A novel type of aberrant recombination in immunoglobulin genes and its implications for V-J joining mechanism.

Functional kappa light chain genes are formed during B-lymphocyte differentiation by the joining of initially separate V and J gene segments. It has been suggested that the intervening DNA is deleted, however the recent reports of what appear to be the reciprocal products of V and J recombination (back-to-back conserved V and J flanking sequences, called f-fragments) in DNA from mature lymphocytes make a simple deletion model unlikely. An alternative scheme involving unequal sister chromatid exchange has been proposed, supported by the evidence that the f-fragments seem to have segregated from the chromosome carrying the reciprocal complete kappa light chain gene (this and other schemes are briefly reviewed in ref. 8). We report here the analysis of a mouse myeloma (MOPC 41), in which a productive (kappa+) and a non-productive (kappa-) rearrangement has occured, which may help to clarify the mechanism of V-J joining. The aberrant rearrangement has led to the joining of a J1 gene segment to a sequence unrelated to any V gene (L10), and which in the germ line is flanked by a sequence resembling a V region recombination signal sequence. In this case no segregation of the reciprocal recombination products (kappa-41 and f41), which is a required step in sister chromatid exchange models, has taken place. An inversion model provides the simplest explanation of this J rearrangement.

Recombined flanks of the variable and joining segments of immunoglobulin genes.

The mechanism of generating immunoglobulin light chain genes by rearrangement of variable (V), joining (J), and constant (C) gene segments is still unknown. It has been discussed mostly in terms of excision and deletion of the DNA between the recombined V and J gene segments. However, the finding of DNA digests from the mouse myeloma T of a fragment (called f-T) that contains the 3' flank of a V kappa and the 5' flank of a J1 gene segment argued against a simple deletion mechanism [Steinmetz, M., Altenburger, W. & Zachau, H. G. (1980) Nucleic Acids Res. 8, 1709--1720]. The origin of fragment f-T has now been investigated by cloning and determining the sequence of the germ-line V gene segment that apparently participated in its formation. Moreover, analogous fragments containing flanking sequences were isolated from the myelomas MOPC 173 and 41 (f-173 and f-41) and studied by sequence analysis. The f fragments appear to be recombination products of V--J rearrangements reciprocal to rearranged kappa genes but, at least in the cases of f-T and f-173, not of the rearranged V genes present in the same tumor cell. This fact is best explained by a sister chromatid exchange mechanism of V--J recombination because, by this model, the rearranged V genes and the reciprocal flank recombination products would segregate into different cells during the following mitosis. The possibility is suggested that there exists in lymphocyte differentiation more than one mechanism of V--J recombination.

Cloning of V region fragments from mouse liver DNA and localization of repetitive DNA sequences in the vicinity of immunoglobulin gene segments.

Two different kappa light chain genes have previously been isolated from one mouse myeloma. The V (variable, abbreviations in ref. 2) gene segments of the two genes were now used to identify their germline counterparts in EcoRI digests of mouse liver DNA. In addition two sets of related V gene segments were found which hybridize with either of the two DNA probes. Five of the V region fragments of one set were cloned in a lambda phage vector and partially characterized by restriction mapping and Southern blot hybridization. Repetitive DNA sequences were found on each of the five fragments as well as on other cloned immunoglobulin gene containing fragments. Cross-hybridization between some but not all of the regions containing repetitive DNA sequences was observed.