We and others recently reported that mutations in the NMNAT1 gene are a common cause of Leber congenital amaurosis (LCA), accounting for ~5% of cases (1-5). NMNAT1 encodes an essential enzyme that generates NAD+ both in a biosynthetic pathway from nicotinic acid mononucleotide (NaMN) and in a salvage pathway from nicotinamide mononucleotide (NMN) (6). Three functionally non-redundant mammalian NMNAT isoforms encoded by different genes have been identified within distinct cellular compartments, where NMNAT1, 2 and 3 localize, respectively, to the nucleus, Golgi complex, and mitochondria (7,8). The mitochondrial isoform, NMNAT3, regenerates NAD+ for cellular energetics, whereas NMNAT1 is involved in nuclear NAD+ homeostasis necessary for both DNA metabolism and cell signaling (Figure 1) (8).
Figure 1. NMNAT proteins and NAD+ metabolism. The major reactions of NAD(P)-mediated signaling and potential compartmentation of the final steps of NAD(P) biosynthesis are summarized. Abbreviations: ART,mono-ADP-ribosyltransferase; TCA, tricarboxylic acid cycle; cADPR, cyclic ADP-ribose; ER, endoplasmic reticulum; NAADP, nicotinic acid adenine dinucleotide phosphate; NADase, bifunctional NAD glycohydrolase/ADP-ribosyl cyclase; NMN, nicotinamide mononucleotide; NMNAT, nicotinamide mononucleotide adenylyltransferase; PARP, poly-ADP-ribose polymerase. Modified from Berger et al 2004.
The identification of NMNAT1 as an LCA disease gene raises the intriguing question of how mutations in a widely expressed NAD+ biosynthetic protein lead to a retina-specific phenotype. The answer to this question is of particular interest because the majority of patients with NMNAT1-LCA have atrophic macular lesions, suggesting that macular cones are especially dependent upon NMNAT1 activity (Figure 2) (2-5). Data from studies performed to date suggest the mutations identified in NMNAT1 lead to decreased NAD+ biosynthetic activity (2,3). We therefore hypothesize that the retinal degeneration caused by mutations in NMNAT1 results primarily from decreased nuclear NAD+ synthesis and the associated alteration in NAD+ homeostasis in the nuclei of retinal cells. This hypothesis also suggests that therapies directed at restoring NAD+ biosynthesis should be beneficial for NMNAT1 LCA. Since cells with decreased NMNAT1 enzyme activity do not appear to be able to utilize NAD+ precursors to increase NAD+ levels, increasing NMNAT1 enzyme activity via gene therapy is an attractive treatment approach (2).
Of interest, NMNAT1 is the principle component of the Wallerian degeneration slow (WldS) fusion protein, which also includes a 70 amino acid N-terminal sequence from the Ube4b multi-ubiquitination factor. The chimeric WldS gene and protein were identified through study of a line of spontaneous mutant mice in which axon stumps that are distal to an injury survive ten times longer than normal; that is, the mice exhibit slow Wallerian degeneration (9-11). It has since been determined that the axonal protection activity of the WldS protein in mice requires both the Ube4b component and an enzymatically active Nmnat1 portion of the chimeric protein (12). Further, expression of the normally nuclear Nmnat1 protein in extranuclear locations can protect axons, suggesting that extranuclear Nmnat enzymatic activity is required for axon protection (13-16). Despite the robust effects of the WldS protein on axon degeneration, it does not appear that WldS protects neuron cell bodies from degeneration (9). For example, retinal ganglion cell axons in WldS mice are protected following induction of glaucoma or optic nerve crush, but RGC cell bodies are not (17,18). Since the retinal degeneration caused by mutations in NMNAT1 causes notable retinal cell loss, especially in the macula, we are focusing our experiments on the potential role of nuclear NAD+ in retinal biology, rather than on the axon protection activity of NMNAT1.
Figure 2. Atrophic macular lesion in patient with NMNAT1 LCA. From Falk, Zhang et al 2012.
We are currently:
- Investigating the pathogenesis of NMNAT1 disease via biochemical studies and gene targeted Nmnat1 mutant mice
- Working to develop adeno-associated virus (AAV)-mediated gene augmentation therapy for NMNAT1 disease
Nature Genetics publication: http://www.ncbi.nlm.nih.gov/pubmed/22842227
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