DNA rearrangements affecting both variable and constant regions of Ig H chain genes in MPC11 mouse myeloma variants

We have examined the mechanisms that account for short Ig H chain production in two variants of the mouse myeloma cell line MPC11 (IgG2b, κ) by mRNA sequencing and restriction enzyme mapping. One variant, F5.5, has a thymidine residue inserted into the (CH₃) domain, of the Ig H chain, resulting in premature termination and translation of a γ2b H chain of 50,000 m.w. A second variant, E5.7A12, contains γ2a-derived sequences that extend from near the 3' end of the CH₂ domain to the intervening sequence between the CH₂ and CH₃ domains, consistent with a microrecombination event (defined as either a double cross-over or gene conversion event). In this variant, the 5' end of the CH₃ domain has been deleted, but the remainder of the γ2b(CH₃) domain is present, resulting in the translation of a γ2b-γ2a-γ2b H chain of 52,000 m.w. Additional rearrangements affecting sequences in or adjacent to the variable region accompany H chain constant region alterations in these cell lines and subclones of these cell lines. In F5.5, novel sequences have recombined within one of two duplicated copies of the V(H) gene. In a sister clone of E5.7A12 that has ceased H chain production (E5.7A14), new sequences have recombined within 300 bp 5' of the enhancer element. Both F5.5 and E5.7A12, like their immediate unstable precursor cells, fail to assemble H-H dimers, halting the Ig assembly process at the heavy-light stage, and do not secrete H chains. We speculate that defects in H chain assembly and secretion, as exemplified by the parents of these variants (i.e., intermediates of these secondary variants), reactivate the Ig gene rearrangement machinery and result in formation of these putatively equally unstable secondary variants.