Understanding the Role of NAD Deficiency in a Mendelian Disorder of Tryptophan Metabolism

PROJECT SUMMARY/ABSTRACT Genetic disruptions of the kynurenine pathway (KP) have been linked to congenital nicotinamide adenine dinucleotide (NAD) deficiencies in families with a history of birth defects and recurrent miscarriages. Mammals synthesize NAD from two pathways. The KP synthesizes NAD de novo from dietary tryptophan, whereas the Preiss-Handler pathway (PHP) bypasses the KP and utilizes dietary niacin. Birth defects associated with congenital NAD deficiency disorders (CNDD) include vertebral, anal, cardiac, tracheoesophageal, renal, and limb anomalies. Some individuals with CNDD also have global developmental delays, learning disorders, and autism. Through the Undiagnosed Diseases Network at Baylor College of Medicine (BCM), we identified a patient with biallelic variants in kynurenine 3-monooxygenase (KMO). KMO encodes an enzyme that catalyzes a key step in the KP and has not been previously associated with CNDD. The proband presents with congenital anomalies, short stature, neurodevelopmental delays, low plasma levels of NAD, and extremely elevated plasma levels of metabolites upstream of KMO (kynurenine and kynurenate). While indicative of a deficiency in KMO, the relative contribution of low NAD levels versus elevated upstream metabolites to her phenotypes is unclear. Moreover, it is not known which phenotypes associated with this disorder may be prevented by supplementing dietary niacin. I propose to use two mouse models of KMO deficiency and dietary manipulations to better understand the metabolic mechanisms underlying the phenotypes associated with KMO deficiency. My central hypothesis is that KMO deficiency is a novel CNDD that increases the risk for congenital anomalies and neurodevelopmental phenotypes, some of which are preventable with niacin supplementation. Overall, the findings from these studies will provide insights into strategies for preventing and treating the phenotypes associated with KMO deficiency and other CNDDs. To understand the risk for prenatal and postnatal phenotypes associated with CNDD and the role of NAD during embryonic development, I am utilizing a mouse model with a global KMO deficiency (Kmo-/-) and a mouse model with a liver-specific deletion of Kmo. My preliminary data show that Kmo-/- embryos are at a higher risk for developing congenital anomalies in the setting of a low niacin diet than their heterozygous littermates. In this proposal, I will analyze skeletal phenotypes in Kmo-/- and Kmo+/- embryos and assess metabolic alterations in these embryos. I will also investigate the role of kynurenine and kynurenate derived from the liver in causing neurodevelopmental phenotypes in Kmo-/- mice fed a niacin- sufficient diet. The long-term goal of these studies is to benefit individuals with CNDD as well as provide insights into the role of NAD and kynurenine metabolism in normal prenatal and postnatal development. The trainee’s environment is primed to accomplish these aims with mentorship from the sponsor and co-sponsor, and resources from the BCM CPMM, the BCM Advanced Technology Cores, and collaborators.