Sorbitol dehydrogenase (SORD) deficiency, due to biallelic loss-of-function variants in the SORD gene, is a recently recognized cause of autosomal recessive hereditary neuropathy. Specific diagnosis is difficult on clinical grounds alone, and molecular genetic testing of SORD is complicated by the presence of a pseudogene. Biochemical testing of serum sorbitol is suggested as a potential biomarker. We report a novel urine biochemical profile of elevated excretion of sorbitol and a second polyol, xylitol, to aid in the identification of individuals with SORD-related neuropathy.

Congenital pancreatic lipase deficiency (CPLD, OMIM #614338) is a rare exocrine pancreatic disorder presenting in late infancy with steatorrhea, fat-soluble vitamin deficiency, and low pancreatic lipase activity. Variants of the pancreatic triglyceride lipase (PNLIP) gene have been linked to CPLD. Six children from four Amish families exhibited CPLD symptoms and two had decreased fecal elastase levels when tested. A novel homozygous PNLIP variant, c.869G>A (p.S290N), was identified in these children. This study aimed to characterize the PNLIP variant to understand its mechanism underlying CPLD. The variant impact was first evaluated using computational modeling. Functional analyses included activity assays, cellular PNLIP partition assessments, and endoplasmic reticulum (ER) stress evaluation in transfected cells. Computational modeling showed that p.Ser290 is highly conserved across species and the variant causes steric hindrance resulting in protein misfolding. Functional assays revealed that the PNLIP variant had a complete loss of activity compared to the wild type (WT), with defects in catalytic function and secretion. Immunoblotting showed reduced PNLIP variant in the medium and increased accumulation in the detergent-insoluble fraction consistent with protein misfolding. Variant-expressing cells had elevated levels of BiP, an ER stress marker, and increased Xbp1 mRNA splicing, suggesting an elevated ER stress and unfolded protein response (UPR). In conclusion, the PNLIP p.S290N variant causes CPLD through a loss-of-function mechanism, characterized by loss of enzymatic activity and defective secretion due to protein misfolding. Further studies are needed to determine whether the misfolding variant protein induces proteotoxicity, potentially increasing the risk of pancreatic injury including chronic pancreatitis.

The Amish of Lancaster County, PA has been the focus of genetic studies for many years due to its demographic history and unique genetic makeup that includes a historical bottleneck event and subsequent genetic drift, resulting in a marked decrease in genetic diversity and increased frequency of some variants that have substantially shaped the health of the community. To characterize the coding variation in the Amish genome, we sequenced the exomes of 7221 adult community members, and in this report, we contrast genetic diversity between the Amish and Europeans from the UK Biobank. Exome sequences of 7221 Amish contained only 14% as many variants as the same number of UKB participants. This reduced genetic diversity has substantial clinical implications. We identified pathogenic (P) and likely pathogenic (LP) variants from ClinVar and a population-specific genetic screening panel and found that most of the variants present in the Amish were highly enriched, resulting in 5.2% of Amish individuals being homozygous for a recessive P/LP variant and 25.6% being heterozygous for at least one dominant P/LP variant. In 43.6% of the 2141 Amish spouse-pairs in our sample, at least one spouse was heterozygous for a P/LP dominant variant, and 24.3% of couples were autosomal recessive disease carrier couples, meaning that each of their children was at ~25% risk of inheriting two copies of that variant. Gene discovery efforts in other founder communities will likely uncover distinct P (and beneficial) variants impacting the health of these communities, with implications for all of human health.

Complement factor I (CFI) deficiency is an ultra-rare inborn disorder of complement regulation that manifests with protean infectious, vasculitic, and neuroinflammatory symptoms. To functionally validate a previously unrecognized, disease-associated CFI variant (Y459S) and determine variant enrichment in the Old Order Amish population.

Partial RAG deficiency (pRD) can manifest with systemic and tissue-specific immune dysregulation, with inflammatory bowel disease (IBD) in 15% of the patients. We aimed at identifying the immunopathological and microbial signatures associated with IBD in patients with pRD and in a mouse model of pRD (Rag1^w/w) with spontaneous development of colitis. pRD patients with IBD and Rag1^w/w mice showed a systemic and colonic Th1/Th17 inflammatory signature. Restriction of fecal microbial diversity, abundance of pathogenic bacteria, and depletion of microbial species producing short-chain fatty acid were observed, which were associated with impaired induction of lamina propria peripheral Treg cells in Rag1^w/w mice. The use of vedolizumab in Rag1^w/w mice and of ustekinumab in a pRD patient were ineffective. Antibiotics ameliorated gut inflammation in Rag1^w/w mice, but only bone marrow transplantation (BMT) rescued the immunopathological and microbial signatures. Our findings shed new light in the pathophysiology of gut inflammation in pRD and establish a curative role for BMT to resolve the disease phenotype.

Major depressive disorder (MDD) is a leading cause of disability worldwide. Risk for MDD is heritable, and the genetic structure of founder populations enables investigation of rare susceptibility alleles with large effect. In an extended Old Order Mennonite family cohort, we identified a rare missense variant in GPR156 (c.1599G>T, p.Glu533Asp) associated with a two-fold increase in the relative risk of MDD. GPR156 is an orphan G protein–coupled receptor localized in the medial habenula, a region implicated in mood regulation. Insertion of a human sequence containing c.1599G>T into the murine Gpr156 locus induced medial habenula hyperactivity and abnormal stress-related behaviors. This work reveals a human variant that is associated with depression, implicates GPR156 as a target for mood regulation, and introduces informative murine models for investigating the pathophysiology and treatment of affective disorders.

Classic maple syrup urine disease (MSUD) results from biallelic mutations in genes that encode the branched-chain α-ketoacid dehydrogenase E1α (BCKDHA), E1β (BCKDHB), or dihydrolipoamide branched-chain transacylase (DBT) subunits, which interact to form the mitochondrial BCKDH complex that decarboxylates ketoacid derivatives of leucine, isoleucine, and valine. MSUD is an inborn error of metabolism characterized by recurrent life-threatening neurologic crises and progressive brain injury that can only be managed with an exacting prescription diet or allogeneic liver transplant. To develop a gene replacement therapy for MSUD, we designed a dual-function recombinant adeno-associated virus serotype 9 (rAAV9) vector to deliver codon-optimized BCKDHA and BCKDHB (rAAV9.hA-BiP-hB) to the liver, muscle, heart, and brain. rAAV9.hA-BiP-hB restored coexpression of BCKDHA and BCKDHB as well as BCKDH holoenzyme activity in BCKDHA^−/− HEK293T cells and did not perturb physiologic branched-chain amino acid homeostasis in wild-type mice at a systemic dose of 2.7 × 10¹⁴ vector genomes per kilogram. In two models of severe MSUD (Bckdha^−/− and Bckdhb^−/− mice) and a newborn calf homozygous for BCKDHA c.248C>T, one postnatal injection prevented perinatal death, normalized growth, restored coordinated expression of BCKDHA and BCKDHB in the skeletal muscle, liver, heart, and brain, and stabilized MSUD biomarkers in the face of high protein ingestion. In summary, we developed a one-time BCKDHA-BCKDHB systemic dual-gene replacement strategy that holds promise as a therapeutic alternative to prescription diet and liver transplant for treatment of MSUD types 1A and 1B, the two most common forms of MSUD in humans.