Faculty Bibliography

Hirschsprung disease (HSCR) exhibits extensive genetic heterogeneity, with 72% of cases involving pathogenic variants in 10 genes forming a gene regulatory network (GRN) essential for enteric nervous system (ENS) development. The receptor tyrosine kinase gene RET is the most significant contributor, implicated in 12%-50% of individuals depending on the phenotype. RET plays a critical role in ENS precursor proliferation and migration, and defects in these processes lead to HSCR. However, the functional impact of RET pathogenic variants and their mechanisms of disease remain poorly understood. To address this, we investigated proliferative and migratory phenotypes in a RET-dependent neural crest-derived cell line harboring one of five missense (c.166C>A [p.Leu56Met]; c.532G>C [p.Glu178Gln]; c.2372A>T [p.Tyr791Phe]; c.2765C>A [p.Ser922Tyr]; or c.2994T>A [p.Phe998Leu]) or three nonsense (c.612C>A, c.2308C>T, or c.2943C>G) heterozygous pathogenic RET variants. Using cDNA- and CRISPR-based prime reverse insertion mechanism engineering (PRIME) editing coupled with quantitative proliferation and migration assays, we observed significant losses in proliferation and migration in three missense (c.612C>A [p.Tyr204^∗]; c.2308C>T [p.Arg770^∗]; and c.2943C>G [p.Tyr981^∗]) and all nonsense variants. Notably, the c.2372A>T (p.Tyr791Phe) missense variant, whose pathogenicity has been debated, appears benign. Importantly, the severity of migration loss did not consistently correlate with proliferation defects, and the phenotypic severity of nonsense variants was independent of their position within the RET protein. This study highlights the necessity of targeted functional assays to accurately assess the pathogenicity of HSCR-associated variants rather than relying solely on bioinformatics predictions, which could be refined by incorporating functional data.

Coding and enhancer variants of the RET receptor tyrosine kinase gene contribute to ~50% of Hirschsprung disease (HSCR) risk, a congenital disorder of disrupted enteric nervous system (ENS) development. The greatest contribution of this risk is from a common variant (rs2435357) in an ENS-active, SOX10-bound RET enhancer (MCS+9.7) that reduces RET gene expression in vivo and triggers expression changes in other ENS genes in the human fetal gut. To uncover the cellular basis of RET-mediated aganglionosis, we used CRISPR/Cas9 to delete (Δ) the homologous mouse enhancer (mcs+9.7). We used single cell RNA sequencing and high-resolution immunofluorescence to demonstrate four significant features of the developing E14.5 gut of Δmcs+9.7/Δmcs+9.7 embryos: (1) a small (5%) yet significant reduction in Ret gene expression in only two major cell types - early differentiating neurons and fate-restricted inhibitory motor neurons; (2) no significant cellular loss in the ENS; and, (3) loss of expression of 19 cell cycle regulator genes suggesting a proliferative defect. To identify the Ret functional threshold for normal ENS development, we also generated, in combination with the Ret CFP null allele, (4) Δmcs+9.7/CFP double heterozygote mice which reduced Ret gene expression in the ENS to 42% with severe loss of inhibitory motor neurons, an effect restricted to the hindgut and driven by proliferative loss. Thus, Ret gene expression drives proliferation of ENS progenitor cells and hindgut-specific inhibitory motor neuron development, and that HSCR aganglionosis arises from a cascade of cellular defects triggered by >50% loss of Ret function.

Hirschsprung disease (HSCR) is associated with deficiency of the receptor tyrosine kinase RET, resulting in loss of cells of the enteric nervous system (ENS) during fetal gut development. The major contribution to HSCR risk is from common sequence variants in RET enhancers with additional risk from rare coding variants in many genes. Here, we demonstrate that these RET enhancer variants specifically alter the human fetal gut development program through significant decreases in gene expression of RET, members of the RET-EDNRB gene regulatory network (GRN), other HSCR genes, with an altered transcriptome of 2,382 differentially expressed genes across diverse neuronal and mesenchymal functions. A parsimonious hypothesis for these results is that beyond RET's direct effect on its GRN, it also has a major role in enteric neural crest-derived cell (ENCDC) precursor proliferation, its deficiency reducing ENCDCs with relative expansion of non-ENCDC cells. Thus, genes reducing RET proliferative activity can potentially cause HSCR. One such class is the 23 RET-dependent transcription factors enriched in early gut development. We show that their knockdown in human neuroblastoma SK-N-SH cells reduces RET and/or EDNRB gene expression, expanding the RET-EDNRB GRN. The human embryos we studied had major remodeling of the gut transcriptome but were unlikely to have had HSCR: thus, genetic or epigenetic changes in addition to those in RET are required for aganglionosis.

