Mutation of the CH1 Domain in the Histone Acetyltransferase CREBBP Results in Autism-Relevant Behaviors in Mice.

Autism spectrum disorders (ASDs) are a group of neurodevelopmental afflictions characterized by repetitive behaviors, deficits in social interaction, and impaired communication skills. For most ASD patients, the underlying causes are unknown. Genetic mutations have been identified in about 25 percent of ASD cases, including mutations in epigenetic regulators, suggesting that dysregulated chromatin or DNA function is a critical component of ASD. Mutations in the histone acetyltransferase CREB binding protein (CBP, CREBBP) cause Rubinstein-Taybi Syndrome (RTS), a developmental disorder that includes ASD-like symptoms. Recently, genomic studies involving large numbers of ASD patient families have theoretically modeled CBP and its paralog p300 (EP300) as critical hubs in ASD-associated protein and gene interaction networks, and have identified de novo missense mutations in highly conserved residues of the CBP acetyltransferase and CH1 domains. Here we provide animal model evidence that supports this notion that CBP and its CH1 domain are relevant to autism. We show that mice with a deletion mutation in the CBP CH1 (TAZ1) domain (CBPΔCH1/ΔCH1) have an RTS-like phenotype that includes ASD-relevant repetitive behaviors, hyperactivity, social interaction deficits, motor dysfunction, impaired recognition memory, and abnormal synaptic plasticity. Our results therefore indicate that loss of CBP CH1 domain function contributes to RTS, and possibly ASD, and that this domain plays an essential role in normal motor function, cognition and social behavior. Although the key physiological functions affected by ASD-associated mutation of epigenetic regulators have been enigmatic, our findings are consistent with theoretical models involving CBP and p300 in ASD, and with a causative role for recently described ASD-associated CBP mutations.

Epigenetic modifications in the nervous system and their impact upon cognitive impairments.

Epigenetic regulation has been long considered to be a critical mechanism in the control of key aspects of cellular functions such as cell division, growth, and cell fate determination. Exciting recent developments have demonstrated that epigenetic mechanisms can also play necessary roles in the nervous system by regulating, for example, neuronal gene expression, DNA damage, and genome stability. Despite the fact that postmitotic neurons are developmentally less active then dividing cells, epigenetic regulation appears to provide means of both long-lasting and very dynamic regulation of neuronal function. Growing evidence indicates that epigenetic mechanisms in the central nervous system (CNS) are important for regulating not only specific aspects of individual neuronal metabolism but also for maintaining function of neuronal circuits and regulating their behavioral outputs. Multiple reports demonstrated that higher-level cognitive behaviors, such as learning and memory, are subject to a sophisticated epigenetic control, which includes interplay between multiple mechanisms of neuronal chromatin modification. Experiments with animal models have demonstrated that various epigenetic manipulations can affect cognition in different ways, from severe dysfunction to substantial improvement. In humans, epigenetic dysregulation has been known to underlie a number of disorders that are accompanied by mental impairment. Here, we review some of the epigenetic mechanisms that regulate cognition and how their disruption may contribute to cognitive dysfunctions. Due to the fact that histone acetylation and DNA methylation are some of the best-studied and critically important epigenomic modifications our research team has particularly strong expertise in, in this review, we are going to concentrate on histone acetylation, as well as DNA methylation/hydroxymethylation, in the mammalian CNS. Additional epigenetic modifications, not surveyed here, are being discussed in depth in the other review articles in this issue of Neuropharmacology.

Mutations in KAT6B, encoding a histone acetyltransferase, cause Genitopatellar syndrome.

Genitopatellar syndrome (GPS) is a skeletal dysplasia with cerebral and genital anomalies for which the molecular basis has not yet been determined. By exome sequencing, we found de novo heterozygous truncating mutations in KAT6B (lysine acetyltransferase 6B, formerly known as MYST4 and MORF) in three subjects; then by Sanger sequencing of KAT6B, we found similar mutations in three additional subjects. The mutant transcripts do not undergo nonsense-mediated decay in cells from subjects with GPS. In addition, human pathological analyses and mouse expression studies point to systemic roles of KAT6B in controlling organismal growth and development. Myst4 (the mouse orthologous gene) is expressed in mouse tissues corresponding to those affected by GPS. Phenotypic differences and similarities between GPS, the Say-Barber-Biesecker variant of Ohdo syndrome (caused by different mutations of KAT6B), and Rubinstein-Taybi syndrome (caused by mutations in other histone acetyltransferases) are discussed. Together, the data support an epigenetic dysregulation of the limb, brain, and genital developmental programs.

