Patterns of somatic mutations in immunoglobulin genes.

Somatic hypermutation of immunoglobulin genes is an important contributor to antibody diversity. Lack of information about somatic hypermutation in regions other than productively rearranged V(D)J exons has hampered efforts to determine what causes the mutations. In order to propose viable models of the mechanism, the identity of the "hypermutable immunoglobulin gene" must be established and the pattern of mutations in that gene must be determined. Although rearranged immunoglobulin genes were known to mutate, the unrearranged J region had not been excluded as another location for hypermutation. To determine if this region could mutate, a large population of unrearranged genes from hybridoma DNA derived from mutating splenic B lymphocytes was amplified and sequenced for evidence of mutation. The frequency of mutations in the unrearranged genes, combined with the lack of mutations in unrearranged V segments, indicated that rearranged allele is the substrate needed for complete activation of the mutational mechanism. The pattern of mutations provides valuable information on where the mutator mechanism is most active. Examination of large populations of productively rearranged genes revealed important findings. Mutations were unevenly distributed in the JC intron, concentrated next to the V(D)J exon. When the size of the JC intron changed, the pattern of mutations relative to the V(D)J exon remained the same. This means that the rearranged V segment, the rearranged J segment and/or something 5$\sp\prime$ of V positions mutations within the locus. Mutations were also unevenly distributed 5$\sp\prime$ of V. Higher frequencies of mutation were found farther upstream of V when the distance between V and the promoter increased. Mutations were skewed, not centered around V(D)J. I have used the observations from my thesis research and results from other laboratories to suggest two models of the hypermutation mechanism.PhDMicrobiology and ImmunologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103333/1/9308476.pdfDescription of 9308476.pdf : Restricted to UM users only