You will be presented with the abstract, introduction, and discussion section from a biomedical research article. Your task is to create a summary that a layperson can understand, capturing the essence of the research article. Keep essential scientific terms, but ensure that the language remains clear and the concepts are explained in an uncomplicated manner.

Title: Mutations in HYAL2, Encoding Hyaluronidase 2, Cause a Syndrome of Orofacial Clefting and Cor Triatriatum Sinister in Humans and Mice}
Article:
Abstract:
Orofacial clefting is amongst the most common of birth defects, with both genetic and environmental components. Although numerous studies have been undertaken to investigate the complexities of the genetic etiology of this heterogeneous condition, this factor remains incompletely understood. Here, we describe mutations in the HYAL2 gene as a cause of syndromic orofacial clefting. HYAL2, encoding hyaluronidase 2, degrades extracellular hyaluronan, a critical component of the developing heart and palatal shelf matrix. Transfection assays demonstrated that the gene mutations destabilize the molecule, dramatically reducing HYAL2 protein levels. Consistent with the clinical presentation in affected individuals, investigations of Hyal2-/- mice revealed craniofacial abnormalities, including submucosal cleft palate. In addition, cor triatriatum sinister and hearing loss, identified in a proportion of Hyal2-/- mice, were also found as incompletely penetrant features in affected humans. Taken together our findings identify a new genetic cause of orofacial clefting in humans and mice, and define the first molecular cause of human cor triatriatum sinister, illustrating the fundamental importance of HYAL2 and hyaluronan turnover for normal human and mouse development.
Introduction:
Orofacial clefts, which include cleft lip( CL) with or without cleft palate( CLP) and cleft palate alone( CP), collectively referred to as CL/P, are amongst the most common birth defects in all populations worldwide with many syndromic and non-syndromic forms described. The majority of cases are of unknown molecular cause[1]. The average incidence of CL/P is one in 700 newborns, with wide variability across racial and ethnic groups and socioeconomic status[2, 3]. The frequency of CL/P also differs by gender and laterality, there being a 2:1 male to female ratio for CLP, and a 1:2 male to female ratio for CP[2, 4]. Orofacial clefts arise from a failure of the intricate molecular and cellular processes that regulate bilateral fusion of the future lip and palate during craniofacial development[1]. They are clinically categorized dependent upon the absence( non-syndromic CL/P; 70% of cases) or presence( syndromic CL/P; 30% of cases) of additional congenital anomalies[5, 6]. For non-syndromic CL/P, 17 genetic risk loci have been defined and replicated with genome wide association studies worldwide[7–11]. As well as environmental influences, more recent evidence implicates non-coding or regulatory genomic regions in non-syndromic CL/P[12–14], perhaps explaining why re-sequencing of candidate genes or exome sequencing strategies rarely identify mutations in these cases[15]. For syndromic forms of CL/P, >300 different syndromes have been described in which CL/P is a prominent feature[16]. Congenital heart disease( CHD) is commonly associated with syndromic CL/P with a reported incidence of 13%-27%[15, 17–22]. Cor triatriatum is a rare congenital cardiac anomaly reported in 0. 1–0. 4% of individuals with congenital heart disease[23, 24]. It is characterized by division of either the left( cor triatriatum sinister) or less commonly the right atrium( cor triatriatum dexter)[25, 26]. To date, no molecular cause has been identified for cor triatriatum in humans. In the current study, we identify mutations in the gene HYAL2 as a new genetic cause of orofacial clefting in humans and mice and describe the first molecular basis for cor triatriatum sinister in humans. Our findings illustrate the fundamental importance of hyaluronan( HA) turnover for normal human and mouse development.
Discussion:
The molecular basis of human CLP is incompletely understood, despite its frequent occurrence and associated morbidity. Our findings demonstrate that a deficiency of the HA-degrading enzyme HYAL2 is a novel cause of syndromic CLP in humans and SMCP in mice, and define the first molecular explanation for cor triatriatum sinister in humans. All but one affected individual exhibited orofacial clefting, and craniofacial dysmorphism was a consistent finding. Myopia and other ocular abnormalities, pectus excavatum, single palmar creases, cor triatriatum sinister, a persistent left superior vena cava and other cardiac features were variably present. Our studies show that Hyal2-/- mice universally exhibit facial dysmorphism, an underdevelopment of the viscerocranium and cardiac valve thickening as well as variably penetrant hearing loss, partial or SMCP( 83%), cor triatriatum sinister( 50%) and atrial dilatation( 50%). The significant parity between the human and mouse phenotypes provides strong evidence that HYAL2-deficiency is the cause of these developmental abnormalities. Our data indicate that the differences between the human and mouse phenotypes may be largely explained by the complete absence of HYAL2 in Hyal2-/- mice, while the destabilizing and deactivating p. K148R and p. P250L HYAL2 substitutions identified in human affected individuals lead primarily to profound reductions in HYAL2 levels, which may still permit limited residual functionality. In surviving Hyal2-/- mice progressive accumulation of HA in the circulation, pulmonary fibrosis, and cardiac dysfunction lead to premature heart failure[36], while the absence of elevated circulating HA in the affected Amish individuals may partially or completely protect them from these cardiopulmonary complications. The residual activity associated with the amino acid substitutions in human subjects likely supports ongoing constitutive turnover of HA, but is