Mutations of the histone linker H1-4 in neurodevelopmental disorders and functional characterization of neurons expressing C-terminus frameshift mutant H1.4.

Rahman syndrome (RMNS) is a rare genetic disorder characterized by mild to severe intellectual disability, hypotonia, anxiety, autism spectrum disorder, vision problems, bone abnormalities and dysmorphic facies. RMNS is caused by de novo heterozygous mutations in the histone linker gene H1-4; however, mechanisms underlying impaired neurodevelopment in RMNS are not understood. All reported mutations associated with RMNS in H1-4 are small insertions or deletions that create a shared frameshift, resulting in a H1.4 protein that is both truncated and possessing an abnormal C-terminus frameshifted tail (H1.4 CFT). To expand understanding of mutations and phenotypes associated with mutant H1-4, we identified new variants at both the C- and N-terminus of H1.4. The clinical features of mutations identified at the C-terminus are consistent with other reports and strengthen the support of pathogenicity of H1.4 CFT. To understand how H1.4 CFT may disrupt brain function, we exogenously expressed wild-type or H1.4 CFT protein in rat hippocampal neurons and assessed neuronal structure and function. Genome-wide transcriptome analysis revealed ~ 400 genes altered in the presence of H1.4 CFT. Neuronal genes downregulated by H1.4 CFT were enriched for functional categories involved in synaptic communication and neuropeptide signaling. Neurons expressing H1.4 CFT also showed reduced neuronal activity on multielectrode arrays. These data are the first to characterize the transcriptional and functional consequence of H1.4 CFT in neurons. Our data provide insight into causes of neurodevelopmental impairments associated with frameshift mutations in the C-terminus of H1.4 and highlight the need for future studies on the function of histone H1.4 in neurons.

Enterovirus 2Apro Cleavage of the YTHDF m6A Readers Implicates YTHDF3 as a Mediator of Type I Interferon-Driven JAK/STAT Signaling.

Enteroviruses (EV) deploy two proteases that mediate viral polyprotein cleavage and host cell manipulation. Here, we report that EV 2A proteases cleave all three members of the YTHDF protein family, cytosolic N6-methyladenosine (m6A) "readers" that regulate target mRNA fate. YTHDF protein cleavage occurs very early during infection, before viral translation is detected or cytopathogenic effects are observed. Preemptive YTHDF protein depletion enhanced viral translation and replication but only in cells with restrained viral translation, signs of inefficient 2A protease activity, and protective innate host immune responses. This effect corresponded with repression of interferon (IFN)-stimulated gene (ISG) induction, while type I/III IFN production was not significantly altered. Moreover, YTHDF3 depletion impaired JAK/STAT signaling in cells treated with type I, but not type II, IFN. YTHDF3 depletion's stimulatory effect on viral dynamics was dampened by JAK/STAT blockade and enhanced by type I IFN pretreatment of cells. We propose that EV 2A proteases cleave YTHDF proteins to antagonize ISG induction in infected cells.IMPORTANCE It is believed that ∼7,000 messenger RNAs (mRNAs) are subject to N6-methyladenosine modification. The biological significance of this remains mysterious. The YTHDF m6A readers are three related proteins with high affinity for m6A-modified mRNA, yet their biological functions remain obscure. We discovered that polio/enteroviruses elicit very early proteolysis of YTHDF1 to 3 in infected cells. Our research demonstrates that YTHDF3 acts as a positive regulator of antiviral JAK/STAT signaling in response to positive single-strand RNA virus infection, enabling type I interferon (IFN)-mediated gene regulatory programs to unfurl in infected cells. Our observation of viral degradation of the YTHDF proteins demonstrates that they are key response modifiers in the innate antiviral immune response.

DNA Methylation and Susceptibility to Autism Spectrum Disorder.

The prevalence of autism spectrum disorder (ASD) has been increasing steadily over the last 20 years; however, the molecular basis for the majority of ASD cases remains unknown. Recent advances in next-generation sequencing and detection of DNA modifications have made methylation-dependent regulation of transcription an attractive hypothesis for being a causative factor in ASD etiology. Evidence for abnormal DNA methylation in ASD can be seen on multiple levels, from genetic mutations in epigenetic machinery to loci-specific and genome-wide changes in DNA methylation. Epimutations in DNA methylation can be acquired throughout life, as global DNA methylation reprogramming is dynamic during embryonic development and the early postnatal period that corresponds to the peak time of synaptogenesis. However, technical advances and causative evidence still need to be established before abnormal DNA methylation and ASD can be confidently associated.

Epigenetic dysregulation of Oxtr in Tet1-deficient mice has implications for neuropsychiatric disorders.

OXTR modulates a variety of behaviors in mammals, including social memory and recognition. Genetic and epigenetic dysregulation of OXTR has been suggested to be implicated in neuropsychiatric disorders, including autism spectrum disorder (ASD). While the involvement of DNA methylation is suggested, the mechanism underlying epigenetic regulation of OXTR is largely unknown. This has hampered the experimental design and interpretation of the results of epigenetic studies of OXTR in neuropsychiatric disorders. From the generation and characterization of a new line of Tet1 mutant mice - by deleting the largest coding exon 4 (Tet1Δe4) - we discovered for the first time to our knowledge that Oxtr has an array of mRNA isoforms and a complex transcriptional regulation. Select isoforms of Oxtr are significantly reduced in the brain of Tet1Δe4-/- mice. Accordingly, CpG islands of Oxtr are hypermethylated during early development and persist into adulthood. Consistent with the reduced express of OXTR, Tet1Δe4-/- mice display impaired maternal care, social behavior, and synaptic responses to oxytocin stimulation. Our findings elucidate a mechanism mediated by TET1 protein in regulating Oxtr expression by preventing DNA hypermethylation of Oxtr. The discovery of epigenetic dysregulation of Oxtr in TET1-deficient mouse brain supports the necessity of a reassessment of existing findings and a value of future studies of OXTR in neuropsychiatric disorders.

Epigenetics and autism spectrum disorder: A report of an autism case with mutation in H1 linker histone HIST1H1E and literature review.

Genetic mutations in genes encoding proteins involved in epigenetic machinery have been reported in individuals with autism spectrum disorder (ASD), intellectual disability, congenital heart disease, and other disorders. H1 histone linker protein, the basic component in nucleosome packaging and chromatin organization, has not been implicated in human disease until recently. We report a de novo deleterious mutation of histone cluster 1 H1 family member e (HIST1H1E; c.435dupC; p.Thr146Hisfs*50), encoding H1 histone linker protein H1.4, in a 10-year-old boy with autism and intellectual disability diagnosed through clinical whole exome sequencing. The c.435dupC at the 3' end of the mRNA leads to a frameshift and truncation of the positive charge in the carboxy-terminus of the protein. An expression study demonstrates the mutation leads to reduced protein expression, supporting haploinsufficiency of HIST1H1E protein and loss of function as an underlying mechanism of dysfunction in the brain. Taken together with other recent cases with mutations of HIST1H1E in intellectual disability, the evidence supporting the link to causality in disease is strong. Our finding implicates the deficiency of H1 linker histone protein in autism. The systematic review of candidate genes implicated in ASD revealed that 42 of 215 (19.5%) genes are directly involved in epigenetic regulations and the majority of these genes belong to histone writers, readers, and erasers. While the mechanism of how haploinsufficiency of HIST1H1E causes autism is entirely unknown, our report underscores the importance of further study of the function of this protein and other histone linker proteins in brain development.