Oculo-facio-cardio-dental Research Articles

Hepatoblastoma in a 13-month-old Male With Oculofaciocardiodental Syndrome.

Oculofaciocardiodental (OFCD) syndrome is an X-linked dominant condition that is typically lethal in males. A 13-month-old male patient with OFCD syndrome presented with hepatoblastoma. He received chemotherapy per standard of care and had a surgical resection with few complications. He received sodium thiosulfate for otoprotection and dexrazoxane for cardioprotection. Five years after diagnosis, he remains well and in remission. Malignancies have been reported in patients with OFCD syndrome; however, our patient is the only male with OFCD syndrome described with hepatoblastoma. Given the rarity of this condition, malignancy may be part of the spectrum of disease.

Dental abnormalities observed in the oculo-facio-cardio-dental (OFCD) syndrome present in two Czech families bearing novel de novo BCOR pathogenic variants

BackgroundThe oculo-facio-cardio-dental syndrome (OFCD) is an ultra-rare multiple congenital anomaly. This report describes clinical findings emphasising dental phenotype in five, molecularly confirmed, female cases from two Czech families.Case presentationDental examinations were carried out. An orthopantomogram was taken in three patients, and all patients’ intraoral cavities and teeth were photographed. Exome sequencing was performed in both probands. Results were validated by Sanger DNA sequencing which was also used to follow segregation of the variants in first-degree relatives. Dental abnormalities and congenital cataracts were present in all five cases, whilst other signs were variable and included facial dysmorphism, microphthalmia, and cardiac and skeletal abnormalities. Two individuals had cleft lip and/or cleft palate. Radiculomegaly occurred in three patients with permanent teeth and was diagnosed on orthopantomograms. Two patients had agenesis of permanent teeth. Malocclusion was also present in two patients due to crowding and a Class III malocclusion and mandibular overjet. De novo novel pathogenic variants in the BCOR gene were identified; c.2382del p.(Lys795Argfs*12) and c.3914dup p.(Gln1306Alafs*20) and co-segregated with the disease in each family.ConclusionsThe OFCD syndrome has a unique dental phenotype and dentists should be aware of signs of this ultra-rare genetic disorder. All patients with congenital cataracts and dental abnormalities, including those without a family history, should be referred for genetic testing and indicated to specialised dental care.

Radiculomegaly as a key clinical feature in oculo-facio-cardio-dental (OFCD) syndrome: a case report with a novel truncating variant in BCOR gene.

Radiculomegaly is a rare dental anomaly characterised by the enlargement of the root canals of teeth. It is usually associated with oculo-facio-cardio-dental (OFCD) syndrome due to truncating variants in BCL-6 transcriptional corepressor (BCOR) (MIM*300485). We present the case of a 21-year-old female patient who was referred to genetics for a polymalformative syndrome including bilateral glaucoma and dental anomalies, especially radiculomegaly. Some others dysmorphic features were right superior lip notch, ogival palate, long philtrum, difficulty in pronation, café-au-lait spots, II-III toe bilateral syndactyly, and macrocephaly. Cone-beam CT confirmed radiculomegaly. The genetic analysis identified a heterozygous pathogenic variant NM_001123385.1:c.2093del (p.Pro698Glnfs*17) in the BCOR gene. After genetic diagnosis of OFCD syndrome, cardiac CT-scan revealed a large asymptomatic atrial septal defect that was subsequently surgically closed. Reviews of the literature have previously highlighted the prevalence of radiculomegaly in OFCD syndrome with a positive predictive value of 88.23% and a sensitivity of 75.94%. This case report highlights the importance of radiculomegaly as a clinical sign of OFCD syndrome, emphasising the rarity of non-syndromic radiculomegaly and the benefits of its diagnosis in clinical management, especially in cardiac screening.

