Adrenoleukodystrophy (ALD) is an X-linked disease affecting 1/20,000 males either as cerebral ALD in childhood or as adrenomyeloneuropathy (AMN) in adults. Childhood ALD is the more severe form, with onset of neurological symptoms between 5-12 years of age. Central nervous system demyelination progresses rapidly and death occurs within a few years. AMN is a milder form of the disease with onset at 15-30 years of age and a more progressive course. Adrenal insufficiency (Addison's disease) may remain the only clinical manifestation of ALD. The principal biochemical abnormality of ALD is the accumulation of very-long-chain fatty acids (VLCFA) because of impaired beta-oxidation in peroxisomes. The normal oxidation of VLCFA-CoA in patients' fibroblasts suggested that the gene coding for the VLCFA-CoA synthetase could be a candidate gene for ALD. Here we use positional cloning to identify a gene partially deleted in 6 of 85 independent patients with ALD. In familial cases, the deletions segregated with the disease. An identical deletion was detected in two brothers presenting with different clinical ALD phenotypes. Candidate exons were identified by computer analysis of genomic sequences and used to isolate complementary DNAs by exon connection and screening of cDNA libraries. The deduced protein sequence shows significant sequence identity to a peroxisomal membrane protein of M(r) 70K that is involved in peroxisome biogenesis and belongs to the 'ATP-binding cassette' superfamily of transporters.
Dendritic abnormalities are the most consistent anatomical correlates of mental retardation (MR). Earliest descriptions included dendritic spine dysgenesis, which was first associated with unclassified MR, but can also be found in genetic syndromes associated with MR. Genetic disorders with well-defined dendritic anomalies involving branches and/or spines include Down, Rett and fragile-X syndromes. Cytoarchitectonic analyses also suggest dendritic pathology in Williams and Rubinstein-Taybi syndromes. Dendritic abnormalities appear to have syndrome-specific pathogenesis and evolution, which correlate to some extent with their cognitive profile. The significance of dendritic pathology in synaptic circuitry and the role of animal models in the study of MR-associated dendritic abnormalities are also discussed. Finally, a model of genotype to neurologic phenotype pathway in MR, centered in dendritic abnormalities, is postulated.
The peroxisome biogenesis disorders (PBDs) are lethal recessive diseases caused by defects in peroxisome assembly. We have isolated PXR1, a human homologue of the yeast P. pastoris PAS8 (peroxisome assembly) gene. PXR1, like PAS8, encodes a receptor for proteins with the type-1 peroxisomal targeting signal (PTS1). Mutations in PXR1 define complementation group 2 of PBDs and expression of PXR1 rescues the PTS1 import defect of fibroblasts from these patients. Based on the observation that PXR1 exists both in the cytosol and in association with peroxisomes, we propose that PXR1 protein recognizes PTS1-containing proteins in the cytosol and directs them to the peroxisome.
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