Characterization of complement C6 deficiency in a PVG/c rat strain.

Complement C6 plays an important role in the effector phase of complement-mediated cell lysis. Recently, a PVG/c rat strain deficient in haemolytic C6 activity was discovered. In the present study we show that these rats lack both antigenic and functional C6, and that repetitive immunization of these rats with PVG/c+ serum results in generation of specific anti-rat C6 antibodies. The observed absence of rat C6 was further investigated at the genomic and transcriptional level using a 492-bp cDNA of rat C6, cloned from a rat liver cDNA library using full length human C6 as a probe. Northern blot analysis revealed the presence of C6 mRNA in livers of both PVG/c- and PVG/c+ rats, corresponding to a size of approximately 3.3 kb, although the level of C6 mRNA expression was approximately 100-fold less in PVG/c- rats. In addition, using rat C6-specific primers, positive signals were obtained in kidneys of both rat strains by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Southern blot analysis of digested genomic DNA did not reveal evidence for large C6 gene deletions. We conclude that the lack of C6 protein in the PVG/c- rat strain is not due to a (large) C6 gene deletion, but presumably is caused by an unstable mRNA or a point mutation in the C6 gene resulting in an aberrant transcription of the C6 gene. Alternatively, a gene coding for a product involved in C6 biosynthesis that acts in trans may carry a mutation.