Metachromatic leukodystrophy in the Navajo Indian population: a splice site mutation in intron 4 of the arylsulfatase A gene.

Metachromatic leukodystrophy (MLD) is an autosomal recessive disorder of myelin metabolism, resulting from the inability to properly degrade 3-sulfogalactosylceramide (sulfatide). This metabolic block is often due to defective functioning of the lysosomal enzyme arylsulfatase A (ARSA). Unmetabolized sulfatide accumulates in the white matter of the CNS and in the peripheral nerves, leading to progressive demyelination and death. Late infantile, juvenile and adult clinical variants of MLD have been described. A Navajo Indian child was diagnosed with late infantile MLD (LIMLD), and his ARSA gene was amplified in three overlapping regions by the PCR and sequenced. A single mutation was found: a G-->A transition in the first nucleotide of intron 4 (IVS4nt1), which abolishes the 5' splice site consensus sequence. Negligible amounts of ARSA mRNA were observed in Northern blots. However, PCR amplification and sequencing of the ARSA cDNA showed that all of the mRNA species from the patient have exon 4 deleted. A new reading frame is thus established which results in a premature stop codon within exon 5. A minority of transcripts had additional splicing errors. Both parents carry this mutation, and the father also carries the pseudodeficiency (PD) allele. Three additional unrelated Navajo LIMLD patients were found to be homozygous for the same MLD-causing mutation by allele-specific oligonucleotide (ASO) hybridization. This method could be used for carrier and patient identification in this population.