Nonsense-mediated mRNA decay affects hyperactive root formation in oculo-facio-cardio-dental syndrome via up-frameshift protein 1

ObjectivesOculo-facio-cardio-dental (OFCD) syndrome is a rare X-linked genetic disorder caused by mutations in the BCL6 co-repressor (BCOR) and is mainly characterized by radiculomegaly (elongated dental roots). All BCOR mutations reported to date have been associated with premature termination codons, indicating that nonsense-mediated mRNA decay (NMD) might play a vital role in the pathogenesis of OFCD syndrome. However, the molecular mechanisms underlying NMD remain unclear. In this study, we investigated the involvement of up-frameshift protein 1 (UPF1), which plays a central role in NMD, in the hyperactive root formation caused by BCOR mutations. MethodsPeriodontal ligament cells, isolated from a Japanese woman with a c.3668delC frameshift mutation in BCOR, and primary human periodontal ligament fibroblasts (HPdLFs) were used for an RNA immunoprecipitation assay to confirm the binding of UPF1 to mutated BCOR. Additionally, the effects of UPF1 on the BCOR transcription levels and corresponding gene expression were determined by performing relative quantitative real-time polymerase chain reactions. ResultsRNA immunoprecipitation revealed that UPF1 binds to exon 9 of mutated BCOR. Additionally, UPF1 knockdown via siRNA upregulated the transcription of BCOR, whereas overexpression of wild-type and mutated BCOR with the same frameshift mutation in HPdLFs altered bone morphogenetic protein 2 (BMP2) expression. ConclusionsOur findings indicate that BCOR mutations regulate the transcription of BCOR via UPF1, which may in turn regulate the expression of BMP2. NMD, caused by a c.3668delC mutation, potentially leads to an OFCD syndrome phenotype, including elongated dental roots.

Oculo-facio-cardio-dental (OFCD) syndrome: a case report

BackgroundOculo-facio-cardio-dental (OFCD) syndrome is a rare condition that affects the eyes, face, heart, and teeth of patients. One notable dental characteristic of OFCD is radiculomegaly, or root gigantism, which highlights the role of dentists in detecting this syndrome. OFCD is an X-linked dominant syndrome that results from a variant in the BCOR gene. Our study presents the first documented case of OFCD in Vietnam and reports a novel BCOR gene variant observed in this case.Case presentationA 19-year-old Vietnamese female patient with an extremely long root with an abscess was clinically examined for the expression of OFCDs. The radiograph and the variant in BCOR gene were also evaluated. We identified abnormalities in the teeth, as well as ocular, facial, and cardiac features, with radiculomegaly of the canines being a specific symptom for OFCDs. The patient’s genetic analysis revealed a pathogenic heterozygous deletion at intron 11 of the BCOR gene, representing a novel variant.ConclusionOculo-facio-cardio-dental syndrome (OFCD) is an extremely rare condition characterized by abnormalities in the eyes, face, heart, and teeth, often caused by variants in the BCOR gene. Radiculomegaly, or enlarged dental roots, is a key diagnostic feature of OFCD, and early detection is crucial for preventing future dental complications.

A Case of Oculo-facio-cardio-dental （OFCD） Syndrome Treated with Orthognathic Surgery Considering the Unique Characteristics of the Condition

A Case of Oculo-facio-cardio-dental （OFCD） Syndrome Treated with Orthognathic Surgery Considering the Unique Characteristics of the Condition

Ocular severe involvement in oculofaciocardiodental syndrome: Description of a case series.

Oculofaciocardiodental (OFCD) syndrome is a rare genetic disorder affecting ocular, facial, dental, and cardiac systems, being an X-linked condition caused by pathogenic variants in the BCL-6 corepressor gene (BCOR). We report a case series of three female patients with OFCD syndrome with severe glaucoma. Three female patients with OFCD syndrome with different variants involving BCOR gene, in heterozygosity: a seven-years-old girl with an insertion (c.2037_2038dupCT), a nine years-old girl with a microdeletion in the X (p21.2-p11.4)) spanning the BCOR gene; and a 25 years-old female with a deletion (c.3858_3859del). Systemic involvement is variable among patients ranging from one patient mainly with ocular and dental involvement to one with associated intra-auricular and intra-ventricular defects. All the patients presented with congenital cataracts diagnosed in the first days of life. Cataract surgery was performed without incidents between 6 and 16 weeks of age in all the patients. Postoperatively, the three patients developed ocular hypertension and glaucoma with the need for surgical interventions, including trabeculectomy, Ahmed valve implantation, and cyclophotocoagulation. OFCD syndrome characterizes by a severe ocular involvement with glaucoma as a characteristic feature. Ocular hypertension after cataract surgery in these patients is challenging, almost always needing surgery during childhood. Therefore, we consider BCOR disruption may predispose to a higher incidence of glaucoma due to its aggressiveness and early onset on our case series. The awareness of these complications is crucial to an adequate follow-up of the patients.

