NMNAT1 E257K variant, associated with Leber Congenital Amaurosis (LCA9), causes a mild retinal degeneration phenotype

Abstract

NMNAT1 (nicotinamide mononucleotide adenylyltransferase 1) encodes a rate-limiting enzyme that catalyzes the biosynthesis of NAD and plays a role in neuroprotection. Mutations in NMNAT1 have been identified to cause a recessive, non-syndromic early form of blindness genetically defined as Leber Congenital Amaurosis 9 (LCA9). One of the most common alleles reported so far in NMNAT1 is the c.769G > A (E257K) missense mutation, which occurs in 70% of all LCA9 cases. However, given its relatively high population frequency and the observation of individuals with homozygous E257K variant without phenotype, the pathogenicity of this allele has been questioned. To address this issue, we have studied the pathogenic effects of this allele by generating a knock-in mouse model. Interestingly, no obvious morphological or functional defects are observed in Nmnat1 E257K homozygous mice up to one year old, even after light-damage. Together with the previous clinical reports, we propose that the E257K allele is a weak hypomorphic allele that has significantly reduced penetrance in the homozygous state. In contrast, compound heterozygous Nmnat1 mice exhibit photoreceptor defects which are exacerbated upon exposure to light. Furthermore, retina tissue- specific Nmnat1 conditional knockout mice exhibit photoreceptor degeneration before the retina has terminally differentiated. These findings suggest that NMNAT1 plays an important role in photoreceptors and is likely involved in both retinal development and maintenance of photoreceptor integrity.

SARM1 depletion rescues NMNAT1 dependent photoreceptor cell death and retinal degeneration

“…In mutant animals in which NMNAT1 was excised, we observed a complete loss of both scotopic (rod-driven responses) and photopic (cone-driven responses) responses, indicating the loss of Nmnat1 in mature retina causes severe photoreceptor dysfunction ( Figure 3D-F). This is consistent with previous reports showing developmental retinal defects in the tissue specific NMNAT1 knockout mice 11,12 . While previous reports show that NMNAT1 is necessary for appropriate retinal development, our pathological and functional analyses of two-month-old mice show that NMNAT1 is necessary for photoreceptor cell maintenance and mature retinal functions.…”

“…We analyzed the retinas of Nmnat1 fl/fl :Rhodopsin-Cre mice at 6-weeks-of-age. Similar to previous findings 11,12 , histological analysis revealed severe thinning of the ONL in these mutant mice ( Figure 4A, B), with a significant reduction in cell number as detected by nuclear counts ( Figure 4E). There was also a much smaller decrease in the number of cells in the INL.…”

“…Among the three mammalian NMNAT isoforms, NMNAT1 is the only enzyme localized to the nucleus 24 . However, in photoreceptors NMNAT1 also localizes near the cilia basal body 11 , consistent with an additional, extra-nuclear role of NMNAT1 in photoreceptor cells. This is of particular interest because engineered non-nuclear variants of enzymatically-active NMNAT1 can potently inhibit pathological axon degeneration, which is commonly observed in the early stages of many neurodegenerative disorders [25][26][27] .…”

“…in mouse retinal ganglion cells (RGCs) robustly protects against ischemic and glaucomatous loss of the axon and soma 10 , while conditional ablation in the developing mouse retina causes severe retinal dystrophy and loss of retinal function 11,12 . Mice harboring Nmnat1 mutations (V9M and D243G) exhibit severe retinal degeneration while the most common LCA9 mutation (E257K), which is not fully penetrant 13 , induces a milder retinal degeneration phenotype 11,14 .…”

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

“…Further, Category 03 gene Nmnat1 encodes an enzyme that synthesizes nicotinamide adenine dinucleotide in the nucleus, which may regulate the large-scale polyADP-ribosylation of protein targets at sites of DNA damage [402]. Mutations in the genes encoding these proteins all result in PR cell loss [230,384,385,400,401,[403][404][405][406][407][408][409][410][411]. Mutations in five of these genes as included in Figure 6 (Cwc27, Ercc1, Ercc6, Sirt6, and Ubb) caused moderate to slow progression of PR cell loss (D 50 ≥ 2 months), consistent with a steady accumulation of unresolved DNA damage with age.…”