Cell-cycle-regulated DNA double-stranded breaks in somatic hypermutation of immunoglobulin genes

Does S. pombe exploit the intrinsic asymmetry of DNA synthesis to imprint daughter cells for mating-type switching?

Typically cell division is envisaged to be symmetrical, with both daughter cells being identical. However, during development and cellular differentiation, asymmetrical cell divisions have a crucial role. In this article, we describe a model of how Schizosaccharomyces pombe exploits the intrinsic asymmetry of DNA replication machinery--the difference between the replication of the leading strand and the lagging strand--to establish an asymmetrical mating-type switching pattern. This is the first system where the direction of DNA replication is involved in the formation of differentiated chromosomes. The discovery raises the possibility that DNA replication might be more generally involved in the establishment of asymmetric cellular differentiation.

Impact of age on hypermutation of immunoglobulin variable genes in humans

Chronological aging is associated with an accumulation of DNA mutations that results in cancer formation. The effect of aging on spontaneous mutations in humans is difficult to study because mutations are infrequent in the overall genome and tumors are relatively rare. In contrast, somatic mutations in immunoglobulin variable genes are abundant and can be studied in peripheral blood lymphocytes. To determine if aging alters the frequency and pattern of hypermutation, we sequenced 331 cDNA clones with rearranged V(H)6 genes and compared 452 mutations from young humans to 570 mutations from old humans. There were more mutated clones in the young population compared to the old population. Among the mutated clones, the frequency, location, and types of substitutions were similar between the young and the old groups. However, the ratio of replacement-to-silent mutations was much higher in the complementarity-determining regions of heavy chains from old people, which indicates that their B cells had been selected by antigen. Among individuals, there was variability in the frequency of tandem mutations, which we have observed in mice defective for the PMS2 mismatch repair protein. Microsatellite variability in DNA, which is caused by impaired mismatch repair, was then measured, and there was a strong correlation between the frequency of tandem mutations and microsatellite alterations. The data suggest that individuals vary in their mismatch repair capacity, which can affect the mutational spectra in their antibodies.

Somatic hypermutation of immunoglobulin and non-immunoglobulin genes

Somatic hypermutation (SHM) of immunoglobulin (Ig) genes is a highly specific mechanism restricted to B lymphocytes during only a few cell generations. Data presented here suggest that transcription of the target genes is required, but not sufficient for SHM. Presumably, cis-acting elements, such as those present in the Ig enhancers, are required to target a mutator factor (MuF) to Ig and human BCL-6 genes. It is postulated that the MuF travels with the transcribing RNA polymerase and is deposited on the target gene when the polymerase pauses. Point mutations, and rare deletions and insertions, are created by the combined actions of MuF and certain DNA polymerases. A subset of the mutations is corrected during SHM by DNA mismatch repair.