Missense substitutions at a conserved 14-3-3 binding site in HDAC4 cause a novel intellectual disability syndrome

Characterization of a novel HDAC2 pathogenetic variant: a missing puzzle piece for chromatinopathies

Histone deacetylases (HDACs) are enzymes pivotal for histone modification (i.e. acetylation marks removal), chromatin accessibility and gene expression regulation. Class I HDACs (including HDAC1, 2, 3, 8) are ubiquitously expressed and they often participate in multi-molecular protein complexes. To date, three neurodevelopmental disorders caused by mutations in genes encoding for HDACs (HDAC4, HDAC6 and HDAC8) and thus belonging to the group of chromatinopathies, have been described. We performed whole exome sequencing (WES) for a patient (#249) clinically diagnosed with the chromatinopathy Rubinstein-Taybi syndrome (RSTS) but negative for mutations in RSTS genes, identifying a de novo frameshift variant in HDAC2 gene. We then investigated its molecular effects in lymphoblastoid cell lines (LCLs) derived from the patient compared to LCLs from healthy donors (HD). As the variant was predicted to be likely pathogenetic and to affect the sequence of nuclear localization signal, we performed immunocytochemistry and lysates fractionation, observing a nuclear mis-localization of HDAC2 compared to HD LCLs. In addition, HDAC2 total protein abundance resulted altered in patient, and we found that newly identified variant in HDAC2 affects also acetylation levels, with significant difference in acetylation pattern among patient #249, HD and RSTS cells and in expression of a known molecular target. Remarkably, RNA-seq performed on #249, HD and RSTS cells shows differentially expressed genes (DEGs) common to #249 and RSTS. Interestingly, our reported patient was clinically diagnosed with RSTS, a chromatinopathy which known causative genes encode for enzymes antagonizing HDACs. These results support the role of HDAC2 as causative gene for chromatinopathies, strengthening the genotype-phenotype correlations in this relevant group of disorders.

Deciphering the roles of subcellular distribution and interactions involving the MEF2 binding region, the ankyrin repeat binding motif and the catalytic site of HDAC4 in Drosophila neuronal morphogenesis

BACKGROUND

Dysregulation of nucleocytoplasmic shuttling of histone deacetylase 4 (HDAC4) is associated with several neurodevelopmental and neurodegenerative disorders. Consequently, understanding the roles of nuclear and cytoplasmic HDAC4 along with the mechanisms that regulate nuclear entry and exit is an area of concerted effort. Efficient nuclear entry is dependent on binding of the transcription factor MEF2, as mutations in the MEF2 binding region result in cytoplasmic accumulation of HDAC4. It is well established that nuclear exit and cytoplasmic retention are dependent on 14-3-3-binding, and mutations that affect binding are widely used to induce nuclear accumulation of HDAC4. While regulation of HDAC4 shuttling is clearly important, there is a gap in understanding of how the nuclear and cytoplasmic distribution of HDAC4 impacts its function. Furthermore, it is unclear whether other features of the protein including the catalytic site, the MEF2-binding region and/or the ankyrin repeat binding motif influence the distribution and/or activity of HDAC4 in neurons. Since HDAC4 functions are conserved in Drosophila, and increased nuclear accumulation of HDAC4 also results in impaired neurodevelopment, we used Drosophila as a genetic model for investigation of HDAC4 function.

RESULTS

Here we have generated a series of mutants for functional dissection of HDAC4 via in-depth examination of the resulting subcellular distribution and nuclear aggregation, and correlate these with developmental phenotypes resulting from their expression in well-established models of neuronal morphogenesis of the Drosophila mushroom body and eye. We found that in the mushroom body, forced sequestration of HDAC4 in the nucleus or the cytoplasm resulted in defects in axon morphogenesis. The actions of HDAC4 that resulted in impaired development were dependent on the MEF2 binding region, modulated by the ankyrin repeat binding motif, and largely independent of an intact catalytic site. In contrast, disruption to eye development was largely independent of MEF2 binding but mutation of the catalytic site significantly reduced the phenotype, indicating that HDAC4 acts in a neuronal-subtype-specific manner.

