Phospholemman (PLM) regulates the cardiac Na pump. Unphosphorylated PLM inhibits the pump, and PLM phosphorylation at S63 and T69 (by PKC), and S68 (by PKA or PKC) causes pump activation. Abnormal phosphorylation of PLM is well established in cardiac hypertrophy and failure. We investigated whether genetic polymorphisms in PLM underlie left ventricular hypertrophy (LVH) by sequencing PCR-amplified PLM exons from anonymised genomic DNA from the Go-DARTS database. Inclusion criteria were age <55, systolic blood pressure <140mmHg, BMI <30kgm-2, and moderate-severe LVH according to echocardiography. 2 out of 43 individuals possessed the same non-synonymous missense coding variant: a C to T transition in position 1 of the codon encoding arginine 70 in PLM exon 7, resulting in the mutation R70C. The R70C PLM phenotype was investigated by expressing wild type (WT) and R70C PLM in HEK293 cells. Phosphorylation induced by activating PKA (10µM forskolin) and PKC (300nM PMA) was measured by phosphospecific immunoblotting, and expressed relative to WT or R70C-expressing cells treated with vehicle. Phosphorylation of WT and R70C PLM by PKA at PLM-S68 was unchanged (2.0±0.3 fold increase over vehicle-treated for WT, 1.8±0.1 fold increase for R70C, mean±SEM, n=4). PKC-induced phosphorylation of PLM-S63 was also unchanged, but R70C PLM was phosphorylated less than WT at S68 following activation of PKC (1.8±0.2 for WT, 1.0±0.2 for R70C, n=4, p<0.05, t-test), suggesting mutation of the arginine at position +2 to S68 had altered this consensus PKC phosphorylation site. We also measured cell surface localisation (membrane-impermeable biotinylation reagents) and palmitoylation (resin-assisted capture of acylated proteins) of WT and R70C PLM. A greater fraction of R70C PLM than WT was found at the cell surface (R70C 3.8±2 fold more enriched than WT, n=3), suggesting a proposed ER retention motif in PLM was also disrupted. A greater fraction of R70C PLM was palmitoylated than WT (53±10% vs 29±5%, n=4, p<0.05), possibly due to this enhanced traffic to the cell surface. Rs61753924, the single nucleotide polymorphism (SNP) underlying PLM R70C, occurs at 0.1% frequency in the 1000 Genomes Project. We investigated the frequency of this SNP in LVH case and non-LVH controls: 19 out of 393 LVH cases (4.8%) and 10 out of 266 (3.7%) controls were positive for rs61753924 (odds ratio = 1.14, p>0.05). In conclusion, PLM R70C shows reduced phosphorylation at S68 by PKC. In vivo PLM is significantly phosphorylated at rest by PKC, so the reduced phosphorylation and enhanced cell surface localisation and palmitoylation of R70C PLM may lead to reduced Na efflux via the pump. Preliminary analysis indicates that the presence of SNP rs617539241 is not a risk factor for the development of LVH, but is enriched in the local population.