Project Summary
Nicotinamide adenine dinucleotide (NAD+) is a cofactor required for glycolysis, the tricarboxylic acid cycle (TCA)
and enzymatic reaction in electron transport chain (ETC). In mammalian cells, NAD+ salvage pathway, where
nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme, is the predominant pathway for
NAD+ biosynthesis. Although the dysregulation of NAD+ in aging and neurodegerative diseases has been
reported, genetic diseases caused by NAMPT variants have not been clinically recognized and understood. Here
we identified the first case of an inherited neurological disease caused by a homozygous single nucleotide
polymorphism (SNP), i.e., a P158A mutation in the coding region of NAMPT gene. The major clinical features of
patients include impaired motor coordination, muscle weakness, atrophy of lower extremities, positive Babiński
sign. The patients were diagnosed as hereditary motor and sensory neuropathy involving axonal degeneration
and neuromuscular junction (NMJ) dysfunction. Using skin-derived patient fibroblasts (p-FBs), our preliminary
studies found that P158A mutation causes reduced bioenergetics, mitochondrial dysfunction, and decreased
enzymatic activity of NAMPT for NAD+ biosynthesis compared with healthy control fibroblasts (c-FBs). The
results indicate the pathological conditions related to the patients is initially resulted from bioenergetic stress and
ultimately from neuronal and muscular degeneration. Thus, our project goal is to understand the pathogenesis
and the mechanism of neuronal and muscular degeneration of this new disease. To achieve our goal, we
generated many molecular tools including P158A-NAMPT mutant mice, c- & p-FBs-derived induced pluripotent
stem cells (c- & p-iPSCs including isogenic and patient like p-iPSCs), and iPSC-induced motor neurons (c- & p-
iMNs). We propose three Specific Aims. Aim 1 will test the hypothesis P158A mutation in NAMPT causes
mitochondrial and synaptic dysfunction of p-iMNs. Using iMNs, we will study the effect of P158A mutation
on cellular bioenergetics, glycolytic metabolism and mitochondrial respiration. We will also conduct combined
metabolomic and transcriptional profiling to determine the molecular base of metabolic changes caused by
P158A mutation. Aim 2 will test the hypothesis that P158A mutation in NAMPT causes MN degeneration.
Using the mutant mice, we will study disease progression, upper and lower MN degeneration. We will use
electrophysiological and two-photon (2-P) imaging to study the effect of P158A mutation on sensory response
and cytosolic and mitochondrial Ca2+ signaling. Aim 3 will test the hypothesis that P158A mutation in NAMPT
causes NMJ abnormalities and muscle degeneration. We will assess structural and functional abnormalities
of NMJs and muscle contractile response of semitendinosus muscles isolated from the symptomatic mutant
mice. A human disease caused by NAMPT mutation has not been reported so far. Our application represents a
first in-depth study on the pathogenesis and mechanism of motor neuron and muscle degeneration of a new
neurological disease caused by a mutation in NAMPT gene.
