A point mutation in the 5' splice region of intron 7 causes a deletion of exon 7 in adenosine deaminase mRNA.

An adenosine deaminase (ADA;EC 3.5.4.4)-deficient B lymphoblastoid cell line BAD05 derived from a Japanese patient with severe combined immunodeficiency was characterized. As previously reported, one allele of BAD05 expresses undetectable ADA mRNA, and the other allele produces an aberrant mRNA without exon 7. Genomic ADA DNA of BAD05 spanning from a portion of exon 6 to a portion of exon 8 was amplified by PCR. The amplified fragments were cloned into a vector, and 8 clones were isolated and sequenced. The analytical result showed a single base change of G to A at the invariant 5' GT of intron 7 of ADA gene in one allele of BAD05, which accounts for the elimination of exon 7 during splicing.