CBP histone acetyltransferase activity regulates embryonic neural differentiation in the normal and Rubinstein-Taybi syndrome brain.

Increasing evidence indicates that epigenetic changes regulate cell genesis. Here, we ask about neural precursors, focusing on CREB binding protein (CBP), a histone acetyltransferase that, when haploinsufficient, causes Rubinstein-Taybi syndrome (RTS), a genetic disorder with cognitive dysfunction. We show that neonatal cbp(+/-) mice are behaviorally impaired, displaying perturbed vocalization behavior. cbp haploinsufficiency or genetic knockdown with siRNAs inhibited differentiation of embryonic cortical precursors into all three neural lineages, coincident with decreased CBP binding and histone acetylation at promoters of neuronal and glial genes. Inhibition of histone deacetylation rescued these deficits. Moreover, CBP phosphorylation by atypical protein kinase C zeta was necessary for histone acetylation at neural gene promoters and appropriate differentiation. These data support a model in which environmental cues regulate CBP activity and histone acetylation to control neural precursor competency to differentiate, and indicate that cbp haploinsufficiency disrupts this mechanism, thereby likely causing cognitive dysfunction in RTS.

Aberrant forms of histone acetyltransferases in human disease.

One of the major mechanisms through which eukaryotic cells respond to developmental and environmental signals is by changing their gene expression patterns. This complex and tightly regulated process is largely regulated at the level of RNA polymerase II-mediated transcription. Within this process an important class of transcriptional regulators are the histone acetyltransferases (HATs), proteins that acetylate histones and non-histone substrates. While hyperacetylation of histones is generally associated with active genes, the effect of acetylation of nonhistone proteins varies between substrates resulting in for example alterations in (sub-nuclear) protein localization or protein stability. Given the central role of HATs in transcriptional regulation and other cellular processes, it may not be surprising that genetic alterations in the genes encoding HATs, resulting in aberrant forms of these regulatory proteins, have been linked with various human diseases, including congenital developmental disorders and various forms of cancer, including leukaemia. Here we will review mutations found in genes encoding human HATs and discuss the (putative) functional consequences on the function of these proteins. So far the lessons learned from naturally occurring mutations in humans have proven to be invaluable and recapitulating such genetic alterations in various experimental systems will extend our knowledge even further. This seems particularly relevant given the wide range of diseases in which acetyltransferases have been implicated and may help to open up new therapeutic avenues.

Defect of histone acetyltransferase activity of the nuclear transcriptional coactivator CBP in Rubinstein-Taybi syndrome.

CREB-binding protein (CBP) is a transcriptional coactivator that has intrinsic histone acetyltransferase (HAT) activity. CBP is the causative gene of Rubinstein-Taybi syndrome (RTS). To investigate the relationships between CBP HAT activity and RTS, we analyzed 16 RTS patients. A microdeletion was identified in one patient by fluorescent in situ hybridization analysis. Heteroallelic mutations were identified in five patients by reverse transcriptase-polymerase chain reaction-single-strand conformation polymorphism analysis and sequencing. These included a 2 bp deletion between nucleotides 4319 and 4320, an 11 bp deletion between nucleotides 4898 and 4908, a 14 bp insertion (CCTCGGTCCTGCAC) between nucleotides 5212 and 5213, a 2 bp deletion between nucleotides 5222 and 5223, and a missense mutation from guanine (G) to cytosine (C) at nucleotide 4951 that changed codon 1378 from CGG (arginine) to CCG (proline). The identical missense mutation was introduced into the recombinant mouse CBP. It abolished the HAT activity of CBP and the ability of CBP to transactivate cyclic AMP-response element binding protein (CREB), in HAT assays and in microinjection experiments, respectively. These results suggest that the loss of the HAT activity of CBP may cause RTS, as the first example of a defect of HAT activity in a human disease. Our findings raise the possibility that treatment of RTS patients with histone deacetylase inhibitors might have beneficial effects.