V(D)J recombination in scid mice

The scid mouse has a defect in its ability to repair ds DNA damage resulting in defective V(D)J recombination and, consequently, lymphoid development. The p450^cs of DNA dependent protein kinase (DPK) is strongly implicated as the scid gene product. The exact nature of the scid mutation is still unknown, however, it may not be a null mutation as scid mice are "leaky". In order to obtain some insight into the nature of the scid mutation, the extent of attempted (unresolved RSS intermediates), and endogenous Ig rearrangement was assessed using LM-PCR & quantitative PCR assays, respectively. Using LM-PCR, SCID B220⁺ cells were shown to have 10x more unresolved JH RSS intermediates than C.B-17 B220⁺IgM^- cells. Unresolved Dfl16.1 & Jκ cleavage products were found to be 1-10% of those in C.B-17 B220⁺cells. PCR assays that detect completed coding joints show scid VDJ joining to occur at 1-10% of wt among different scid mice with unequal usage of the JH elements. In one mouse, a single rearrangement was 100% the frequency of the normal C.B-17 corresponding structure. Scid kappa rearrangement occurs at 1-5% wt levels, and aberrantly sized structures are observed at a high frequency. Some of these VDJH and K structures are potentially productive, and exhibit an unusually high incidence of N/P addition, especially for the κ locus, suggesting that κ rearrangement may be attempted in the absence of productively rearranged IgH while B lineage cells are Tdt⁺. The aberrant κ structures appear to arise from joining by homology.