Genetic cause of leukocyte adhesion molecule deficiency: Abnormal splicing and a missense mutation in a conserved region of CD18 impair cell surface expression of β2 integrins

Patients with leukocyte adhesion molecule (CD11/ CD18, β2 integrins) deficiency have structural defects in the common β subunit (CD18), which prevent heterodimer formation and normal cell surface expression of these receptors, leading to life-threatening bacterial infections. To elucidate the nature of these defects in a patient with partial (type H) deficiency, abnormal CD18 cDNA clones were isolated, using the polymerase chain reaction to amplify the patient's B cell-derived cDNAs. Sequence analysis revealed two mutant alleles. cDNA clones, representing a maternal allele, contained both a 12-base pair insertion resulting in an in-frame addition of four amino acids between P²⁴⁷ and E²⁴⁸ and a C¹⁷⁵⁶→T nucleotide transition, resulting in an R⁵⁸⁶→ W substitution in the normal CD18 protein. The inframe insertion arose by a single nucleotide C→A transversion in the 3′ terminus of an intron, generating an aberrant splice acceptor site. Other cDNA clones contained an A¹⁰⁵²→G nucleotide transition not present in either parent which resulted in an N³⁵¹→S substitution. To determine the functional importance of these changes, cDNA encoding a normal α chain (CD11b) was cotransfected into COS with CD18 cDNAs encoding for wild-type, maternal mutant allele, or CD18 containing N³⁵¹→S substitution. Immunostaining of transfectants with anti-CD18 monoclonal antibodies revealed no cell surface expression of the maternal mutant CD18, and 22% surface expression of N³⁵¹→S CD18. Both the insertion and the N³⁵¹→S mutations occurred in a 250 amino acid extracellular region of CD18 that is highly conserved among β integrins supporting a role for this region in heterodimer formation.