Etiological involvement of KCND1 variants in an X-linked neurodevelopmental disorder with variable expressivity.

Utilizing trio whole-exome sequencing and a gene matching approach, we identified a cohort of 18 male individuals from 17 families with hemizygous variants in KCND1, including two de novo missense variants, three maternally inherited protein-truncating variants, and 12 maternally inherited missense variants. Affected subjects present with a neurodevelopmental disorder characterized by diverse neurological abnormalities, mostly delays in different developmental domains, but also distinct neuropsychiatric signs and epilepsy. Heterozygous carrier mothers are clinically unaffected. KCND1 encodes the α-subunit of Kv4.1 voltage-gated potassium channels. All variant-associated amino acid substitutions affect either the cytoplasmic N- or C-terminus of the channel protein except for two occurring in transmembrane segments 1 and 4. Kv4.1 channels were functionally characterized in the absence and presence of auxiliary ß subunits. Variant-specific alterations of biophysical channel properties were diverse and varied in magnitude. Genetic data analysis in combination with our functional assessment shows that Kv4.1 channel dysfunction is involved in the pathogenesis of an X-linked neurodevelopmental disorder frequently associated with a variable neuropsychiatric clinical phenotype.

Heterozygous variants in CTR9, which encodes a major component of the PAF1 complex, are associated with a neurodevelopmental disorder.

PURPOSE: CTR9 is a subunit of the PAF1 complex (PAF1C) that plays a crucial role in transcription regulation by binding CTR9 to RNA polymerase II. It is involved in transcription-coupled histone modification through promoting H3K4 and H3K36 methylation. We describe the clinical and molecular studies in 13 probands, harboring likely pathogenic CTR9 missense variants, collected through GeneMatcher. METHODS: Exome sequencing was performed in all individuals. CTR9 variants were assessed through 3-dimensional modeling of the activated human transcription complex Pol II-DSIF-PAF-SPT6 and the PAF1/CTR9 complex. H3K4/H3K36 methylation analysis, mitophagy assessment based on tetramethylrhodamine ethyl ester perchlorate immunofluorescence, and RNA-sequencing in skin fibroblasts from 4 patients was performed. RESULTS: Common clinical findings were variable degrees of intellectual disability, hypotonia, joint hyperlaxity, speech delay, coordination problems, tremor, and autism spectrum disorder. Mild dysmorphism and cardiac anomalies were less frequent. For 11 CTR9 variants, de novo occurrence was shown. Three-dimensional modeling predicted a likely disruptive effect of the variants on local CTR9 structure and protein interaction. Additional studies in fibroblasts did not unveil the downstream functional consequences of the identified variants. CONCLUSION: We describe a neurodevelopmental disorder caused by (mainly) de novo variants in CTR9, likely affecting PAF1C function.

Loss-of-function variants in SRRM2 cause a neurodevelopmental disorder.

PURPOSE: SRRM2 encodes the SRm300 protein, a splicing factor of the SR-related protein family characterized by its serine- and arginine-enriched domains. It promotes interactions between messenger RNA and the spliceosome catalytic machinery. This gene, predicted to be highly intolerant to loss of function (LoF) and very conserved through evolution, has not been previously reported in constitutive human disease. METHODS: Among the 1000 probands studied with developmental delay and intellectual disability in our database, we found 2 patients with de novo LoF variants in SRRM2. Additional families were identified through GeneMatcher. RESULTS: Here, we report on 22 patients with LoF variants in SRRM2 and provide a description of the phenotype. Molecular analysis identified 12 frameshift variants, 8 nonsense variants, and 2 microdeletions of 66 kb and 270 kb. The patients presented with a mild developmental delay, predominant speech delay, autistic or attention-deficit/hyperactivity disorder features, overfriendliness, generalized hypotonia, overweight, and dysmorphic facial features. Intellectual disability was variable and mild when present. CONCLUSION: We established SRRM2 as a gene responsible for a rare neurodevelopmental disease.

Postmitotic Hoxa5 Expression Specifies Pontine Neuron Positional Identity and Input Connectivity of Cortical Afferent Subsets.

The mammalian precerebellar pontine nucleus (PN) has a main role in relaying cortical information to the cerebellum. The molecular determinants establishing ordered connectivity patterns between cortical afferents and precerebellar neurons are largely unknown. We show that expression of Hox5 transcription factors is induced in specific subsets of postmitotic PN neurons at migration onset. Hox5 induction is achieved by response to retinoic acid signaling, resulting in Jmjd3-dependent derepression of Polycomb chromatin and 3D conformational changes. Hoxa5 drives neurons to settle posteriorly in the PN, where they are monosynaptically targeted by cortical neuron subsets mainly carrying limb somatosensation. Furthermore, Hoxa5 postmigratory ectopic expression in PN neurons is sufficient to attract cortical somatosensory inputs regardless of position and avoid visual afferents. Transcriptome analysis further suggests that Hoxa5 is involved in circuit formation. Thus, Hoxa5 coordinates postmitotic specification, migration, settling position, and sub-circuit assembly of PN neuron subsets in the cortico-cerebellar pathway.

Small chromosomal regions position themselves autonomously according to their chromatin class.

