Identification of novel genetic determinants of erythrocyte membrane fatty acid composition among Greenlanders

Fatty acids (FAs) are involved in cellular processes important for normal body function, and perturbation of FA balance has been linked to metabolic disturbances, including type 2 diabetes. An individual's level of FAs is affected by diet, lifestyle, and genetic variation. We aimed to improve the understanding of the mechanisms and pathways involved in regulation of FA tissue levels, by identifying genetic loci associated with inter-individual differences in erythrocyte membrane FA levels. We assessed the levels of 22 FAs in the phospholipid fraction of erythrocyte membranes from 2,626 Greenlanders in relation to single nucleotide polymorphisms genotyped on the MetaboChip or imputed. We identified six independent association signals. Novel loci were identified on chromosomes 5 and 11 showing strongest association with oleic acid (rs76430747 in ACSL6, beta (SE): -0.386% (0.034), p = 1.8x10-28) and docosahexaenoic acid (rs6035106 in DTD1, 0.137% (0.025), p = 6.4x10-8), respectively. For a missense variant (rs80356779) in CPT1A, we identified a number of novel FA associations, the strongest with 11-eicosenoic acid (0.473% (0.035), p = 2.6x10-38), and for variants in FADS2 (rs174570), LPCAT3 (rs2110073), and CERS4 (rs11881630) we replicated known FA associations. Moreover, we observed metabolic implications of the ACSL6 (rs76430747) and CPT1A (rs80356779) variants, which both were associated with altered HbA1c (0.051% (0.013), p = 5.6x10-6 and -0.034% (0.016), p = 3.1x10-4, respectively). The latter variant was also associated with reduced insulin resistance (HOMA-IR, -0.193 (0.050), p = 3.8x10-6), as well as measures of smaller body size, including weight (-2.676 kg (0.523), p = 2.4x10-7), lean mass (-1.200 kg (0.271), p = 1.7x10-6), height (-0.966 cm (0.230), p = 2.0x10-5), and BMI (-0.638 kg/m2 (0.181), p = 2.8x10-4). In conclusion, we have identified novel genetic determinants of FA composition in phospholipids in erythrocyte membranes, and have shown examples of links between genetic variants associated with altered FA membrane levels and changes in metabolic traits

Carnitine acetyltransferase: A new player in skeletal muscle insulin resistance?

Carnitine acetyltransferase (CRAT) deficiency has previously been shown to result in muscle insulin resistance due to accumulation of long-chain acylcarnitines. However, differences in the acylcarnitine profile and/or changes in gene expression and protein abundance of CRAT in myotubes obtained from obese patients with type 2 diabetes mellitus (T2DM) and glucose-tolerant obese and lean controls remain unclear. The objective of the study was to examine whether myotubes from obese patients with T2DM express differences in gene expression and protein abundance of CRAT and in acylcarnitine species pre-cultured under glucose and insulin concentrations similar to those observed in healthy individuals in the over-night fasted, resting state. Primary myotubes obtained from obese persons with or without T2DM and lean controls (n=9 in each group) were cultivated and harvested for LC-MS-based profiling of acylcarnitines. The mRNA expression and protein abundance of CRAT were determined by qPCR and Western Blotting, respectively. Our results suggest that the mRNA levels and protein abundance of CRAT were similar between groups. Of the 14 different acylcarnitine species measured by LC-MS, the levels of palmitoylcarnitine (C16) and octadecanoylcarnitine (C18) were slightly reduced in myotubes derived from T2DM patients (p<0.05) compared to glucose-tolerant obese and lean controls. This suggests that the CRAT function is not the major contributor to primary insulin resistance in cultured myotubes obtained from obese T2DM patients

HbA1c association for <i>ACSL6</i> (rs76430747) and <i>CPT1A</i> (rs80356779).

<p>Data are shown as raw means stratified by genotype, and as effect sizes estimated without assuming an additive effect model.</p

Association and conditional plot combined for the <i>FADS2</i> and <i>CPT1A</i> loci with erythrocyte membrane 11-eicosenoic acid (20:1 ω-9).

<p>The association results of the unconditional analysis are colored according to the LD, which was calculated separately for the two regions in each plot for the candidate SNPs, rs174570 and rs80356779, respectively. The p-values are based on imputation data. Green dots represent the results of the conditional analyses, and the circle denotes the SNP conditioned on A) rs174570 and B) rs80356779.</p

Association and conditional plot of the <i>ACSL6</i> locus with erythrocyte membrane oleic acid (18:1 ω-9).

<p>The association results of the unconditional analysis are colored according to the LD, which is calculated for the candidate SNP in the region, rs76430747. Green dots represent the results of the conditional analysis, and the circle denotes the SNP conditioned on (rs76430747). The p-values are based on imputation data.</p

Overview of FA-synthesis pathways and candidate-SNP associations.

<p>Effects of candidate SNPs from the six identified erythrocyte membrane FA-associated loci are shown for each of the 22 assessed FAs. <i>ACSL6</i>, rs76430747; <i>FADS2</i>, rs174570; <i>CPT1A</i>, rs80356779; <i>LPCAT3</i>, rs2110073; <i>CERS4</i>, rs11881630; <i>DTD1</i>, rs6035106.</p

<i>FADS2-CPT1A</i> linkage pattern.

<p><i>FADS2-CPT1A</i> linkage pattern.</p