Developmental roles of histone H3 variants and their chaperones.

Animal development and lifetime potential exploit a balance between the stability and plasticity of cellular identity. Within the nucleus, this is controlled by an interplay involving lineage-specific transcription factors and chromatin dynamics. Histone H3 variants contribute to chromatin dynamics through the timing and sites of their incorporation, promoted by dedicated histone chaperones. Moreover, their individual modifications and binding partners provide distinct features at defined genomic loci. We highlight here the importance of the H3.3 replacement variant for the nuclear reprogramming that occurs during gametogenesis, fertilization, and germline establishment. Furthermore, we describe how the recently characterized H3.3 dynamics associated with gastrulation, myogenesis, or neurogenesis underline the role of chromatin changes in cell differentiation. Finally, we discuss the challenges of maintaining centromeric identity through propagation of the centromeric CenH3 variant in different cell types. Future challenges will be to gain a comprehensive picture of H3 variants and their chaperones during development and differentiation.

HIRA dependent H3.3 deposition is required for transcriptional reprogramming following nuclear transfer to Xenopus oocytes.

BACKGROUND: Nuclear reprogramming is potentially important as a route to cell replacement and drug discovery, but little is known about its mechanism. Nuclear transfer to eggs and oocytes attempts to identify the mechanism of this direct route towards reprogramming by natural components. Here we analyze how the reprogramming of nuclei transplanted to Xenopus oocytes exploits the incorporation of the histone variant H3.3. RESULTS: After nuclear transplantation, oocyte-derived H3.3 but not H3.2, is deposited on several regions of the genome including rDNA, major satellite repeats, and the regulatory regions of Oct4. This major H3.3 deposition occurs in absence of DNA replication, and is HIRA-and transcription-dependent. It is necessary for the shift from a somatic- to an oocyte-type of transcription after nuclear transfer. CONCLUSIONS: This study demonstrates that the incorporation of histone H3.3 is an early and necessary step in the direct reprogramming of somatic cell nuclei by oocyte. It suggests that the incorporation of histone H3.3 is necessary during global changes in transcription that accompany changes in cell fate.

A developmental requirement for HIRA-dependent H3.3 deposition revealed at gastrulation in Xenopus.

Discovering how histone variants that mark distinct chromatin regions affect a developmental program is a major challenge in the epigenetics field. To assess the importance of the H3.3 histone variant and its dedicated histone chaperone HIRA, we used an established developmental model, Xenopus laevis. After the early rapid divisions exploiting a large maternal pool of both replicative H3.2 and replacement H3.3, H3.3 transcripts show a distinct peak of expression at gastrulation. Depletion of both H3.2 and H3.3 leads to an early gastrulation arrest. However, with only H3.3 depletion, defects occur at late gastrulation, impairing further development. Providing exogenous H3.3 mRNAs, but not replicative H3.2 mRNAs, rescues these defects. Notably, downregulation of the H3.3 histone chaperone HIRA similarly impairs late gastrulation, and we find a global defect in H3.3 incorporation into chromatin comparable to H3.3 depletion. We discuss how specific HIRA-dependent H3.3 deposition is required for chromatin dynamics during gastrulation.

The double face of the histone variant H3.3.

Histone proteins wrap DNA to form nucleosome particles that compact eukaryotic genomes while still allowing access for cellular processes such as transcription, replication and DNA repair. Histones exist as different variants that have evolved crucial roles in specialized functions in addition to their fundamental role in packaging DNA. H3.3--a conserved histone variant that is structurally very close to the canonical histone H3--has been associated with active transcription. Furthermore, its role in histone replacement at active genes and promoters is highly conserved and has been proposed to participate in the epigenetic transmission of active chromatin states. Unexpectedly, recent data have revealed accumulation of this specific variant at silent loci in pericentric heterochromatin and telomeres, raising questions concerning the actual function of H3.3. In this review, we describe the known properties of H3.3 and the current view concerning its incorporation modes involving particular histone chaperones. Finally, we discuss the functional significance of the use of this H3 variant, in particular during germline formation and early development in different species.

CC2D2A mutations in Meckel and Joubert syndromes indicate a genotype-phenotype correlation.

Meckel-Gruber syndrome (MKS) is a lethal fetal disorder characterized by diffuse renal cystic dysplasia, polydactyly, a brain malformation that is usually occipital encephalocele, and/or vermian agenesis, with intrahepatic biliary duct proliferation. Joubert syndrome (JBS) is a viable neurological disorder with a characteristic "molar tooth sign" (MTS) on axial images reflecting cerebellar vermian hypoplasia/dysplasia. Both conditions are classified as ciliopathies with an autosomal recessive mode of inheritance. Allelism of MKS and JBS has been reported for TMEM67/MKS3, CEP290/MKS4, and RPGRIP1L/MKS5. Recently, one homozygous splice mutation with a founder effect was reported in the CC2D2A gene in Finnish fetuses with MKS, defining the 6th locus for MKS. Shortly thereafter, CC2D2A mutations were also reported in JBS. The analysis of the CC2D2A gene in our series of MKS fetuses, identified 14 novel truncating mutations in 11 cases. These results confirm the involvement of CC2D2A in MKS and reveal a major contribution of CC2D2A to the disease. We also identified three missense CC2D2A mutations in two JBS cases. Therefore, and in accordance with the data reported regarding RPGRIP1L, our results indicate phenotype-genotype correlations, as missense and presumably hypomorphic mutations lead to JBS while all null alleles lead to MKS.