A FRAME-SHIFT MUTATION IN A HOT-SPOT REGION OF THE NUCLEAR AUTOANTIGEN LA (SS-B)

A hot spot region was identified in the exon 7 of the nuclear autoanti gen La (SS-B). Two La cDNAs were identified which contained a frame sh ift mutation in the hot spot region. One La cDNA was isolated from a c DNA library made from peripheral blood lymphocytes of an autoimmune pa tient with primary Sjogren's Syndrome, the other La cDNA was isolated from a human liver cDNA Library. The patient's La cDNA had a deletion and the liver La cDNA had an insert of an (A)-residue at the same posi tion. Inserts of 4, 16 and 24 more or less homogeneous (A)-residues we re found at the same site in the three La retropseudogenes. The hot sp ot region located in one of the major autoepitope regions of the La an tigen. Both frame shift mutations resulted in premature stop codons. Z n case of the human liver La cDNA, the premature stop codon located a single amino acid downstream of the frame shift mutation, while it loc ated eleven amino acids downstream of the frame shift mutation in the patient's La cDNA. In consequence, only the sequence of the La peptide encoded by the patient's La cDNA markedly differed from the correspon ding La peptide sequence. Translation of the patients mutant La mRNA i n transfected mouse cells resulted in a C-terminally truncated La pept ide. Due to the lack of the nuclear location signal it remained in the cytoplasm. The modified La peptide shared homology with (i) La protei n itself and (ii) a series of DNA binding proteins including other aut oantigens and viral proteins such as topoisomerase I, RNA dependent RN A polymerase of influenza virus and reverse transcriptase. The self-ho mology region includes the amino acids which the La protein shares wit h B1 Laminin. It represents a putative neo-epitope that could be invol ved in triggering of the autoimmune response. (C) 1996 Academic Press Limited