DNA Repair

Edited by: Karen Young Kreeger T. Blunt, N.J. Finnie, G.E. Taccioli, G.C.M. Smith, J. Demengeot, T.M. Gottlieb, R. Mizuta, A.J. Varghese, F.W. Alt, P.A. Jeggo, S.P. Jackson, "Defective DNA-dependent protein kinase activity is linked to V(D)J recombination and DNA repair defects associated with the murine SCID mutation," Cell, 80:813-23, 1995. (Cited in almost 120 publications as of December 1996) Comments by Frederick W. Alt, Howard Hughes Medical Institute, Children's Hospital, Boston, and Harvard Medical School; and Guillermo E. Taccioli, Boston University School of Medicine Photo: Barbara Stunir QUICK FIX: From left, Frederick Alt and Guillermo Taccioli and their colleagues identified DNA-dependent kinase as a key enzyme involved in DNA repair. The way that DNA repairs itself recently has been the object of study in many labs. One collaborative effort among researchers in the United Kingdom, Canada, and the United States has shed light on an enzyme that catalyzes this process. "This work shows that an enzyme that is used in all cells for repairing DNA double-strand breaks is also used for repairing specific DNA breaks that occur in the development of the immune system," says Frederick W. Alt, a Howard Hughes Medical Institute investigator and a professor of genetics and pediatrics at Children's Hospital in Boston and Harvard Medical School. Such breaks, which can be introduced by agents such as X-rays or certain kinds of drugs, will fragment chromosomes and be lethal to the cell if they are not repaired. "The most important finding of the Cell paper was that it identified the DNA-dependent protein kinase enzyme [DNA-PK] as a component of the DNA double-strand break repair system," explains Alt. The researchers compared DNA-PK activity in two mutant cell lines. They used cells from severe combined immune deficient (SCID) mice and hamster cells that were similarly deficient in the process of recombining immune-system-specific DNA sequences. During the development of lymphocytes, genes that encode the variable regions of immunoglobulin and T-cell receptors are created by piecing together chromosomal DNA segments in a process called recombination. These segments are known as the variable (V), diversity (D), and joining (J) regions. The recombination of the V(D)J pieces is facilitated by DNA-PK and other proteins. Coauthors Stephen Jackson, a molecular biologist with the Wellcome/Cancer Research Campaign Institute in Cambridge, U.K., and Penny Jeggo, a biologist at the Mutation Research Unit in Sussex, U.K., and their colleagues have been major contributors to the mutant cell approach to studying DNA repair, according to Alt. The researchers pinpointed DNA-PK as the key enzyme by showing that both mutant cell types lack activity for it. This is due to a defect in DNA-PK's catalytic subunit. "This work shows that even though V(D)J gene segments are taken apart by lymphocyte-specific enzymes, enzymes expressed in all cells are used to put them back together," says Alt. In the field of immunology, he notes, "trying to understand what the defect was in SCID mice was a long-standing challenge. This paper basically solves that problem." Alt speculates that this and other aspects of the research account for the paper's high number of citations. The PI3 kinase family of proteins-of which DNA-PK is a member-has also been a hot area of research, he says. This line of work "opens many doors in the study of cancer and immunodeficiency diseases," adds Guillermo E. Taccioli, an assistant professor of microbiology at Boston University School of Medicine, who participated in this study while a postdoc in Alt's lab. Alt and Taccioli note that many of their ongoing studies are now aimed at finding the mechanisms by which DNA-PK puts the V(D)J gene segments back together during recombination.

DNA Repair

The Scientist Staff

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