X-linked adrenoleukodystrophy (X-ALD; McKusick 300100) is an X-chromosome linked peroxisomal disorder that affects the white matter of the nervous system and the adrenal cortex. This disease is estimated to affect approxi­mately one in every 20,000 newborns in Serbia. Three main phenotypes are seen in males: i) the childhood cere­bral form (CCALD) manifests most commonly between 4 and 8 years of age in individuals who develop normally for several years but then present with progressive behavioral, visual and auditory disturbances and an abnormal gait, usually leading to death within 3 years; ii) adrenomyelo­neuropathy (AMN) manifests most commonly in the late 20s as progressive paraparesis, sphincter disturbances and varying degrees of distal sensory loss, and progresses slowly over decades; iii) “Addison’s Disease only” pres­ents with primary adrenocortical insufficiency between 2 years of age and adulthood without evidence of neuro­logical abnormality [1]. There is, at present, no satisfac­tory therapy for X-ALD. Some documented benefits result from bone marrow transplantation when performed at an early stage of the disease [2]. Recently, the cholesterol lowering drug lovastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A (HMGCoA) inhibitor, was proposed as a therapeutic agent for X-ALD [3,4].

      Diagnosis of X-Linked Adrenoleukodystrophy. The X-ALD gene (ABCD1) was identified using positional cloning strategies [5]. It is approximately 20 kb long and contains 10 exons. It came as a surprise that its protein product, adrenoleukodystrophy protein (ALDP), is not a very long chain acyl-CoA synthetase (VLCS) as had been anticipated. Immunocytochemical studies demonstrated that ALDP is a peroxisomal membrane protein [6,7], in agreement with the observed deficiency of peroxisomal β oxidation of saturated very long chain fatty acids (VLCFA) in X-ALD patients.

      The function of ALDP and its role in either VLCFA metabolism and/or VLCS activity remains to be deter­mined. Because of its similarity to transport proteins it is assumed that ALDP is a transporter. But its potential sub­strate remains unidentified.

      The most important laboratory assay is the measure­ment of the concentration of VLCFAs in plasma. In virtu­ally all male patients with X-ALD the plasma concentra­tion of hexacosanoic acid (C26:0) and thetetracosanoic/ docosanoic (C24:0/C22:0) and hexacosanoic/docosanoic (C26:0/C22:0) ratios are clearly increased: exceptionally the plasma C26:0 is borderline normal or mildly increased, but the ratios are abnormal, and the diagnosis is estab­lished by elevated concentrations of VLCFAs in cultured skin fibroblasts. The most frequently used procedure is that developed by Moser et al. [8,9]. This involves prepa­ration of a total lipid extract, followed by preparation of the fatty acid methylesters, and chromatography on thin layer plates with subsequent analysis on a gas chromato­graph, with or without mass spectrometry detection. Rarely, molecular genetic testing (sequence analysis or Southern blotting) is required to confirm the diagnosis when measurement of VLCFAs is inconclusive and for investigation of hemizygotes or obligate heterozygotes.

      Pathogenesis. The relationship between the deficient oxidation of VLCFAs and the pathogenesis of the disease has remained enigmatic. Studies have suggested that the adrenal insufficiency and atrophy is due to an increase in membrane microviscosity in the cortical cells, brought about by increased concentration of VLCFAs [10].

      While adrenal dysfunction results from accumulation of cholesterol esterified with VLCFA, pathogenesis of the central nervous system is more complex. Two types of nervous system pathology occur [11]. The first involves parts of axons most distant from the cell body and is asso­ciated with loss of myelin in tracts that contain these axons. It presents as slowly progressive spasticity of the lower extremities in young adult and progresses slowly over decades. It is referred to as adrenomyeloneuropathy (AMN). In the second there is rapidly progressive demy­elination, which affects mainly parieto-occipital lobes of the brain and spares the axons at least initially. There is intensive perivascular infiltration by T cells, B cells and macrophages and prominent immunoreactivity for cyto­kines such as tumor necrosis factor in astrocytes and macro­phages. This type occurs mainly in young boys and progresses rapidly, often leading to an apparently vegeta­tive state within 2 years [12]. It has been suggested that the myelinolysis in adrenoleukodystrophy results from the high content of VLCFAs in myelin lipids [13]. Since gan­gliosides are substantial components of myelin and neu­ronal membranes, those that are rich in VLCFAs may play a major role in the induction of myelin instability. It has been argued [11] that VLCFA-containing gangliosides are highly immunogenic, elicit an immune reaction, with mi­gration of macrophages and lymphocytes which attack the myelinated fibres remaining and the oligodendrocytes, thus leading to a fulminating and widespread lymphocyte-mediated destruction of cerebral myelin. These two pheno­types often occur within the same family.

      Genetic Counseling and Prenatal Diagnosis. X-ALD is inherited in an X-linked recessive manner. The ABCD1 gene is the only gene associated with X-ALD. A female carrier has a 50 % chance of transmitting the ABCD1 mutation with each pregnancy. All sons who inherit the mutation become affected, daughters who inherit the muta­tion are carriers and are usually not seriously affected.

      The range of phenotypic expression in X-ALD and the prognosis for an affected male is unpredictably variable and can not be predicted through levels of VLCFA in plas­ma and cultured fibroblasts [14], the residual VLCFA beta-oxidation activity in skin fibroblasts, the family his­tory or the nature of the mutation identified in the patients ABCD1 gene [15]. More than 800 different mutations have been identified in the ABCD1 gene [16]. The same mutation can be associated with each of the known clinical phenotypes. Mild phenotypes may be associated with large delations that abolish formation of the gene product, and severe phenotypes occur with missense mutations in which normal amounts of ALDP protein is produced. The most common ABCD1 mutation, a two base pair deletion in exon 5 found in approximately 10 % of X-ALD families, has been associated with each of the known X-ALD clini­cal phenotypes. Segregation analysis suggests that the phenotypic variability is due to an autosomal modifier gene [17,18]. It is important for couples at risk to be aware that widely varying phenotypes often coexist in the same kindred or sibship. Thus, families that have experienced the relatively mild phenotypes need to be advised that affected offspring may display the severe phenotype.

      Prenatal testing is possible for pregnant women who is carrier. In these the risk of having an affected male is 25 % (or 50 % if the fetus is known to be male). The usual procedure is to determine sex by karyotyping fetal cells obtained by chorionic villus sampling (CVS) at about 10-12 weeks gestation or by amniocentesis at 16-18 weeks gestation. If the karyotype is 46,XY (male) and if the dis­ease causing mutation has been identified in family mem­ber, DNA from fetal cells can be analyzed for the muta­tion. If mutation analysis is not possible, VLCFA can be measured in cultured amniocytes or cultured chorionic villus cells but false negative results are possible and are related to technical factors.

      Concluding Remarks. The pathogenesis of X-ALD is still not completely understood, much progress has been made in its diagnosis and our better understanding of the disease has lead to several therapeutic options. Therefore, it is hoped and feasible that in the near future gene therapy may become available for those affected by this severe and potentially lethal disease.