Sortilin as a novel regulator of plasma cholesterol, very-low density lipoprotein secretion and LDL catabolism

Elevated low-density lipoprotein cholesterol (LDL-C) is associated with increased risk of cardiovascular disease. Genome-wide association studies (GWAS) have identified a variant at a locus on chromosome 1p13 harboring the gene SORT1, which encodes the protein sortilin, as very strongly associated with a greater than 10-fold increase in hepatic expression of SORT1, reduced plasma levels of LDL-C, and reduced risk of myocardial infarction (MI). Through a series of overexpression, knockout and knockdown studies we have interrogated a role for Sort1 in lipid metabolism, and found that increased hepatic Sort1 expression is associated with reduced plasma cholesterol, reduced secretion of the LDL precursor very-low density lipoprotein (VLDL), and increased LDL catabolism. We further demonstrate that increased Sort1 expression reduces VLDL secretion and promotes LDL catabolism by directly binding to VLDL/LDL particles and directing them to the endolysosomal system for degradation. We also show by using liver specific Sort1 small-interfering RNAs (siRNAs) that reductions in Sort1 expression in mouse liver increases plasma cholesterol, while total body Sort1 deficiency either does not affect plasma cholesterol or leads to a paradoxical reduction in plasma cholesterol. Loss-of-function studies demonstrate that sortilin serves as a bona fide receptor for LDL, but is also associated with reduced VLDL secretion. We thus provide functional evidence that genetically-increased hepatic sortilin both reduces hepatic VLDL secretion and increases LDL catabolism, providing dual mechanisms for the very strong association between increased hepatic sortilin expression and reduced plasma LDL-C levels in humans

Sortilin as a novel regulator of plasma cholesterol, very-low density lipoprotein secretion and LDL catabolism

Elevated low-density lipoprotein cholesterol (LDL-C) is associated with increased risk of cardiovascular disease. Genome-wide association studies (GWAS) have identified a variant at a locus on chromosome 1p13 harboring the gene SORT1, which encodes the protein sortilin, as very strongly associated with a greater than 10-fold increase in hepatic expression of SORT1, reduced plasma levels of LDL-C, and reduced risk of myocardial infarction (MI). Through a series of overexpression, knockout and knockdown studies we have interrogated a role for Sort1 in lipid metabolism, and found that increased hepatic Sort1 expression is associated with reduced plasma cholesterol, reduced secretion of the LDL precursor very-low density lipoprotein (VLDL), and increased LDL catabolism. We further demonstrate that increased Sort1 expression reduces VLDL secretion and promotes LDL catabolism by directly binding to VLDL/LDL particles and directing them to the endolysosomal system for degradation. We also show by using liver specific Sort1 small-interfering RNAs (siRNAs) that reductions in Sort1 expression in mouse liver increases plasma cholesterol, while total body Sort1 deficiency either does not affect plasma cholesterol or leads to a paradoxical reduction in plasma cholesterol. Loss-of-function studies demonstrate that sortilin serves as a bona fide receptor for LDL, but is also associated with reduced VLDL secretion. We thus provide functional evidence that genetically-increased hepatic sortilin both reduces hepatic VLDL secretion and increases LDL catabolism, providing dual mechanisms for the very strong association between increased hepatic sortilin expression and reduced plasma LDL-C levels in humans

Phenotypes of undiagnosed adults with actionable OTC and GLA variants

Summary: Inherited metabolic disorders (IMDs) are variably expressive, complicating identification of affected individuals. A genotype-first approach can identify individuals at risk for morbidity and mortality from undiagnosed IMDs and can lead to protocols that improve clinical detection, counseling, and management. Using data from 57,340 participants in two hospital biobanks, we assessed the frequency and phenotypes of individuals with pathogenic/likely pathogenic variants (PLPVs) in two IMD genes: GLA, associated with Fabry disease, and OTC, associated with ornithine transcarbamylase deficiency. Approximately 1 in 19,100 participants harbored an undiagnosed PLPV in GLA or OTC. We identified three individuals (2 male, 1 female) with PLPVs in GLA, all of whom were undiagnosed, and three individuals (3 female) with PLPVs in OTC, two of whom were undiagnosed. All three individuals with PLPVs in GLA (100%) had symptoms suggestive of mild Fabry disease, and one individual (14.2%) had an ischemic stroke at age 33, likely indicating the presence of classic disease. No individuals with PLPVs in OTC had documented hyperammonemia despite exposure to catabolic states, but all (100%) had chronic symptoms suggestive of attenuated disease, including mood disorders and migraines. Our findings suggest that GLA and OTC variants identified via a genotype-first approach are of high penetrance and that population screening of these genes can be used to facilitate stepwise phenotyping and appropriate care

Molecular diagnosis and novel genes and phenotypes in a pediatric thoracic insufficiency cohort