A rare genotype of biallelic mosaic variants in BCOR gene causing a bilateral ocular anterior segment dysgenesis and cataracts.

Oculofaciocardiodental (OFCD) syndrome is a rare X-linked dominant syndrome characterized by the involvement of the eyes, face, teeth, and heart with variable expressivity. The syndrome is caused by loss-of-function variants in the BCOR gene located on the X chromosome. OFCD affects only females with presumed embryonic lethality among males. We report a first case of a female with biallelic mosaic variants in BCOR gene, leading to a severe ocular phenotype including anterior segment dysgenesis, cataracts, and retinal involvement. The unique condition of biallelic mosaic loss-of-function mutations leads to a variable expression of an allele with the pathogenic variant, independent of the X-Inactivation pattern. This novel mechanism of co-existent biallelic mosaicism should be suspected in unexplained severe cases of OFCD.

Molecular mechanism of hyperactive tooth root formation in oculo-facio-cardio-dental syndrome.

Mutations in the B-cell lymphoma 6 (BCL6) interacting corepressor (BCOR) cause oculo-facio-cardio-dental (OFCD) syndrome, a rare X-linked dominant condition that includes dental radiculomegaly among other characteristics. BCOR regulates downstream genes via BCL6 as a transcriptional corepressor. However, the molecular mechanism underlying the occurrence of radiculomegaly is still unknown. Thus, this study was aimed at identifying BCOR-regulated genetic pathways in radiculomegaly. The microarray profile of affected tissues revealed that the gene-specific transcriptional factors group, wherein nucleus factor 1B, distal-less homeobox 5, and zinc finger protein multitype 2 (ZFPM2) were the most upregulated, was significantly expressed in periodontal ligament (PDL) cells of the diseased patient with a frameshift mutation (c.3668delC) in BCOR. Wild-type BCOR overexpression in human periodontal ligament fibroblasts cells significantly hampered cellular proliferation and ZFPM2 mRNA downregulation. Promoter binding assays showed that wild-type BCOR was recruited in the BCL6 binding of the ZFPM2 promoter region after immunoprecipitation, while mutant BCOR, which was the same genotype as of our patient, failed to recruit these promoter regions. Knockdown of ZFPM2 expression in mutant PDL cells significantly reduced cellular proliferation as well as mRNA expression of alkaline phosphatase, an important marker of odontoblasts and cementoblasts. Collectively, our findings suggest that BCOR mutation-induced ZFPM2 regulation via BCL6 possibly contributes to hyperactive root formation in OFCD syndrome. Clinical data from patients with rare genetic diseases may aid in furthering the understanding of the mechanism controlling the final root length.

Expression of Normal or Mutated X-Linked BCOR Transcripts in OFCD iPSCs

Reprogramming diseased cells with mutated genes into induced pluripotent stem cells (iPSCs) can allow studies of disease mechanism and correct the mutation. Oculofaciocardiodental (OFCD) syndrome is a developmental disorder caused by heterozygous mutations in the X-linked BCL-6 corepressor (BCOR) gene. In this present study, we aimed to reprogram stem cells from a tooth apical papilla (SCAP) of a patient with OFCD, termed SCAP-O, into iPSCs. The SCAP-O carry a copy of the BCOR gene having 1 nucleotide deletion in 1 of the alleles, therefore harboring a mixture of cells expressing either normal (SCAP-OBCOR-WT) or mutated (SCAP-OBCOR-mut) BCOR transcripts. We subcloned SCAP-O and separated SCAP-OBCOR-WT and SCAP-OBCOR-mut as verified by sequencing. The selected subclone SCAP-OBCOR-mut expressed only the mutated BCOR transcripts and remained in such condition after multiple passages. We reprogrammed SCAP-O and subclone SCAP-OBCOR-mut into transgene-free iPSCs using an excisable lentiviral vector system (hSTEMCCA-loxP) carrying 4 reprogramming factors in a single cassette, followed by removal of transgenes via Cre-mediated excision. We found that after reprogramming SCAP-O or subclone SCAP-OBCOR-mut into iPSCs, some of the iPSC clones expressed either solely the normal BCOR-WT or BCOR-mut transcripts, while other clones expressed both BCOR-WT and BCOR-mut transcripts. This is our first step toward establishing OFCD study models by generating isogenic control BCOR-WT iPSCs versus BCOR-mut iPSCs.

