Docosahexaenoic acid in red blood cells of patients with X-linked retinitis pigmentosa.

PURPOSE: Abnormalities in the distribution of long-chain polyunsaturated fatty acids (LCPUFA) have been documented in plasma of patients with X-linked retinitis pigmentosa (XLRP). In this study, fatty acid profiles of red blood cells (RBC) were used as an index for LCPUFA metabolism in patients with XLRP because RBC lipids reflect membrane-associated fatty acids. Correlations between LCPUFA content and electroretinographic (ERG) function were assessed.
METHODS: Mean ages for the XLRP group (n = 18) and control group (n = 28) were 22 +/- 18 years and 24 +/- 16 years, respectively. Electroretinographic assessment included the International Society for the Clinical Electrophysiology of Vision standard protocol. Methyl esters of RBC fatty acids were analyzed by capillary column gas chromatography.
RESULTS: The content of the omega 3 fatty acid, docosahexaenoic acid (DHA), was 40% lower in the group with XLRP (23.1 +/- 5.9 micrograms/ml RBC [mean +/- 1 SD]) than in normal subjects (38.6 +/- 9.4 micrograms/ml RBC, t = 6.24, P < 0.0001). Total omega 3 LCPUFA content in patients with XLRP was reduced by 30% from normal levels compared to a 10% reduction in omega 6 LCPUFA levels. Elongation reactions for omega 3, omega 6, saturated fatty acids, and monounsaturated fatty acids were markedly lower for patients with XLRP than for normal subjects. Multiple regression analysis revealed that RBC-DHA was a significant determinant for amplitude and implicit time of cone ERG responses.
CONCLUSIONS: The overwhelming majority of patients with XLRP have lower levels of DHA in RBCs compared to normally sighted control subjects. An analysis of fatty acid profiles suggests a metabolic defect in fatty acid chain elongation mechanisms. The significant association between DHA content and cone ERG response parameters is consistent with an effect of lipid abnormalities on membrane environment and physiology in retinal photoreceptors.