The receptor tyrosine kinase RET plays a critical role in the fate specification of enteric neural crest-derived cells (ENCDCs) during enteric nervous system (ENS) development. RET loss of function (LoF) is associated with Hirschsprung disease (HSCR), which is marked by aganglionosis of the gastrointestinal (GI) tract. Although the major phenotypic consequences and the underlying transcriptional changes from Ret LoF in the developing ENS have been described, cell type- and state-specific effects are unknown. We performed single-cell RNA sequencing on an enriched population of ENCDCs from the developing GI tract of Ret null heterozygous and homozygous mice at embryonic day (E)12.5 and E14.5. We demonstrate four significant findings: 1) Ret-expressing ENCDCs are a heterogeneous population comprising ENS progenitors as well as glial- and neuronal-committed cells; 2) neurons committed to a predominantly inhibitory motor neuron developmental trajectory are not produced under Ret LoF, leaving behind a mostly excitatory motor neuron developmental program; 3) expression patterns of HSCR-associated and Ret gene regulatory network genes are impacted by Ret LoF; and 4) Ret deficiency leads to precocious differentiation and reduction in the number of proliferating ENS precursors. Our results support a model in which Ret contributes to multiple distinct cellular phenotypes during development of the ENS, including the specification of inhibitory neuron subtypes, cell cycle dynamics of ENS progenitors, and the developmental timing of neuronal and glial commitment.

The major genetic risk factors for Hirschsprung disease (HSCR) are three common polymorphisms within cis-regulatory elements (CREs) of the receptor tyrosine kinase gene RET, which reduce its expression during enteric nervous system (ENS) development. These risk variants attenuate binding of the transcription factors RARB, GATA2, and SOX10 to their cognate CREs, reduce RET gene expression, and dysregulate other ENS and HSCR genes in the RET-EDNRB gene regulatory network (GRN). Here, we use siRNA, ChIP, and CRISPR-Cas9 deletion analyses in the SK-N-SH cell line to ask how many additional HSCR-associated risk variants reside in RET CREs that affect its gene expression. We identify 22 HSCR-associated variants in candidate RET CREs, of which seven have differential allele-specific in vitro enhancer activity, and four of these seven affect RET gene expression; of these, two enhancers are bound by the transcription factor PAX3. We also show that deleting multiple variant-containing enhancers leads to synergistic effects on RET gene expression. These, coupled with our prior results, show that common sequence variants in at least 10 RET enhancers affect HSCR risk, seven with experimental evidence of affecting RET gene expression, extending the known RET-EDNRB GRN to reveal an extensive regulatory code modulating disease risk at a single gene.

BACKGROUND:Previous research in autism and other neurodevelopmental disorders (NDDs) has indicated an important contribution of protein-coding (coding) de novo variants (DNVs) within specific genes. The role of de novo noncoding variation has been observable as a general increase in genetic burden but has yet to be resolved to individual functional elements. In this study, we assessed whole-genome sequencing data in 2671 families with autism (discovery cohort of 516 families, replication cohort of 2155 families). We focused on DNVs in enhancers with characterized in vivo activity in the brain and identified an excess of DNVs in an enhancer named hs737. RESULTS:, OR = 1.87) involved in gene regulation. Individuals with coding DNVs have greater phenotypic severity (hypotonia, ataxia, and delayed development syndrome [HADDS]) in comparison to individuals with noncoding DNVs that have autism and hypotonia. CONCLUSIONS:In this study, we identify DNVs in the hs737 enhancer in individuals with autism. Through multiple approaches, we find hs737 targets the gene EBF3 that is genome-wide significant in NDDs. By assessment of noncoding variation and the genes they affect, we are beginning to understand their impact on gene regulatory networks in NDDs.

The development of the gut from endodermal tissue to an organ with multiple distinct structures and functions occurs over a prolonged time during embryonic days E10.5-E14.5 in the mouse. During this process, one major event is innervation of the gut by enteric neural crest cells (ENCCs) to establish the enteric nervous system (ENS). To understand the molecular processes underpinning gut and ENS development, we generated RNA-sequencing profiles from wild-type mouse guts at E10.5, E12.5, and E14.5 from both sexes. We also generated these profiles from homozygous Ret null embryos, a model for Hirschsprung disease (HSCR), in which the ENS is absent. These data reveal 4 major features: 1) between E10.5 and E14.5 the developmental genetic programs change from expression of major transcription factors and its modifiers to genes controlling tissue (epithelium, muscle, endothelium) specialization; 2) the major effect of Ret is not only on ENCC differentiation to enteric neurons but also on the enteric mesenchyme and epithelium; 3) a muscle genetic program exerts significant effects on ENS development; and 4) sex differences in gut development profiles are minor. The genetic programs identified, and their changes across development, suggest that both cell autonomous and nonautonomous factors, and interactions between the different developing gut tissues, are important for normal ENS development and its disorders.

Disruptions in gene regulatory networks (GRNs), driven by multiple deleterious variants, potentially underlie complex traits and diseases. Hirschsprung disease (HSCR), a multifactorial disorder of enteric nervous system (ENS) development, is associated with at least 24 genes and 7 chromosomal loci, with RET and EDNRB as its major genes. We previously demonstrated that RET transcription in the ENS is controlled by an extensive GRN involving the transcription factors (TF) RARB, GATA2 and SOX10 and other HSCR genes. We now demonstrate, using human and mouse cellular and animal models, that EDNRB is transcriptionally regulated in the ENS by GATA2, SOX10 and NKX2.5 TFs. Significantly, RET and EDNRB expression is regulated by their shared use of GATA2 and SOX10 and, in turn, these TFs are controlled by EDNRB and RET in a dose-dependent manner. This study expands the ENS development GRN to include both RET and EDNRB, uncovers the mechanistic basis for RET-EDNRB epistasis and emphasizes how functionally different genes associated with a complex disorder can be united through a common GRN.