Congenital cataracts in females caused by BCOR mutations; report of six further families demonstrating clinical variability and diverse genetic mechanisms

BackgroundPathogenic variants in the BCOR gene have been identified in males with X-linked recessive microphthalmia and in females with X-linked dominant oculofaciocardiodental (OFCD) syndrome. This latter condition has previously been regarded as rare but the increased availability of genetic testing in recent years has led to the identification of a greater number of patients. MethodsWe report the clinical and molecular findings in a series of 10 patients with pathogenic BCOR variants from 5 families, all seen in a single institution over a two year period. ResultsWe emphasize the phenotypic variability in this cohort and the diverse genetic mechanisms involved which included point mutations and deletions of BCOR as well as the occurrence of gonadal and somatic mosaicism. ConclusionIn this report we demonstrate the novel findings of four newly identified variants in BCOR associated with an OFCD phenotype, and suggest that the frequency of this condition in females presenting with congenital cataract, including unilateral cataract, is more common than anticipated. We demonstrate the utility of screening for genetic causes of congenital cataract. Although gonadal mosaicism in OFCD had previously been reported, we demonstrate the presence of somatic mosaicism where BCOR mutations may only be detected in DNA from tissues other than blood such as buccal cells.

Nuclear import of transcriptional corepressor BCOR occurs through interaction with karyopherin α expressed in human periodontal ligament

Mutations in the gene encoding BCL-6 corepressor (BCOR) are responsible for oculofaciocardiodental (OFCD) syndrome, which is a rare X-linked dominant disorder characterized by radiculomegaly of permanent teeth as the most typical symptom. To function as a transcriptional corepressor, BCOR needs to enter the nucleus; however, the molecular pathway for its nuclear translocation during dental root formation remains unclear. The purpose of this study was to determine the mechanism underlying BCOR transport into the nucleus. Our results showed that human periodontal ligament (PDL) cells expressed karyopherin α (KPNA)2, KPNA4, and KPNA6 belonging to a family of nuclear import proteins, which interacted with BCOR in the immunoprecipitation assay. Site-directed mutagenesis targeting the two nuclear localization signals (NLSs) within BCOR reduced its nuclear translocation; however, co-expression of KPNA2, KPNA4, or KPNA6 with BCOR carrying a previously described mutation which eliminated one of the two NLSs significantly increased nuclear accumulation of the mutant BCOR, indicating participation of KPNA in BCOR nuclear translocation. Comparative expression profiling of PDL cells isolated from normal and OFCD patients revealed significant downregulation of SMAD4, GLI1, and nuclear factor 1-C (NFIC) mRNA expression, suggesting that BCOR mutations cause hyperactive root formation in OFCD syndrome by inhibiting SMAD4-Hedgehog-NFIC signaling implicated in dental root development. Our study contributes to understanding of the mechanisms providing nuclear import of BCOR during root formation.

Radiculomegaly of canines in oculofaciocardiodental syndrome

Oculofaciocardiodental (OFCD) syndrome is a rare genetic disease, first reported by Hayward in 1980. This syndrome presents with various ocular, facial, cardiac, and dental symptoms, including congenital cataract, dysmorphic facial features, congenital heart disease, and enlarged roots, respectively. The most important criteria for the diagnosis of OFCD syndrome are dental abnormalities, especially extreme elongation of canine roots. Here, we report detailed analysis of the dentofacial region, as well as ocular, facial, cardiac, and dental findings in a female with OFCD syndrome. To the best of our knowledge, the patient in this case is the first such patient reported in South Korea.