insufficient for the rapid and regulated turnover required during development. Further support for this is provided by our study of Hyal2+/- mice where a small number showed developmental abnormalities in the absence of elevated circulating HA[36]. However, this may be an overly simplistic interpretation as circulating HA levels are approximately 10–20 fold higher in the mouse, despite similar rates of turnover in mice and humans[38]. The identification of additional HYAL2-deficient individuals would undoubtedly provide a more complete understanding of the phenotypic spectrum associated with HYAL2-deficiency. We identified SMCP as a likely cause of early pre-weaning lethality in Hyal2-/- mice. The underdeveloped viscerocranium likely weakens the palate, and in combination with the facial dysmorphism, presents significant feeding difficulties for Hyal2-/- suckling pups. Neurological abnormalities could also contribute to death, as the cribriform plate of the ethmoid is important in supporting the olfactory bulb and in preventing the leakage of cerebrospinal fluid into the nasal cavity. The bones of the viscerocranium have not yet been examined in affected individuals with the HYAL2 variant, but defects of the vomer are common in affected individuals with CLP[39]. The identification of developmental abnormalities in a small number of heterozygous( Hyal2+/-) mice suggests that haploinsufficiency of HYAL2 may also pose the risk of a similar outcome in humans. Consistent with the notion that altered HYAL2 levels may affect palatal development, decreased HA has previously been associated with increased risk for CP in the Tbx1-/- mouse[40], whereas increased HA has been associated with CP in Sim2-/- mice[41]. Thus, HYAL2 variants may be considered as candidates, even in the heterozygous form, to confer risk for these developmental abnormalities. However screening a cohort of non-syndromic CL/P individuals, provided little evidence for a major role for HYAL2 in this cohort( personal communication- P. Stanier). The cohort consisted of 380 individuals( 176 with unilateral or bilateral cleft lip and palate, 38 with cleft lip only, 121 with cleft palate only and 46 with submucous cleft palate) of a mixture of ethnicities the majority of which( 72%) were of white European origin recruited from the North East Thames region of the UK. Approximately 22% of cases were familial with affected first or second-degree relatives. While this study cohort is too small to draw definitive genotype-phenotype conclusions, this finding remains consistent with HYAL2 mutation being likely only associated with a rare, autosomal recessive syndrome. Eloquent studies in mice deficient in HA synthesis( Has2-/-) demonstrated that high molecular mass HA was essential for epithelial to mesenchymal transition( EMT) in the developing heart, whereas HA oligosaccharides inhibited EMT and promoted angiogenesis[42–44]. HYAL2 could modulate the size of HA to inhibit EMT and promote differentiation, in which case its deficiency would result in increased levels of high molecular mass HA, an overproduction of mesenchymal cells, and a decrease in differentiation. Indeed, the thickened cardiac valves and excess mesenchymal cells in some parts of the head of Hyal2-/- mice, suggest that EMT is upregulated in Hyal2-/- mice, and that HYAL2 normally plays a role in inhibiting this process. In histological sections, encompassing the nasopharynx and secondary palate, there is an abundance of cartilage but a deficiency in bone. The deficient ossification in the skull of Hyal2-/- mice, and underdevelopment of the viscerocranium, suggests a failure in the development of osteoblasts, as has been seen in Tbx22-/- mice[45]. In addition to the direct effects of HA on signaling pathways, it is possible that an overabundance of HA in Hyal2-/- tissues leads to alterations in the gradients of morphogens or signal transduction factors, impacting development. Clearly further studies are needed to investigate this possibility. Treatment strategies for this developmental disorder deserve further research, as HA synthesis can be inhibited using 4-methylumelliferone, an FDA-approved drug in some parts of the world[46]. Further, commercial preparations of human hyaluronidase that function at a neutral pH could be used to remove extracellular HA; these enzymes are available for human use[47], and formulations that are stable in the circulation are under development[48]. With the Hyal2-/- mouse model already available, testing of such strategies may enable the development of a pre-natal treatment for this malformation syndrome. Taken together, our findings highlight hyaluronidase enzymes as playing a vital role in both human and mouse development, and in particular have revealed a previously unrecognized pathway involved in the pathogenesis of orofacial clefting, myopia and cor triatriatum sinister in humans, which may ultimately be amenable to treatment therapies.
Cleft lip and palate ( CLP ) is one of the most common congenital anomalies worldwide . Most of the time the cause is unknown . Affected individuals have multiple problems including difficulties with feeding and speech and have to undergo several operations to correct the orofacial cleft . We describe mutations in an enzyme involved in the development of the palate and heart ( Hyaluronidase 2 , HYAL2 ) as the cause of a syndrome of CLP and a congenital heart anomaly ( cor triatriatum sinister , CTS ) in families from the Amish community and Northern Saudi Arabia of Arabic ethnicity . Mice who lack this enzyme show similar features . HYAL2 is involved in the metabolism of hyaluronic acid , which is important in many tissues of the body . By illustrating a lack of HYAL2 causes CLP and CTS we have demonstrated the importance of HYAL2 for the normal development of the palate and heart , improving our understanding of the complex mechanisms involved in the pathogenesis of CLP and CTS which may ultimately contribute to the development of novel therapies to treat these congenital anomalies .