Characterization of mutations and phenotypes associated with HYAL2 deficiency
Date
2017
Authors
Hasan, S M Naimul
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Abstract
Hyaluronidase 2 (HYAL2) is an endoglycosidase involved in the cleavage of hyaluronic acid (HA). HA is abundantly present in the extracellular matrix (ECM) and reported to have a functional role during embryonic development. Abundant HA has been found in the provisional matrix of the developing skeleton, craniofacial mesenchyme, endocardial cushions and smooth muscles of the gastrointestinal tract of the mouse. Cardiac cushion explant experiments showed that high molecular mass HA promoted epithelial to mesenchymal transition (EMT) while small HA fragments inhibited EMT signaling and induced cellular proliferation. Recently through linkage analysis, followed by next generation sequencing and computational analysis, two separate HYAL2 mutations (K148R and P250L) were identified in Amish and Saudi Arabian families with the syndromic cleft palate. We hypothesize that mutations in HYAL2 cause deficiency of HYAL2 enzyme and increase HA levels in the craniofacial region. Elevated HA in the craniofacial area interferes with the differentiation of osteoblasts leading to decreased bone development.
Comparison of the phenotypes of patients with HYAL2-mutations to the phenotype of the Hyal2-/- mouse from our lab showed similarities including cleft palate (submucosal with underdeveloped viscerocranial bones in mice), heart abnormalities and hearing loss. Functional studies of the mutated forms of HYAL2 expressed in HYAL2-deficient mouse embryonic fibroblasts (MEFs) showed that both mutations destabilized the HYAL2 protein and significantly reduced the HYAL2 level compared to the wild type. Histological analysis of the craniofacial region showed increased HA and reduced condensation of the matrix in the nasopharynx and palate region of HYAL2 deficient compared to wild type mice. Further, assessment of some markers of bone development and differentiation using immunohistochemistry revealed decreased development of the palatine, vomer and maxillary bones in the HYAL2 deficient mice. Our results indicate that HYAL2 deficiency affects HA turnover during craniofacial development, causing syndromic cleft lip/palate (CLP) and suggesting a possible role for HA in regulating osteoblast differentiation and development.
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Keywords
Hyaluronidase 2, Hyaluronic acid, Mutation, Extracellular matrix, Cleft palate