MAPPING THE UPSTREAM BOUNDARY OF SOMATIC MUTATIONS IN REARRANGED IMMUNOGLOBULIN TRANSGENES AND ENDOGENOUS GENES

Mammalian B-cell specific somatic hypermutation contributes to affinity maturation of the antibody response. This mutator activity is highly focused on rearranged immunoglobulin variable regions, but the underlying mechanism remains to be elucidated. In an effort to gain insights into the mechanism of somatic hypermutation, the precise distribution and frequency of mutations upstream of murine immunoglobulin genes was determined by examining the same variable gene segments when mutated in different B-cell lines. Immunoglobulin sequences analysed included k light chain transgenes bearing mutated Vk24 variable regions, and the endogenous Vk gene isolated from myeloma MOPC167, which also exhibits mutations in the variable region. In addition, mutated endogenous V(h)1 gene segments of the S107 heavy chain variable gene family were also examined. For both V(h)1 and Vk 24, somatic mutations were generally not found upstream of the leader intron, even in genes which exhibited a high mutation frequency in the variable region itself. The 5' somatic mutation boundary identified in immunoglobulin transgenes overlaps the boundary observed in endogenous genes, suggesting that both share cis-elements required for defining the mutable domain. Furthermore, the location of this 5' boundary appears not to change when these immunoglobulin genes are examined in different cell lines. These data may be indicative of a defined start site for immunoglobulin mutator activity.