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Regis Fallon, Jana Zernant, Takayuki Nagasaki, Marin Gantner, sarah harkins-perry, Kevin Eade, Rando Allikmets, Martin Friedlander; Rare variants in the phosphoglycerate dehydrogenase (PHGDH) gene in MacTel patients lead to decreased enzymatic activity. Invest. Ophthalmol. Vis. Sci. 2018;59(9):5409.
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© ARVO (1962-2015); The Authors (2016-present)
Macular Telangiectasia (MacTel) is a neurodegenerative disease affecting the macula and leading to progressive vision loss. Recent genetic and metabolism work has shown a link between the disease and serine biogenesis. Phosphoglycerate dehydrogenase (PHGDH) is the rate limiting enzyme involved in L-serine synthesis. Genome wide association studies (GWAS) identified the PHGDH locus as one of 5 loci statistically significantly associated with MacTel. However, a direct link between the PHGDH function and MacTel has yet to be determined.
Whole exome sequencing (WES) analysis was performed in 320 MacTel patients to identify specific rare coding variants associated with the disease. To determine the effect of the identified variants on PHGDH activity, we transfected HEK 293T cells with overexpression vectors containing either the WT protein or one of the variants, and collected the cell lysates. Western blots of the lysates confirmed the overexpression compared to endogenous levels of PHGDH. We developed an activity assay that utilizes the reverse reaction of PHGDH, which reduces 3-phosphohydroxypyruvate through NADH oxidation, to spectrophotometrically measure PHGDH activity. Thus, the decrease of NADH absorption over time was used to analyze each variant’s activity.
Analysis of WES data identified 13 rare variants (MAF<0.001) in 18 patients in the coding sequences of PHGDH, all predicted to affect the protein activity. The difference was statistically significant (p= 0.00012) when compared to matched controls from the general population. Of the 13 variants, two introduce stop codons and the rest are missense alleles with predicted strong functional effect. One variant was present in 7 patients, including a patient that had early onset of the disease. Bi-allelic mutations in PHDGH cause severe syndromic features including developmental delay and microcephaly. Heterozygous carriers of these alleles are developmentally normal and may manifest unexpected phenotypes such as MacTel. In vitro analyses showed that, compared to WT PHGDH, the variants carried by the MacTel patients lead to decreased enzymatic activity.
The PHGDH variants observed in MacTel patients are functionally compromised and may lead to decreased serine and glycine production in vivo. Our work provides the first direct link between PHGDH function, serine biosynthesis and MacTel.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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