Keywords: B cells, antibody diversity, somatic hypermutation, class switch recombination.
Somatic hypermutation of immunoglobulin genes occurs at a frequency that is a million times greater than mutation in other genes. Mutations occur in variable genes to increase antibody affinity, and in switch regions before constant genes to cause switching from IgM to IgG. Hypermutation is initiated in activated B cells when the activation-induced deaminase (AID) protein deaminates cytosine in DNA to uracil. Uracils can be processed by either a mutagenic pathway to produce mutations, or a non-mutagenic pathway to remove mutations. We have studied proteins that are involved in the mutagenic pathway by examining mice that are deficient for the proteins, and by analyzing the biochemical interactions between the relevant proteins. We examined the role of mismatch repair proteins, MSH2, MSH3, MSH6, PMS2, and MLH1, since they would recognize mismatches. The MSH2-MSH6 heterodimer is involved in hypermutation by binding to U:G and other mismatches generated during repair synthesis, but the other proteins are not necessary. We then analyzed the role of low fidelity DNA polymerases eta, iota, and theta in synthesizing mutations, and conclude that polymerase eta is the dominant participant by generating mutations at A:T base pairs. Intriguingly, the hypermutation machinery has hijacked certain repair proteins from the mismatch repair and base excision pathways to create mutations, rather than repair them. However, the most profound enigma in hypermutation, and perhaps immunology, is how AID is targeted to a very small region of DNA in the immunoglobulin loci. Current research is focused on (1) detecting and locating the position of uracils in variable regions from activated B cells, (2) targeting AID to the variable and switch regions by the transcription complex, and (3) determining interactions between AID and known DNA repair proteins that are involved in hypermutation.
Gearhart, P.J. and Sen, R. 2008. Regulating antibody diversity: Taming a mutagen. Mol. Cell 31:615-616.
Rajagopal, D., Maul, R.W., Ghosh, A., Chakraborty, T., Khamlichi, A.A., Sen, R., and Gearhart, P.J. 2009. Immunoglobulin switch ? sequence causes RNA polymerase II accumulation and reduces dA hypermutation. J. Exp. Med. 206:1237-1244.
Kohli, R.M., Abrams, S.R., Gajula, K.S., Maul, R.W., Gearhart, P.J., and Stivers, J.T. 2009. A portable hotspot recognition loop transfers sequence preferences from APOBEC family member to activation-induced cytidine deaminase. J. Biol. Chem. 284:22898-22904.
Saribasak, H., Rajagopal, D., Maul, R.W., and Gearhart, P.J. 2009. Hijacked DNA repair proteins and unchained DNA polymerases. Philos. Trans. R. Soc. Lond. B Biol. Sci. 364:605-611.
Maul, R.W. and Gearhart, P.J. 2009. Women, autoimmunity, and cancer: a dangerous liaison between estrogen and activation-induced deaminase? J. Exp. Med. 206:11-13.
Maul, R.W. and Gearhart, P.J. 2010. Controlling somatic hypermutation in immunoglobulin variable and switch regions. Immunol. Res. 47:113-122.
Maul, R.W. and Gearhart, P.J. 2010. AID and somatic hypermutation. Adv. Immunol. 105:159-191.
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