RET enhancer haplotype-dependent remodeling of the human fetal gut development program

Variability in proliferative and migratory defects in Hirschsprung disease-associated RET pathogenic variants

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.

Synergistic effects of Ret coding and enhancer loss-of-function alleles cause progressive loss of inhibitory motor neurons in the enteric nervous system

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.

The Role of de novo and Ultra-Rare Variants in Hirschsprung Disease (HSCR): Extended Gene Discovery for Risk Profiling of Patients

Background

Hirschsprung disease (HSCR) is a rare neurodevelopmental disorder caused by disrupted migration and proliferation of enteric neural crest cells during enteric nervous system development. Genetic studies suggest a complex etiology involving both rare and common variants, but the contribution of ultra-rare pathogenic variants (PAs) remains poorly understood.

Methods

We perform whole-exome sequencing (WES) on 301 HSCR probands and 109 family trios, employing advanced statistical methods and gene prioritization strategies to identify genes carrying de novo and ultra-rare coding pathogenic variants. Multiple study designs, including case-control, de novo mutation analysis and joint test, are used to detect associated genes. Candidate genes are further prioritized based on their biological and functional relevance to disease associated tissues and onset period (i.e., human embryonic colon).

Results

We identify 19 risk genes enriched with ultra-rare coding pathogenic variants in HSCR probands, including four known genes (RET, EDNRB, ZEB2, SOX10) and 15 novel candidates (e.g., COLQ, NES, FAT3) functioning in neural proliferation and neuromuscular synaptic development. These genes account for 17.5% of the population-attributable risk (PAR), with novel candidates contributing 6.5%. Notably, a positive correlation between pathogenic mutational burden and disease severity is observed. Female cases exhibit at least 42% higher ultra-rare pathogenic variant burden than males (P = 0.05).

Conclusions

This first-ever genome-wide screen of ultra-rare variants in a large, phenotypically diverse HSCR cohort highlights the substantial contribution of ultra-rare pathogenic variants to the disease risk and phenotypic variability. These findings enhance our understanding of the genetic architecture of HSCR and provide potential targets for genetic screening and personalized interventions.

Variability in proliferative and migratory defects in Hirschsprung disease-associated RET pathogenic variants

Despite the extensive genetic heterogeneity of Hirschsprung disease (HSCR; congenital colonic aganglionosis) 72% of patients harbor pathogenic variants in 10 genes that form a gene regulatory network (GRN) controlling the development of the enteric nervous system (ENS). Among these genes, the receptor tyrosine kinase gene RET is the most significant contributor, accounting for pathogenic variants in 12%-50% of patients depending on phenotype. RET plays a critical role in the proliferation and migration of ENS precursors, and defects in these processes lead to HSCR. However, despite the gene's importance in HSCR, the functional consequences of RET pathogenic variants and their mechanism of disease remain poorly understood. To address this, we investigated the proliferative and migratory phenotypes in a RET-dependent neural crest-derived cell line harboring one of five missense (L56M, E178Q, Y791F, S922Y, F998L) or three nonsense (Y204X, R770X, Y981X) pathogenic heterozygous variants. Using a combination of cDNA-based and CRISPR-based PRIME editing coupled with quantitative proliferation and migration assays, we detected significant losses in cell proliferation and migration in three missense (E178Q, S922Y, F998L) and all nonsense variants. Our data suggests that the Y791F variant, whose pathogenicity has been debated, is likely not pathogenic. 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 and feasibility of targeted functional assays to accurately assess the pathogenicity of HSCR-associated variants, rather than relying solely on machine learning predictions, which could themselves be refined by incorporating such functional data.

Detailed survey of an in vitro intestinal epithelium model by single-cell transcriptomics

The co-culture of two adult human colorectal cancer cell lines, Caco-2 and HT29, on Transwell is commonly used as an in vitro gut mimic, yet the translatability of insights from such a system to adult human physiological contexts is not fully characterized. Here, we used single-cell RNA sequencing on the co-culture to obtain a detailed survey of cell type heterogeneity in the system and conducted a holistic comparison with human physiology. We identified the intestinal stem cell-, transit amplifying-, enterocyte-, goblet cell-, and enteroendocrine-like cells in the system. In general, the co-culture was fetal intestine-like, with less variety of gene expression compared to the adult human gut. Transporters for major types of nutrients were found in the majority of the enterocytes-like cells in the system. TLR 4 was not expressed in the sample, indicating that the co-culture model is incapable of mimicking the innate immune aspect of the human epithelium.

Clinical and Genetic Correlation in Neurocristopathies: Bridging a Precision Medicine Gap

Neurocristopathies (NCPs) encompass a spectrum of disorders arising from issues during the formation and migration of neural crest cells (NCCs). NCCs undergo epithelial-mesenchymal transition (EMT) and upon key developmental gene deregulation, fetuses and neonates are prone to exhibit diverse manifestations depending on the affected area. These conditions are generally rare and often have a genetic basis, with many following Mendelian inheritance patterns, thus making them perfect candidates for precision medicine. Examples include cranial NCPs, like Goldenhar syndrome and Axenfeld-Rieger syndrome; cardiac-vagal NCPs, such as DiGeorge syndrome; truncal NCPs, like congenital central hypoventilation syndrome and Waardenburg syndrome; and enteric NCPs, such as Hirschsprung disease. Additionally, NCCs' migratory and differentiating nature makes their derivatives prone to tumors, with various cancer types categorized based on their NCC origin. Representative examples include schwannomas and pheochromocytomas. This review summarizes current knowledge of diseases arising from defects in NCCs' specification and highlights the potential of precision medicine to remedy a clinical phenotype by targeting the genotype, particularly important given that those affected are primarily infants and young children.