Two new cases of KYNU deficiency: Further delineation of the phenotypic and biochemical spectrum and exploration of treatment options

Congenital NAD⁺ deficiency disorders (CNDD) represent a novel category of hereditary diseases that affect NAD⁺ biosynthesis from tryptophan. CNDD include kynureninase (KYNU) deficiency with only ten documented patients to date. Here, we report two new cases. In addition to the previously described clinical phenotype of congenital heart defects, skeletal abnormalities, hearing loss and airway malacia, our patients also had hypothyroidism, recurrent infections, and necrotizing enterocolitis. The first patient's exome analysis identified compound heterozygosity for two novel KYNU variants, pathogenicity being supported by extensive metabolic profiling. The second patient was homozygous for a known KYNU missense variant. For the first time we describe a potential treatment of KYNU deficiency as both patients were treated with oral nicotinamide (a B3 vitamer and NAD⁺ precursor) and pyridoxine (a vitamin B6, cofactor of KYNU) from young age, which improved biochemistry without evident short-term adverse effects. Furthermore, we show that maternal supplementation with nicotinamide riboside (another B3 vitamer and NAD⁺ precursor) during pregnancy appears to be safe. In conclusion, KYNU deficiency is an extremely rare disorder associated with severe congenital defects, demonstrating the indispensable role of de novo NAD⁺ biosynthesis in foetal development. Metabolomic profiling is a valuable tool in the analysis of new genetic variants. NAD⁺ precursor supplementation in the form of nicotinamide, combined with pyridoxine, may be a novel therapeutic approach.