DNA repair capacity: inconsistency between effect of over-expression of five NER genes and the correlation to mRNA levels in primary lymphocytes

We have previously shown that high DNA repair capacity protects psoriasis p atients against chemically induced basal cell carcinoma [Dybdahl et al. Mut at. Res. 433 (1999) 15-22], We have used the same study persons to investig ate the correlation between expression of eight genes involved in nucleotid e excision repair and DNA repair capacity, mRNA levels of XPA, XPB, XPC, XP D, XPF, XPG, CSB and ERCC1 in primary lymphocytes from 33 individuals were quantified by dot-blots and normalized to beta -actin. ERCC1 and XPD mRNA q uantities were highly correlated (r = 0.89; P < 10(-11)) while XPA, XPB, XP C, XPG, XPFand CSB mRNAs were moderately correlated (r = 0.2-0.7). Thus, th e mRNA expressions seem to fall in at least two groups. There was a three t o sevenfold variation in the expression levels of the mRNAs. This is in con trast to the more than a hundredfold variation in mRNA levels reported in c ancer patients. DNA repair capacity was measured in a host cell reactivation assay, where p rimary lymphocytes were transfected with an UV-irradiated plasmid encoding firefly-luciferase. Only ERCC1 and XPD mRNA levels correlated with the DNA repair capacity (P < 0.03), In order to see if ERCC1 or XPD activity was li miting for DNA repair, we cotransfected with plasmids encoding NER genes, t hus over-expressing either XPB, XPC, XPD, CSB or ERCC1 in the host cell rea ctivation assay. Only XPB over-expression increased DNA repair capacity. Th us, there is no indication that neither XPD nor ERCC1 limits the DNA repair capacity. However, our results indicate that ERCC1 and XPD mRNA levels may be used as a proxy for DNA repair capacity in lymphocytes. (C) 2000 Elsevi er Science B.V. All rights reserved.