Expanding the phenotype of the X-linked BCOR microphthalmia syndromes.

Two distinct syndromes arise from pathogenic variants in the X-linked gene BCOR (BCL-6 corepressor): oculofaciocardiodental (OFCD) syndrome, which affects females, and a severe microphthalmia ('Lenz'-type) syndrome affecting males. OFCD is an X-linked dominant syndrome caused by a variety of BCOR null mutations. As it manifests only in females, it is presumed to be lethal in males. The severe male X-linked recessive microphthalmia syndrome ('Lenz') usually includes developmental delay in addition to the eye findings and is caused by hypomorphic BCOR variants, mainly by a specific missense variant c.254C > T, p.(Pro85Leu). Here, we detail 16 new cases (11 females with 4 additional, genetically confirmed, affected female relatives; 5 male cases each with unaffected carrier mothers). We describe new variants and broaden the phenotypic description for OFCD to include neuropathy, muscle hypotonia, pituitary underdevelopment, brain atrophy, lipoma and the first description of childhood lymphoma in an OFCD case. Our male X-linked recessive cases show significant new phenotypes: developmental delay (without eye anomalies) in two affected half-brothers with a novel BCOR variant, and one male with high myopia, megalophthalmos, posterior embryotoxon, developmental delay, and heart and bony anomalies with a previously undescribed BCOR splice site variant. Our female OFCD cases and their affected female relatives showed variable features, but consistently had early onset cataracts. We show that a mosaic carrier mother manifested early cataract and dental anomalies. All female carriers of the male X-linked recessive cases for whom genetic confirmation was available showed skewed X-inactivation and were unaffected. In view of the extended phenotype, we suggest a new term of X-linked BCOR-related syndrome.

Radiculomegaly: a case report of this rare dental finding with review of the associated oculo-facio-cardio-dental syndrome

Radiculomegaly, or root gigantism, is a rare dental abnormality with important clinical implications. It is highly specific for oculo-facio-cardio-dental (OFCD) syndrome, which places dentists at the forefront of diagnosis of this syndrome. Only 1 case of nonsyndromic radiculomegaly has been reported in the literature since the description of OFCD syndrome in 1996. We present the second confirmed, nonsyndromic/nonfamilial case and review the literature for dental treatments in patients with this dental finding. A review of the English language literature was performed in PubMed for patients with radiculomegaly or OFCD syndrome. Teeth affected by radiculomegaly, gender, orodental findings, presence of OFCD syndrome, and dental treatment methods were recorded. Sixty-seven cases of radiculomegaly and 92 cases of OFCD syndrome were found in the literature. Only 1 confirmed case of nonsyndromic/nonfamilial radiculomegaly had been reported previously. Ten reports described dental treatment or treatment plan details, and even fewer included specific methods. Because dental anomalies, especially radiculomegaly, are a primary feature of OFCD syndrome, dentists should be aware of the clinical and radiographic features. Radiculomegaly poses a distinct challenge to dentists, and reports of dental therapy provided to these patients are sparse. Early diagnosis of the syndrome may prevent dental challenges and improve prognosis.

