SMCHD1 is involved in de novo methylation of the DUX4-encoding D4Z4 macrosatellite.

The DNA methylation epigenetic signature is a key determinant during development. Rules governing its establishment and maintenance remain elusive especially at repetitive sequences, which account for the majority of methylated CGs. DNA methylation is altered in a number of diseases including those linked to mutations in factors that modify chromatin. Among them, SMCHD1 (Structural Maintenance of Chromosomes Hinge Domain Containing 1) has been of major interest following identification of germline mutations in Facio-Scapulo-Humeral Dystrophy (FSHD) and in an unrelated developmental disorder, Bosma Arhinia Microphthalmia Syndrome (BAMS). By investigating why germline SMCHD1 mutations lead to these two different diseases, we uncovered a role for this factor in de novo methylation at the pluripotent stage. SMCHD1 is required for the dynamic methylation of the D4Z4 macrosatellite upon reprogramming but seems dispensable for methylation maintenance. We find that FSHD and BAMS patient's cells carrying SMCHD1 mutations are both permissive for DUX4 expression, a transcription factor whose regulation has been proposed as the main trigger for FSHD. These findings open new questions as to what is the true aetiology for FSHD, the epigenetic events associated with the disease thus calling the current model into question and opening new perspectives for understanding repetitive DNA sequences regulation.

Rdh10 loss-of-function and perturbed retinoid signaling underlies the etiology of choanal atresia.

Craniofacial development is a complex process that involves sequential growth and fusion of the facial prominences. When these processes fail, congenital craniofacial anomalies can occur. For example, choanal atresia (CA) is a congenital craniofacial anomaly in which the connection between the nasal airway and nasopharynx is completely blocked. CA occurs in approximately 1/5000 live births and is a frequent component of congenital disorders such as CHARGE, Treacher Collins, Crouzon and Pfeiffer syndromes. However, the detailed cellular and molecular mechanisms underpinning the etiology and pathogenesis of CA remain elusive. In this study, we discovered that mice with mutations in retinol dehydrogenase 10 (Rdh10), which perturbs Vitamin A metabolism and retinoid signaling, exhibit fully penetrant CA. Interestingly, we demonstrate Rdh10 is specifically required in non-neural crest cells prior to E10.5 for proper choanae formation, and that in the absence of Rdh10, Fgf8 is ectopically expressed in the nasal fin. Furthermore, we found that defects in choanae development are associated with decreased cell proliferation and increased cell death in the epithelium of the developing nasal cavity, which retards invagination of the nasal cavity, and thus appears to contribute to the pathogenesis of CA. Taken together, our findings demonstrate that RDH10 is essential during the early stages of facial morphogenesis for the formation of a functional nasal airway, and furthermore establish Rdh10 mutant mice as an important model system to study CA.

A review of craniofacial disorders caused by spliceosomal defects.

The spliceosome is a large ribonucleoprotein complex that removes introns from pre-mRNA transcripts. Mutations in EFTUD2, encoding a component of the major spliceosome, have recently been identified as the cause of mandibulofacial dysostosis, Guion-Almeida type (MFDGA), characterized by mandibulofacial dysostosis, microcephaly, external ear malformations and intellectual disability. Mutations in several other genes involved in spliceosomal function or linked aspects of mRNA processing have also recently been identified in human disorders with specific craniofacial malformations: SF3B4 in Nager syndrome, an acrofacial dysostosis (AFD); SNRPB in cerebrocostomandibular syndrome, characterized by Robin sequence and rib defects; EIF4A3 in the AFD Richieri-Costa-Pereira syndrome, characterized by Robin sequence, median mandibular cleft and limb defects; and TXNL4A in Burn-McKeown syndrome, involving specific craniofacial dysmorphisms. Here, we review phenotypic and molecular aspects of these syndromes. Given the apparent sensitivity of craniofacial development to defects in mRNA processing, it is possible that mutations in other proteins involved in spliceosomal function will emerge in the future as causative for related human disorders.

Osteosclerotic bone dysplasia in siblings with a Fam20C mutation.