Abstract Thoracic insufficiency syndromes are a genetically and phenotypically heterogeneous group of disorders characterized by congenital abnormalities or progressive deformation of the chest wall and/or vertebrae that result in restrictive lung disease and compromised respiratory capacity. We performed whole exome sequencing on a cohort of 42 children with thoracic insufficiency to elucidate the underlying molecular etiologies of syndromic and non-syndromic thoracic insufficiency and predict extra-skeletal manifestations and disease progression. Molecular diagnosis was established in 24/42 probands (57%), with 18/24 (75%) probands having definitive diagnoses as defined by laboratory and clinical criteria and 6/24 (25%) probands having strong candidate genes. Gene identified in cohort patients most commonly encoded components of the primary cilium, connective tissue, and extracellular matrix. A novel association between KIF7 and USP9X variants and thoracic insufficiency was identified. We report and expand the genetic and phenotypic spectrum of a cohort of children with thoracic insufficiency, reinforce the prevalence of extra-skeletal manifestations in thoracic insufficiency syndromes, and expand the phenotype of KIF7 and USP9X-related disease to include thoracic insufficiency

Activation of ER stress and mTORC1 suppresses hepatic sortilin-1 levels in obese mice

Recent GWAS have identified SNPs at a human chromosom1 locus associated with coronary artery disease risk and LDL cholesterol levels. The SNPs are also associated with altered expression of hepatic sortilin-1 (SORT1), which encodes a protein thought to be involved in apoB trafficking and degradation. Here, we investigated the regulation of Sort1 expression in mouse models of obesity. Sort1 expression was markedly repressed in both genetic (ob/ob) and high-fat diet models of obesity; restoration of hepatic sortilin-1 levels resulted in reduced triglyceride and apoB secretion. Mouse models of obesity also exhibit increased hepatic activity of mammalian target of rapamycin complex 1 (mTORC1) and ER stress, and we found that administration of the mTOR inhibitor rapamycin to ob/ob mice reduced ER stress and increased hepatic sortilin-1 levels. Conversely, genetically increased hepatic mTORC1 activity was associated with repressed Sort1 and increased apoB secretion. Treating WT mice with the ER stressor tunicamycin led to marked repression of hepatic sortilin-1 expression, while administration of the chemical chaperone PBA to ob/ob mice led to amelioration of ER stress, increased sortilin-1 expression, and reduced apoB and triglyceride secretion. Moreover, the ER stress target Atf3 acted at the SORT1 promoter region as a transcriptional repressor, whereas knockdown of Atf3 mRNA in ob/ob mice led to increased hepatic sortilin-1 levels and decreased apoB and triglyceride secretion. Thus, in mouse models of obesity, induction of mTORC1 and ER stress led to repression of hepatic Sort1 and increased VLDL secretion via Atf3. This pathway may contribute to dyslipidemia in metabolic disease

De novo loss-of-function variants in X-linked MED12 are associated with Hardikar syndrome in females

PURPOSE: Hardikar syndrome (MIM 612726) is a rare multiple congenital anomaly syndrome characterized by facial clefting, pigmentary retinopathy, biliary anomalies, and intestinal malrotation, but with preserved cognition. Only four patients have been reported previously, and none had a molecular diagnosis. Our objective was to identify the genetic basis of Hardikar syndrome (HS) and expand the phenotypic spectrum of this disorder. METHODS: We performed exome sequencing on two previously reported and five unpublished female patients with a clinical diagnosis of HS. X-chromosome inactivation (XCI) studies were also performed. RESULTS: We report clinical features of HS with previously undescribed phenotypes, including a fatal unprovoked intracranial hemorrhage at age 21. We additionally report the discovery of de novo pathogenic nonsense and frameshift variants in MED12 in these seven individuals and evidence of extremely skewed XCI in all patients with informative testing. CONCLUSION: Pathogenic missense variants in the X-chromosome gene MED12 have previously been associated with Opitz-Kaveggia syndrome, Lujan syndrome, Ohdo syndrome, and nonsyndromic intellectual disability, primarily in males. We propose a fifth, female-specific phenotype for MED12, and suggest that nonsense and frameshift loss-of-function MED12 variants in females cause HS. This expands the MED12-associated phenotype in females beyond intellectual disability.status: publishe

De novo variants in RNF213 are associated with a clinical spectrum ranging from Leigh syndrome to early-onset stroke

Purpose: RNF213, encoding a giant E3 ubiquitin ligase, has been recognized for its role as a key susceptibility gene for moyamoya disease. Case reports have also implicated specific variants in RNF213 with an early-onset form of moyamoya disease with full penetrance. We aimed to expand the phenotypic spectrum of monogenic RNF213-related disease and to evaluate genotype-phenotype correlations. Methods: Patients were identified through reanalysis of exome sequencing data of an unselected cohort of unsolved pediatric cases and through GeneMatcher or ClinVar. Functional characterization was done by proteomics analysis and oxidative phosphorylation enzyme activities using patient-derived fibroblasts. Results: We identified 14 individuals from 13 unrelated families with (de novo) missense variants in RNF213 clustering within or around the Really Interesting New Gene (RING) domain. Individuals presented either with early-onset stroke (n = 11) or with Leigh syndrome (n = 3). No genotype-phenotype correlation could be established. Proteomics using patient-derived fibroblasts revealed no significant differences between clinical subgroups. 3D modeling revealed a clustering of missense variants in the tertiary structure of RNF213 potentially affecting zinc-binding suggesting a gain-of-function or dominant negative effect. Conclusion: De novo missense variants in RNF213 clustering in the E3 RING or other regions affecting zinc-binding lead to an early-onset syndrome characterized by stroke or Leigh syndrome