Molecular basis of IgG Fc receptor III defect in a patient with systemic lupus erythematosus

By flow cytometry analysis we could show a decreased expression of Fc gamma receptor type III (Fc gamma RIII) on granulocytes of a patient with systemic lupus erythematosus (SLE). Therefore, we constructed a leukocyte cDNA library from this patient with the aim of investigating this defect on the molecular level. Using an Fc gamma RIII cDNA probe we isolated 15 Fc gamma RIII cDNA clones, which were all characterized by sequencing. Our sequence data revealed that the patient was heterozygous for Fc gamma RIII (NA-1/NA-2). Only clone 5 (NA-2) was a full-length cDNA clone. In contrast to the wild-type Fc gamma RIII the signal sequence is mutated, lacking the hydrophobic region essential for co-translational transport across the endoplasmic reticulum membrane. The predicted transport defect leading to the lack of membrane expression could be confirmed by immunofluorescence staining after expression of this cDNA clone in BHK cells. The cDNA clones 6 and 8 (NA-1) lack the first 45 bp of the signal sequence, but considering the flow cytometry data the signal sequence must be functional allowing the membrane expression of this receptor allele. The part of the cDNA sequence of all isolated clones coding the mature Fc gamma RIII is identical to the wild-type sequence. Therefore, we conclude that the decreased expression of Fc gamma RIII on granulocytes of this SLE patient is due to the transport defect of one of the receptor alleles.