IDENTIFICATION OF MUTATIONS IN THE ALPHA-L-IDURONIDASE GENE (IDUA) THAT CAUSE HURLER AND SCHEIE SYNDROMES

Mucopolysaccharidosis type I (MPS-I) is an autosomal recessive genetic disease caused by a deficiency of the lysosomal glycosidase alpha-L-i duronidase. Hurler (severe), Scheie (mild), and Hurler/Scheie (interme diate) syndromes are clinical subtypes of MPS-I, but it is difficult t o distinguish between these subtypes by biochemical measurements. Muta tion analysis was undertaken to provide a molecular explanation for th e clinical variation seen in MPS-I. Using chemical cleavage and direct PCR sequencing, we have defined four previously undescribed mutations for MPS-I (delG1702, 1060+2t-->c, R89Q, and 678-7g-->a). R89Q and 678 -7g-->a were found to be present in 40% of Scheie syndrome alleles. Ex pression of R89Q demonstrated reduced stability and activity of the mu tant protein. The deleterious effect of R89Q may be potentiated by a p olymorphism (A361T) to produce an intermediate phenotype. 678-7g-->a w as found to be a mild mutation, since it was present in an index Schei e syndrome patient in combination with a severe allele (W402X). This m utation appears to allow a very small amount of normal mRNA to be prod uced from the allele which is likely to be responsible for the mild cl inical phenotype observed. Both the 5' and 3' splice site mutations (1 060+2t-->c and 678-7g-->a, respectively) result in high proportions of mature mRNAs containing introns, which has not been observed for othe r splicing mutations. The frameshift mutation (delG1702) and the 5' sp lice site mutation (1060+2t-->c) are both thought to be associated wit h severe MPS-I. The identification of these MPS-I mutations begins to document the expected genetic heterogeneity in MPS-I and provides the first molecular explanations for the broad range of clinical phenotype s observed.