A backup role of DNA polymerase kappa in Ig gene hypermutation only takes place in the complete absence of DNA polymerase eta.
Patients with the variant form of xeroderma pigmentosum (XPV) syndrome have a genetic deficiency in DNA polymerase (Pol) eta, and display accordingly an increased skin sensitivity to UV light, as well as an altered mutation pattern of their Ig V genes in memory B cells, alteration that consists in a reduced mutagenesis at A/T bases. We previously suggested that another polymerase with a different mutation signature, Pol kappa, is used as backup for Ig gene hypermutation in both humans and mice in cases of complete Pol eta deficiency, a proposition supported in this study by the analysis of Pol eta x Pol kappa double-deficient mice. We also describe a new XPV case, in which a splice site mutation of the first noncoding exon results in a decreased mRNA expression, a mRNA that otherwise encodes a normal Pol eta protein. Whereas the Pol eta mRNA level observed in patient's fibroblasts is one-twentieth the value of healthy controls, it is only reduced to one-fourth of the normal level in activated B cells. Memory B cells from this patient showed a 50% reduction in A/T mutations, with a spectrum that still displays a strict Pol eta signature. Pol eta thus appears as a dominant enzyme in hypermutation, its presence precluding the use of a substitute enzyme even in conditions of reduced availability. Such a dominant behavior may explain the lack of Pol kappa signature in Ig gene mutations of some XPV patients previously described, for whom residual Pol eta activity might exist.
Faili
Ahmad
A
Faculté de Médecine, Institut National de la Santé et de la Recherche Médicale Unite 783 Développement du Système immunitaire, Université Paris Descartes, Site Necker-Enfants Malades, Paris, France.
Stary
Anne
A
Delbos
Frédéric
F
Weller
Sandra
S
Aoufouchi
Said
S
Sarasin
Alain
A
Weill
Jean-Claude
JC
Reynaud
Claude-Agnès
CA
D000328:Adult; D000818:Animals; D001402:B-Lymphocytes / Q000201:enzymology* / Q000276:immunology; D004252:DNA Mutational Analysis; D004259:DNA-Directed DNA Polymerase / Q000235:genetics* / Q000276:immunology; D005260:Female; D006801:Humans; D051379:Mice; D018345:Mice, Knockout; D009154:Mutation; D012333:RNA, Messenger / Q000032:analysis; D020133:Reverse Transcriptase Polymerase Chain Reaction; D027041:Somatic Hypermutation, Immunoglobulin / Q000235:genetics*; D014983:Xeroderma Pigmentosum / Q000201:enzymology / Q000235:genetics*; D000099270:Y-Family DNA Polymerases
D004259:DNA-Directed DNA Polymerase; D012333:RNA, Messenger; C404302:POLK protein, human; D000099270:Y-Family DNA Polymerases; C112338:Rad30 protein