Elucidating the clinical spectrum and molecular basis of HYAL2 deficiency

Hyaluronidase 2 deficiency due to novel compound heterozygous variants in HYAL2: a case report of siblings with HYAL2 deficiency showing different clinical severity and literature review

This study reports the first Asian case of syndromic cleft lip and palate resembling CHARGE-like syndrome, caused by novel compound heterozygous variants of the HYAL2 gene. Hyaluronidase-2 (HYAL2) plays a critical role in hyaluronic acid degradation and tissue remodelling. A 2-year-old Japanese boy presented with growth deficiency, congenital heart disease, craniofacial dysmorphism, micropenis, and developmental delays-features that overlapped with those of CHARGE syndrome. Genetic analysis identified two rare HYAL2 missense variants (c.1133G>A, p.Arg378His; c.1271A>G, p.His424Arg), classified as "likely pathogenic" based on ACMG/AMP criteria. This case highlights the importance of considering HYAL2 deficiency in the syndromic presentation of cleft lip and palate with congenital heart disease, particularly in the absence of CHD7 abnormalities. This study also emphasizes potential primary testicular dysfunction in male patients with HYAL2 deficiency and underscores the need for further research to clarify genotype-phenotype correlations and pathology.

Genetic Deficiencies of Hyaluronan Degradation

Hyaluronan (HA) is a large polysaccharide that is broadly distributed and highly abundant in the soft connective tissues and embryos of vertebrates. The constitutive turnover of HA is very high, estimated at 5 g per day in an average (70 kg) adult human, but HA turnover must also be tightly regulated in some processes. Six genes encoding homologues to bee venom hyaluronidase (HYAL1, HYAL2, HYAL3, HYAL4, HYAL6P/HYALP1, SPAM1/PH20), as well as genes encoding two unrelated G8-domain-containing proteins demonstrated to be involved in HA degradation (CEMIP/KIAA1199, CEMIP2/TMEM2), have been identified in humans. Of these, only deficiencies in HYAL1, HYAL2, HYAL3 and CEMIP have been identified as the cause or putative cause of human genetic disorders. The phenotypes of these disorders have been vital in determining the biological roles of these enzymes but there is much that is still not understood. Deficiencies in these HA-degrading proteins have been created in mice and/or other model organisms where phenotypes could be analyzed and probed to expand our understanding of HA degradation and function. This review will describe what has been found in human and animal models of hyaluronidase deficiency and discuss how this has advanced our understanding of HA's role in health and disease.