LEUKOCYTE ADHESION DEFICIENCY - ABERRANT SPLICING OF A CONSERVED INTEGRIN SEQUENCE CAUSES A MODERATE DEFICIENCY PHENOTYPE

Abstract

Leukocyte adhesion deficiency (LAD) is a heritable deficiency of the LFA-1, Mac-1, p150,95 family of leukocyte .alpha..beta. heterodimers (the leukocyte integrins). We have studied the defect in patients who synthesize an aberrantly small form of the .beta. subunit common to all three proteins. S1 nuclease protection showed the presence of a 90-nucleotide mismatch in RNA from patients and relatives, correlating with inheritance of the disease. Use of the Taq polymerase chain reaction to amplify this region of RNA after first strand cDNA synthesis and sequencing showed an in-frame deletion of 90 nucleotides in the extracellular domain. Thus, this highly conserved region, 63% and 53% identical in amino acid sequence to two other .beta. subunits of the integrin family, is required for association of the .beta. subunit with .alpha. subunits. The 90-nucleotide region corresponds to a single exon present in both the normal and patient genome. The patient DNA has a single G to C substitution in the 5'' splice site. This results in the direct joining of nonconsecutive exons in an unusual type of abnormal RNA splicing. A small amount of normally spliced message, detected by S1 nuclease protection and Taq polymerase chain reaction, encodes a normal sized .beta. subunit which is surface-expressed and accounts for the low levels of leukocyte integrin expression observed in these patients, and hence the moderate phenotype.