Biallelic variants in SLC38A3 encoding a glutamine transporter cause epileptic encephalopathy.

The solute carrier (SLC) superfamily encompasses >400 transmembrane transporters involved in the exchange of amino acids, nutrients, ions, metals, neurotransmitters and metabolites across biological membranes. SLCs are highly expressed in the mammalian brain; defects in nearly 100 unique SLC-encoding genes (OMIM: https://www.omim.org) are associated with rare Mendelian disorders including developmental and epileptic encephalopathy and severe neurodevelopmental disorders. Exome sequencing and family-based rare variant analyses on a cohort with neurodevelopmental disorders identified two siblings with developmental and epileptic encephalopathy and a shared deleterious homozygous splicing variant in SLC38A3. The gene encodes SNAT3, a sodium-coupled neutral amino acid transporter and a principal transporter of the amino acids asparagine, histidine, and glutamine, the latter being the precursor for the neurotransmitters GABA and glutamate. Additional subjects with a similar developmental and epileptic encephalopathy phenotype and biallelic predicted-damaging SLC38A3 variants were ascertained through GeneMatcher and collaborations with research and clinical molecular diagnostic laboratories. Untargeted metabolomic analysis was performed to identify novel metabolic biomarkers. Ten individuals from seven unrelated families from six different countries with deleterious biallelic variants in SLC38A3 were identified. Global developmental delay, intellectual disability, hypotonia, and absent speech were common features while microcephaly, epilepsy, and visual impairment were present in the majority. Epilepsy was drug-resistant in half. Metabolomic analysis revealed perturbations of glutamate, histidine, and nitrogen metabolism in plasma, urine, and CSF of selected subjects, potentially representing biomarkers of disease. Our data support the contention that SLC38A3 is a novel disease gene for developmental and epileptic encephalopathy and illuminate the likely pathophysiology of the disease as perturbations in glutamine homeostasis.

Clinical Manifestations of Noonan Syndrome and Related Disorders.

Noonan syndrome represents a heterogeneous group of genetic disorders caused by mutations in genes of the RAS/MAPK pathway. Related syndromes include cardiofaciocutaneous syndrome, Noonan syndrome with multiple lentigines and Costello syndrome. The common phenotypic features of Noonan syndrome include facial dysmorphisms, short stature, congenital heart defects and genitourinary abnormalities. These and other findings as well as features of related disorders are discussed. In addition we briefly review clinical diagnosis and prenatal findings of these syndromes and genetic counseling implications.

Severe central sleep apnea in a child with biallelic variants in NALCN.

The sodium leak channel, nonselective (NALCN), is necessary for the proper function of the neurons that play an important role in the sleep-wake cycle and regulation of breathing patterns during wakefulness and sleep. We report a 38-month-old male with developmental delay, hypotonia, and severe central sleep apnea with periodic breathing requiring noninvasive ventilation during sleep, who was found to have novel biallelic pathogenic variants in NALCN. A review of the literature illustrates 17 additional children with biallelic variants in the NALCN gene. The clinical and sleep manifestations of these children are discussed. CITATION: Maselli K, Park H, Breilyn MS, Arens R. Severe central sleep apnea in a child with biallelic variants in NALCN. J Clin Sleep Med. 2022;18(10):2507-2513.

Plasma lyso-sphingomyelin levels are positively associated with clinical severity in acid sphingomyelinase deficiency.

INTRODUCTION: A reliable biomarker is urgently needed in the diagnosis and management of acid sphingomyelinase deficiency (ASMD, also known as Niemann Pick A, A/B, and B). Lyso-sphingomyelin (LSM) has previously been proposed as a biomarker for this disease. However, existing studies have not investigated the relationship between LSM levels and clinical subtype or severity. The purpose of this study is to address this gap in knowledge. MATERIAL AND METHODS: We present a cross-sectional study of 28 patients with ASMD, enrolled in an ongoing natural history study at the Icahn School of Medicine at Mount Sinai and The Children's Hospital at Montefiore. Plasma LSM levels from 28 patients were analyzed, including 7 patients with the infantile neurovisceral phenotype (ASMD type A), 3 patients with chronic neurovisceral disease (ASMD type A/B) and 18 patients with chronic visceral ASMD (ASMD type B). The association between LSM levels and clinical subtype, dichotomized as infantile (type A) or chronic (type A/B and B), was analyzed using the Wilcoxon rank sum test. In secondary analysis, the association between LSM levels and clinical severity among the chronic ASMD patients was analyzed using the Kruskal-Wallis test. RESULTS: LSM levels were elevated in all patients with ASMD when compared to a reference range of (0.04-3.8 (ng/mL)). Median LSM levels were higher in patients with infantile ASMD (386 ng/mL [314, 605]) compared to chronic ASMD (133 ng/mL [90, 209]), p < .001. Additionally, among individuals with chronic ASMD there was a positive association between LSM level and clinical severity (p = .01, p for trend <0.001). CONCLUSION: We identified greater LSM elevations in patients with infantile ASMD compared to those with chronic ASMD. Among patients with chronic ASMD, LSM levels were positively associated with clinical severity. These data support investigation of LSM as a biomarker for ASMD. Future studies are required to determine if LSM levels are predictive of phenotype in pre-symptomatic patients and how such levels correlate in response to treatment.

Established and Emerging Treatments for Patients with Inborn Errors of Metabolism.

Inborn errors of metabolism (IEMs) are inherited defects in a metabolic pathway resulting in clinical disease. The overall goal of therapy is to restore metabolic homeostasis while minimizing the deleterious effects of the interruption. Conventional treatments focus on decreasing substrate, providing product, and replacing deficient enzyme or cofactor. We discuss examples of established, novel, and emerging therapies to provide a framework for understanding the principles of management for patients with IEMs.