Loss of BCOR Protein in Development of Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is an aggressive haematological malignancy with a dismal clinical outcome and represents an unmet medical need. However, therapeutic progress will not be made without a greater understanding of the varied molecular pathogenesis of this heterogeneous disease. Recently, the majority of cases of the largest subgroup of AML, AML with a normal karyotype (AML-NK), were found to harbour recurrent loss-of-function mutations in epigenetic regulators.Germline mutations in the BCL6 Co-Repressor (BCOR) gene, situated at Xq27-28, are associated with two overlapping X-linked dominant disorders, the Oculo-facio-cardio-dental (OFCD) syndrome and syndromic micropthalmia. In addition, somatic mutations in BCOR were described within the last few years in adults and children with AML, particularly in cases arising in the context of aplastic anaemia and in patients with antecedent myelodysplastic syndromes.BCOR encodes a ubiquitously expressed protein which is involved in BCL6-mediated transcriptional repression. In contrast to other BCL6 partners, NCOR and SMRT, it was recently demonstrated to occupy the majority of its DNA binding sites independently from BCL6, reflecting its multiple interaction partners. BCOR also forms a Polycomb repressor complex 1 (PRC1)-like complex with PCGF1, KDM2B, RING1, SKP1, RYBP, and RNF2, and can bind to both class I and II histone deacetylases. Its spectrum of protein-binding partners and studies of mutant forms suggest that BCOR is an epigenetic modulator. Therefore, we predict that loss of BCOR in AML results in loss of its complex repressive function and leads to an activation of an important leukaemogenic transcriptional programme.To test this hypothesis, we generated a mouse model of leukaemia using CRISPR-Cas9 genome editing. Using a common endonuclease assay, we validated 6 different gRNA constructs against Bcor targeting the region of known human mutations described in AML. The three most efficient constructs were subsequently used to create a pool of lentiviral vectors. In order to increase the efficiency of targeting the haematopoietic progenitor cells, we harvested lineage depleted bone marrow cells from mice expressing the Cas9 cDNA from the Rosa locus (Rosa-Cas9). In parallel, we performed the same experiments with Rosa-Cas9 mice harbouring a Flt3 internal tandem duplication (Flt3-ITD), a mutation that can co-occur with BCOR mutations in individual AML patients.First, we transduced bone marrow lineage negative cells with lentiviruses carrying the three BCOR sgRNAs at 60-70% efficiency and performed a serial re-plating assay to investigate the clonogenic potential of targeted cells. We observed a significant increase in the colony numbers in both Cas9 only and Cas9 Flt3-ITD targeted cells in comparison to the empty lentiviral vector controls from the second re-plating onwards. Using a standard endonuclease assay we have confirmed the presence of Cas9-induced mutations within the target Bcor exons, indicative of clonal selection for Bcor targeted cells and an increased self-renewal potential of hematopoietic precursors that lack BCOR. Transduced cells were also injected into lethally irradiated recipient mice in the presence of helper cells to investigate development of leukaemia in vivo. We have been able to confirm the presence of the Bcor mutated cells in the blood of tested mice 12 weeks post injection and these mice continue to be monitored for evidence of leukemia.We are currently conducting integrated genomic analysis with ChIP-Seq, RNA-Seq and methylation analysis to link the BCOR mutations and their altered cellular phenotype to aberrant epigenetic control and pre/leukaemic gene expression programmes. Finally, we are also going to assess co-operativity of Flt3-ITD and BCOR mutations in AML leukaemogenesis. We anticipate that our work will inform the biology of AML-NK associated with BCOR, identify potential therapeutic targets and provide platforms for the assessment of novel therapeutics. DisclosuresNo relevant conflicts of interest to declare.

Oculo-facio-cardio-dental (OFCD) syndrome: The first Italian case of BCOR and co-occurring OTC gene deletion

Oculo-facio-cardio-dental (OFCD) syndrome is a rare genetic disorder affecting ocular, facial, dental and cardiac systems. The syndrome is an X-linked dominant trait and it might be lethal in males. This syndrome is usually caused by mutations in the BCL6 interacting co-repressor gene (BCOR). We described a female child with mild phenotype of oculo-facio-cardio-dental syndrome. Array-comparative genomic hybridization (a-CGH) analysis revealed a de novo heterozygous deletion in the Xp11.4 region of approximately 2.3Mb, involving BCOR and ornithine carbamoyl-transferase (OTC) genes. The deletion observed was subsequently confirmed by real time PCR. In this study we report a first case with co-occurrence of BCOR and OTC genes completely deleted in OFCD syndrome.

