Baltimore subsequently discovered the proteins that do the cutting, known as recombination activating gene (RAG) proteins, but Alt speculated that some other mechanism was pasting the Vs, Ds, and Js back together. Knowing that cells have a variety of tools for general gene repair, he theorized that those same tools recombine V, D, and J gene segments for the immune system. In the early 1990's his lab studied numerous different kinds of hamster ovary cells that were known to be defective in gene repair, and added RAG proteins to cut apart the V, D, and J segments. They then investigated which cell types could and couldn't reassemble the segments.
Three of the cell types couldn't complete the VDJ recombination, and further studies of these cells led to the discovery of major components of the non-homologous DNA end-joining pathway. This pathway not only joins the severed V, D, and J segments, but also has a key role in maintaining genomic stability by mending double-strand breaks in DNA molecules. Alt and his collaborators immediately identified three proteins involved in this pathway, and later elucidated roles for two more; a total of six end-joining proteins are known today. Alt's team found that mice deficient in these proteins, in combination with certain other mutations, are dramatically susceptible to cancers of the immune system and other cancers.
Mechanisms of oncogene amplification and translocation
Alt's lab also has demonstrated that when a cell is deficient in both end-joining proteins and a protein known as p53, gene amplification, translocations, and tumor formation are greatly enhanced. Normally, p53 sets up a "checkpoint" that detects cells with unr
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Contact: Susan Craig
susan.craig@childrens.harvard.edu
617-355-6420
Children's Hospital Boston
24-Mar-2004