A functional SNP in proteolipid protein 2 (PLP2) promoter increases ER stress induced apoptosis and confers increased risk of neonatal hypoxic-ischemic brain injury. L. Zhang¹, T. Wang¹, D. Valle^1,2. 1) Dept Human Genetics, IGM, JHMI; 2) Howard Hughes Med. Inst., Baltimore, MD.
   We identified a SNP (-113C>A) in the promoter of proteolipid protein 2 (PLP2) gene that results in significant reduction (> 4 fold) of PLP2 mRNA and protein. PLP2-(-113C>A) is over-represented in males with X-linked mental retardation (frequency: control 1.56%, XLMR 5.86%, P<0.001). To delineate PLP2 function in CNS, we generated PLP2 deficient mice by gene targeting. We found PLP2-deficient MEFs have elevated levels of pro-caspase 12 and cleaved caspase 12, an ER stress specific caspase, as well as increased GRP78, an ER chaperone, suggesting increased basal level of ER stress. By immunofluroescence and electron microscopy, we showed PLP2 deficient MEFs have dilated ER lumen and inefficient protein trafficking from ER to Golgi as visualized by ts-VSVG-GFP, supporting the idea of increased basal ER stress. Furthermore, we found the increased ER stress is specifically associated with enhanced eIF2-alpha signaling, as evident by the increased eIF2-alpha phosphorylation and increased expression of downstream genes, such as ATF4 and CHOP. PLP2-deficient MEFs and cultured primary neurons show increased apoptosis following ER stress induction, but are indistinguishable from control cells in response to intrinsic and extrinsic apoptosis stimuli. In addition, PLP2-deficient neurons show increased sensitivity to pharmacologically induced hypoxia. Using a model of neonatal hypoxia-ischemia, we demonstrated that the hippocampal injury of PLP2 deficient neonatal mice is 10 fold more than the wild type littermates. Finally, we found that male human fibroblasts with PLP2-(-113C>A) also have increased susceptibility to cell death when exposed to ER stressors. In summary, our results suggest that the human PLP2 polymorphism is an important modifier of genetic or environmental stress of the ER and that individuals with PLP2-(-113C>A) have increased risk to neonatal brain injury related to genetic and/or environmental stress on the ER machinery. This mechanism may account for the association of the PLP2 promoter polymorphism and XLMR.