The spatial arrangement of chromatin is linked to the regulation of nuclear processes. One striking aspect of nuclear organization is the spatial segregation of heterochromatic and euchromatic domains. The mechanisms of this chromatin segregation are still poorly understood. In this work, we investigated the link between the primary genomic sequence and chromatin domains. We analyzed the spatial intranuclear arrangement of a human artificial chromosome (HAC) in a xenospecific mouse background in comparison to an orthologous region of native mouse chromosome. The two orthologous regions include segments that can be assigned to three major chromatin classes according to their gene abundance and repeat repertoire: (1) gene-rich and SINE-rich euchromatin; (2) gene-poor and LINE/LTR-rich heterochromatin; and (3) gene-depleted and satellite DNA-containing constitutive heterochromatin. We show, using fluorescence in situ hybridization (FISH) and 4C-seq technologies, that chromatin segments ranging from 0.6 to 3 Mb cluster with segments of the same chromatin class. As a consequence, the chromatin segments acquire corresponding positions in the nucleus irrespective of their chromosomal context, thereby strongly suggesting that this is their autonomous property. Interactions with the nuclear lamina, although largely retained in the HAC, reveal less autonomy. Taken together, our results suggest that building of a functional nucleus is largely a self-organizing process based on mutual recognition of chromosome segments belonging to the major chromatin classes.

CTCF Binding Polarity Determines Chromatin Looping.

CCCTC-binding factor (CTCF) is an architectural protein involved in the three-dimensional (3D) organization of chromatin. In this study, we assayed the 3D genomic contact profiles of a large number of CTCF binding sites with high-resolution 4C-seq. As recently reported, our data also suggest that chromatin loops preferentially form between CTCF binding sites oriented in a convergent manner. To directly test this, we used CRISPR/Cas9 genome editing to delete core CTCF binding sites in three loci, including the CTCF site in the Sox2 super-enhancer. In all instances, CTCF and cohesin recruitment were lost, and chromatin loops with distal, convergent CTCF sites were disrupted or destabilized. Re-insertion of oppositely oriented CTCF recognition sequences restored CTCF and cohesin recruitment, but did not re-establish chromatin loops. We conclude that CTCF binding polarity plays a functional role in the formation of higher-order chromatin structure.

Flexible long-range loops in the VH gene region of the Igh locus facilitate the generation of a diverse antibody repertoire.

The immunoglobulin heavy-chain (Igh) locus undergoes large-scale contraction in pro-B cells, which facilitates VH-DJH recombination by juxtaposing distal VH genes next to the DJH-rearranged gene segment in the 3' proximal Igh domain. By using high-resolution mapping of long-range interactions, we demonstrate that local interaction domains established the three-dimensional structure of the extended Igh locus in lymphoid progenitors. In pro-B cells, these local domains engaged in long-range interactions across the Igh locus, which depend on the regulators Pax5, YY1, and CTCF. The large VH gene cluster underwent flexible long-range interactions with the more rigidly structured proximal domain, which probably ensures similar participation of all VH genes in VH-DJH recombination to generate a diverse antibody repertoire. These long-range interactions appear to be an intrinsic feature of the VH gene cluster, because they are still generated upon mutation of the Eµ enhancer, IGCR1 insulator, or 3' regulatory region in the proximal Igh domain.

Allelic exclusion of the immunoglobulin heavy chain locus is independent of its nuclear localization in mature B cells.

In developing B cells, the immunoglobulin heavy chain (IgH) locus is thought to move from repressive to permissive chromatin compartments to facilitate its scheduled rearrangement. In mature B cells, maintenance of allelic exclusion has been proposed to involve recruitment of the non-productive IgH allele to pericentromeric heterochromatin. Here, we used an allele-specific chromosome conformation capture combined with sequencing (4C-seq) approach to unambigously follow the individual IgH alleles in mature B lymphocytes. Despite their physical and functional difference, productive and non-productive IgH alleles in B cells and unrearranged IgH alleles in T cells share many chromosomal contacts and largely reside in active chromatin. In brain, however, the locus resides in a different repressive environment. We conclude that IgH adopts a lymphoid-specific nuclear location that is, however, unrelated to maintenance of allelic exclusion. We additionally find that in mature B cells-but not in T cells-the distal VH regions of both IgH alleles position themselves away from active chromatin. This, we speculate, may help to restrict enhancer activity to the productively rearranged VH promoter element.

Robust 4C-seq data analysis to screen for regulatory DNA interactions.

Regulatory DNA elements can control the expression of distant genes via physical interactions. Here we present a cost-effective methodology and computational analysis pipeline for robust characterization of the physical organization around selected promoters and other functional elements using chromosome conformation capture combined with high-throughput sequencing (4C-seq). Our approach can be multiplexed and routinely integrated with other functional genomics assays to facilitate physical characterization of gene regulation.

4C technology: protocols and data analysis.

Chromosome conformation capture (3C) technology and its genome-wide derivatives have revolutionized our knowledge on chromatin folding and nuclear organization. 4C-seq Technology combines 3C principles with high-throughput sequencing (4C-seq) to enable for unbiased genome-wide screens for DNA contacts made by single genomic sites of interest. Here, we discuss in detail the design, application, and data analysis of 4C-seq experiments. Based on many hundreds of different 4C-seq experiments, we define criteria to assess data quality and show how different restriction enzymes and cross-linking conditions affect results. We describe in detail the mapping strategy of 4C-seq reads and show advanced strategies for data analysis.