[Peroxisomal diseases--a survey]

Peroxisomes are ubiquitous cytoplasmic structures in mammalian tissues. The metabolic functions of these organelles include synthesis of plasmalogens and other ether lipids, beta-oxidation, especially of very long-chain fatty acids (VLCFAs, > C22) and their derivatives, inactivation of hydrogen peroxide by peroxisomal catalase and involvement in several other metabolic pathways, e.g. gluconeogenesis, catabolism of purines and polyamines and detoxification of ethanol. Peroxisomal diseases which may arise from genetic faults in the biogenesis of the organelle or aberrant targeting of one or more proteins to the peroxisome, are divided into three groups based on the extent of loss of peroxisomal functions. Prototype of the first group is the cerebro-hepato-renal syndrome of Zellweger (ZS) which shows generalised loss of peroxisomal functions and absence of demonstrable mature peroxisomes in the liver. Other syndromes which are briefly discussed include neonatal adrenoleukodystrophy (NALD) and infantile Refsum syndrome (IRS) which may be regarded as milder variants of ZS, and diseases caused by loss of a limited number of peroxisomal functions (rhizomelic chondrodysplasia punctate). However, the group of peroxisomal diseases with the highest incidence are those syndromes where only a single peroxisomal function is impaired. The most common peroxisomal disease, X-linked adrenoleukodystrophy (XALD) belongs to this group. XALD develops as a result of an isolated defect of peroxisomal acyl-CoA synthetase with resultant accumulation of VLCFAs, especially C26:0. Primary hyperoxaluria type 1 is caused by deficient activity of peroxisomal alanine: glyoxylate aminotransferase due to aberrant targeting of this enzyme to mitochondria and not peroxisomes, a unique example of a genetic enzyme trafficking defect. The primary diagnosis of these syndromes is usually based on clinical findings and measurement of accumulated or depleted metabolites in the body e.g. VLCFAs, bile acid intermediates, phytanic acid, pipecolic acid and plasmalogens. Therapy includes dietary adjustments e.g. supplementation with oleic acid derivatives to normalise elevated VLCFAs in XALD. Treatment with hypolipidaemic drugs and certain peroxisomal substrates which induce proliferation of mature peroxisomes offers promise in the therapy of these debilitating and often fatal diseases.