CONCLUSIONS

We found that the impairments to mushroom body and eye development resulting from nuclear accumulation of HDAC4 were exacerbated by mutation of the ankyrin repeat binding motif, whereas there was a differing requirement for the MEF2 binding site and an intact catalytic site. It will be of importance to determine the binding partners of HDAC4 in nuclear aggregates and in the cytoplasm of these tissues to further understand its mechanisms of action.

Genotype-Phenotype Correlations in 2q37-Deletion Syndrome: An Update of the Clinical Spectrum and Literature Review

2q37 microdeletion/deletion syndrome (2q37DS) is one of the most common subtelomeric deletion disorders, caused by a 2q37 deletion of variable size. The syndrome is characterized by a broad and diverse spectrum of clinical findings: characteristic facial dysmorphism, developmental delay/intellectual disability (ID), brachydactyly type E, short stature, obesity, hypotonia in infancy, and abnormal behavior with autism spectrum disorder. Although numerous cases have been described so far, the exact mapping of the genotype and phenotype have not yet been achieved.

MATERIALS AND METHODS

In this study we analyzed nine newly diagnosed cases with 2q37 deletion (3 male/6 female, aged between 2 and 30 years old), and followed up at the Iasi Regional Medical Genetics Centre. All patients were tested first with MLPA using combined kits P036/P070 subtelomeric screening mix and follow-up mix P264; after, the deletion size and location were confirmed via CGH-array. We compared our findings with the data of other cases reported in the literature.

RESULTS

From nine cases, four had pure 2q37 deletions of variable sizes, and five presented deletion/duplication rearrangements (with chromosomes 2q, 9q, and 11p). In most cases, characteristic phenotypic aspects were observed: 9/9 facial dysmorphism, 8/9 global developmental delay and ID, 6/9 hypotonia, 5/9 behavior disorders, and 8/9 skeletal anomalies-especially brachydactyly type E. Two cases had obesity, one case had craniosynostosis, and four had heart defects. Other features found in our cases included translucent skin and telangiectasias (6/9), and a hump of fat on the upper thorax (5/9).

CONCLUSIONS

Our study enriches the literature data by describing new clinical features associated with 2q37 deletion, and possible genotype-phenotype correlations.

HDAC4 in cancer: A multitasking platform to drive not only epigenetic modifications

Controlling access to genomic information and maintaining its stability are key aspects of cell life. Histone acetylation is a reversible epigenetic modification that allows access to DNA and the assembly of protein complexes that regulate mainly transcription but also other activities. Enzymes known as histone deacetylases (HDACs) are involved in the removal of the acetyl-group or in some cases of small hydrophobic moieties from histones but also from the non-histone substrate. The main achievement of HDACs on histones is to repress transcription and promote the formation of more compact chromatin. There are 18 different HDACs encoded in the human genome. Here we will discuss HDAC4, a member of the class IIa family, and its possible contribution to cancer development.

AutoCaSc: Prioritizing candidate genes for neurodevelopmental disorders

Routine exome sequencing (ES) in individuals with neurodevelopmental disorders (NDD) remains inconclusive in >50% of the cases. Research analysis of unsolved cases can identify novel candidate genes but is time-consuming, subjective, and hard to compare between labs. The field, therefore, requires automated and standardized assessment methods to prioritize candidates for matchmaking. We developed AutoCaSc (https://autocasc.uni-leipzig.de) based on our candidate scoring scheme. We validated our approach using synthetic trios and real in-house trio ES data. AutoCaSc consistently (94.5% of all cases) scored the relevant variants in valid novel NDD genes in the top three ranks. In 93 real trio exomes, AutoCaSc identified most (97.5%) previously manually scored variants while evaluating additional high-scoring variants missed in manual evaluation. It identified candidate variants in previously undescribed NDD candidate genes (CNTN2, DLGAP1, SMURF1, NRXN3, and PRICKLE1). AutoCaSc enables anybody to quickly screen a variant for its plausibility in NDD. After contributing >40 descriptions of NDD-associated genes, we provide usage recommendations based on our extensive experience. Our implementation is capable of pipeline integration and therefore allows the screening of large cohorts for candidate genes. AutoCaSc empowers even small labs to a standardized matchmaking collaboration and to contribute to the ongoing identification of novel NDD entities.