Raine syndrome is an autosomal recessive disorder caused by mutations in the FAM20C gene. FAM20C codes for the human homolog of DMP4, a dentin matrix protein highly expressed in odontoblasts and moderately in bone. DMP4 is probably playing a role in the mineralization process. Since the first case reported in 1989 by Raine et al. 21 cases have been published delineating a phenotype which associates dysmorphic features, cerebral calcifications, choanal atresia or stenosis and thoracic/pulmonary hypoplasia. Kan and Kozlowski suggested the name of Raine syndrome to describe this new lethal osteosclerotic bone dysplasia. All the cases described were lethal during the neonatal period except for the last two reported patients aged 8 and 11 years who presented severe mental retardation. Here we describe two sisters, with an attenuated phenotype of Raine syndrome, who present an unexpectedly normal psychomotor development at ages 4 and 1, respectively. Identification of a homozygous mutation in the FAM20C gene confirmed the Raine syndrome diagnosis, thus contributing to the expansion of the Raine syndrome phenotype. This case report also prompted us to revisit the FAM20 gene classification and allowed us to highlight the ancestral status of Fam20C.

CHD8 interacts with CHD7, a protein which is mutated in CHARGE syndrome.

CHARGE syndrome is an autosomal dominant disorder caused in about two-third of cases by mutations in the CHD7 gene. For other genetic diseases e.g. hereditary spastic paraplegia, it was shown that interacting partners are involved in the underlying cause of the disease. These data encouraged us to search for CHD7 binding partners by a yeast two-hybrid library screen and CHD8 was identified as an interacting partner. The result was confirmed by a direct yeast two-hybrid analysis, co-immunoprecipitation studies and by a bimolecular fluorescence complementation assay. To investigate the function of CHD7 missense mutations in the CHD7-CHD8 interacting area on the binding capacity of both proteins, we included three known missense mutations (p.His2096Arg, p.Val2102Ile and p.Gly2108Arg) and one newly identified missense mutation (p.Trp2091Arg) in the CHD7 gene and performed both direct yeast two-hybrid and co-immunoprecipitation studies. In the direct yeast two-hybrid system, the CHD7-CHD8 interaction was disrupted by the missense mutations p.Trp2091Arg, p.His2096Arg and p.Gly2108Arg, whereas in the co-immunoprecipitation studies disruption of the CHD7-CHD8 interaction by the mutations could not be observed. The results lead to the hypothesis that CHD7 and CHD8 proteins are interacting directly and indirectly via additional linker proteins. Disruption of the direct CHD7-CHD8 interaction might change the conformation of a putative large CHD7-CHD8 complex and could be a disease mechanism in CHARGE syndrome.

Endocrine and radiological studies in patients with molecularly confirmed CHARGE syndrome.

CONTEXT: CHARGE syndrome is a complex of congenital malformations, and CHD7 has been reported as a major gene involved in the etiology. OBJECTIVE: We performed endocrine and radiological studies to determine whether endocrinological disorders such as hypogonadotropic hypogonadism, GH deficiency, or hypothyroidism are involved and also whether olfactory bulb hypoplasia and semicircular canal aplasia are major signs in patients with molecularly confirmed CHARGE syndrome. DESIGN: Clinical features, endocrinological assessments, and radiological abnormalities in eight children (five boys and three girls) whose molecular analyses were available were evaluated among 15 children clinically diagnosed with CHARGE syndrome at our institute. RESULTS: We identified heterozygous CHD7 mutations in all patients screened for mutations. Four boys had micropenis and/or cryptorchidism. One was diagnosed with GH deficiency, and the other was diagnosed with hypothyroidism. Computed tomography findings revealed aplasia of the semicircular canals. Magnetic resonance imaging studies of the olfactory bulb region revealed abnormal olfactory sulci and bulb development in all children. CONCLUSION: We suggest that hypogonadism, GH deficiency, and hypothyroidism could be possible endocrinological defects in patients with CHD7 mutations and that olfactory bulb hypoplasia as well as semicircular canal aplasia should be considered as a major sign for CHARGE syndrome and recommend a computed tomography scan of the temporal bone and magnetic resonance imaging study of the olfactory bulb region.