one representative wild-type mouse (A) and three Mpzl2-mutant mice (B–D). The apical (a), middle (m), and basal (b) organs of Corti are boxed in image (A). Close-ups of the organ of Corti from one wild-type mouse (E, I, and M) and three Mpzl2-mutant mice (F–H, J–L, and N–P) are shown. Arrowheads point to inner hair cells (IHCs), blackhead arrows to outer hair cells (OHCs), and arrows to Deiters cells (DCs). BM, basilar membrane; IP, inner pillar cell; and OP, outer pillar cell. Asterisks mark abnormalities. The scale bars represent 500 μm (A–D) and 25 μm (E–P).

Figure 5

Organ-of-Corti Cytoarchitecture of Wild-Type and Mpzl2-Mutant (Mpzl2^ko/ko) Mice Close-ups of the organ of Corti from representative frozen sections (10 μm) prepared from one representative wild-type mouse (A, E, and I) and three Mpzl2-mutant mice (B–D, F–H, and J–L). Hair cells and supporting cells were immunolabeled for Myosin VIIa (green) and SOX2 (red), respectively, in the apical, middle, and basal turns of the cochlea. Actin in the organ of Corti was stained with phalloidin (purple). Arrowheads point to inner hair cells (IHCs), whitehead arrows to outer hair cells (OHCs), and arrows to Deiters cells (DCs). Asterisks mark abnormalities. The scale bar represents 10 μm.

Figure 6

MPZL2 Displayed a Distinct Localization in the Cochlear Organ of Corti and Stria Vascularis at P4 in Wild-Type Mice (A) MPZL2 (red) localizes in the organ of Corti in the basal region of Deiters cells (DCs) present below the three rows of outer hair cells (OHCs) and diffusely in inner hair cells (IHCs), OHCs, and DCs. (B) In a serial section, collagen IV (green) immunostaining marked basement membranes, including the basilar membrane (BM), thereby indicating the localization of MPZL2 at the DC-BM contact region. (C) In the stria vascularis, MPZL2 (red) immunostaining was observed predominantly in the basal cell (BC) region. (D) Co-immunostaining of MPZL2 and Na⁺-K⁺ATPase (green), a marker for marginal cells (MCs), confirms immunostaining of MPZL2 (red) in the basal cells. Cell nuclei were stained with DAPI (blue). Arrowheads point to IHC, whitehead arrows to OHCs, and arrows to DCs. The scale bar (A–D) repsresents 20 μm.