Oculofaciocardiodental syndrome: novel BCOR mutations and expression in dental cells

Oculofaciocardiodental (OFCD) syndrome is a rare X-linked dominant condition. Mutations in BCOR have been described as causal in OFCD syndrome. Almost all BCOR mutations result in premature termination codons (PTCs); therefore, nonsense-mediated mRNA decay (NMD) might have an important role in pathogenesis. The purpose of this study was to identify BCOR mutations in two OFCD patients, if it present, and to clarify the pathogenesis of radiculomegaly using one OFCD patient's pulp and periodontal ligament (PDL) cells. In our genetic analysis, two novel BCOR mutations were found. We also examined the transcript levels and the effects of NMD using cultured pulp and PDL cells from one affected patient. BCOR expression was normal in pulp but reduced in PDL cells, which is consistent with the higher rates of NMD in PDL cells. The mutant PDL cells had unstable mutant transcripts and proliferated faster than did wild-type cells, but mutant pulp cells appeared normal by these measures. In summary, the nonsense and frameshift mutations, which introduce PTCs, were found to contribute to OFCD syndrome in our two patients. Furthermore, in PDL cells, the mutation resulting in a PTC corresponded to greater NMD, unstable mutant transcripts and increased cell proliferation, which may contribute to hyperactive root formation.

A potential molecular pathogenesis of cardiac/laterality defects in Oculo-Facio-Cardio-Dental syndrome

Pitx2 is the last effector of the left–right (LR) cascade known to date and plays a crucial role in the patterning of LR asymmetry. In Xenopus embryos, the expression of Pitx2 gene in the left lateral plate mesoderm (LPM) is directly regulated by Xnr1 signaling, which is mediated by Smads and FoxH1. Previous studies suggest that the suppression of Pitx2 gene in the left LPM is a potential cause of cardiac/laterality defects in Oculo-Facio-Cardio-Dental (OFCD) syndrome, which is known to be caused by mutations in BCL6 co-repressor (BCOR) gene. Recently, our work has revealed that the BCL6/BCOR complex blocks Notch-dependent transcriptional activity to protect the expression of Pitx2 in the left LPM from the inhibitory activity of Notch signaling. These studies indicated that uncontrolled Notch activity in the left LPM caused by dysfunction of BCOR may result in cardiac/laterality defects of OFCD syndrome. However, this Notch-dependent inhibitory mechanism of Pitx2 gene transcription still remains unknown. Here we report that transcriptional repressor ESR1, which acts downstream of Notch signaling, inhibits the expression of Pitx2 gene by binding to a left side-specific enhancer (ASE) region in Pitx2 gene and recruiting histone deacetylase 1 (HDAC1) to this region. Once HDAC1 is tethered, histone acetyltransferase p300 is no longer recruited to the Xnr1-dependent transcriptional complex on the ASE region, leading to the suppression of Pitx2 gene in the left LPM. The study presented here uncovers the regulatory mechanism of Pitx2 gene transcription which may contribute to an understanding of pathogenesis of OFCD syndrome.

Bcl6a function is required during optic cup formation to prevent p53-dependent apoptosis and colobomata

Mutations in BCOR (Bcl6 corepressor) are found in patients with oculo-facio-cardio-dental (OFCD) syndrome, a congenital disorder affecting visual system development, and loss-of-function studies in zebrafish and Xenopus demonstrate a role for Bcor during normal optic cup development in preventing colobomata. The mechanism whereby BCOR functions during eye development to prevent colobomata is not known, but in other contexts it serves as a transcriptional corepressor that potentiates transcriptional repression by B cell leukemia/lymphoma 6 (BCL6). Here, we have explored the function of the zebrafish ortholog of Bcl6, Bcl6a, during eye development, and our results demonstrate that Bcl6a, like Bcor, is required to prevent colobomata during optic cup formation. Our data demonstrate that Bcl6a acts downstream of Vax1 and Vax2, known regulators of ventral optic cup formation and choroid fissure closure, and that bcl6a is a direct target of Vax2. Together, this regulatory network functions to repress p53 expression and thereby suppress apoptosis in the developing optic cup. Furthermore, our data demonstrate that Bcl6a functions cooperatively with Bcor, Rnf2 and Hdac1 in a common gene regulatory network that acts to repress p53 and prevent colobomata. Together, these data support a model in which p53-dependent apoptosis needs to be tightly regulated for normal optic cup formation and that Bcl6a, Bcor, Rnf2 and Hdac1 activities mediate this regulation.