In genetics, crosslinking of DNA occurs when various exogenous or endogenous agents react with two different positions in the DNA. This can occur in either the same strand (intrastrand crosslink) or in the opposite strands of the DNA (interstrand crosslink). Crosslinks also occur between DNA and protein. DNA replication is blocked by crosslinks, which cause replication arrest and cell death if the crosslink is not repaired.
Contents
The RAD51 family plays a role in repair.
Exogenous
Alkylating agents such as 1, 3-bis(2-chloroethyl)-1-nitrosourea (BCNU, carmustine)) and nitrogen mustard which are used in chemotherapy can cross link with DNA at N7 position of guanine on the opposite strands forming interstrand crosslinks.
Cisplatin (cis-diamminedichloroplatinum(II)) and its derivatives forms DNA cross links as monoadduct, interstrand crosslink, intrastrand crosslink or DNA protein crosslink. Mostly it acts on the adjacent N-7 guanine forming 1, 2 intrastrand crosslink.
DNA damage induced by ionizing radiation is similar to that of oxidative stress, and these lesions have been implicated in aging and cancer. Biological effects of single-base damage by radiation or oxidation, such as 8-oxoguanine and thymine glycol, have been extensively studied. Recently the focus has shifted to some of the more complex lesions. Tandem DNA lesions are formed at a substantial frequency by ionizing radiation and metal-catalyzed H2O2 reactions. Under anoxic conditions, the predominant double-base lesion is a species in which the C8 of guanine is linked to the 5-methyl group of an adjacent 3'-thymine (G[8,5- Me]T).
Endogenous
Repair of DNA crosslinks
DNA crosslinks generally cause loss of overlapping sequence information from the two strands of DNA. Therefore, accurate repair of the damage depends on retrieving the lost information from an undamaged homologous chromosome in the same cell. Retrieval can occur by pairing with a sister chromosome produced during a preceding round of replication. In a diploid cell retrieval may also occur by pairing with a non-sister homologous chromosome, as occurs especially during meiosis. Once pairing has occurred, the crosslink can be removed and correct information introduced into the damaged chromosome by the process of homologous recombinational repair.
Treatment of E. coli with psoralen-plus-UV light (PUVA) produces interstrand crosslinks in the cells’ DNA. Cole et al. and Sinden and Cole presented evidence that an homologous recombinational repair process requiring the products of genes uvrA, uvrB, and recA can remove these crosslinks in E. coli. This process appears to be quite efficient. Even though one or two unrepaired crosslinks are sufficient to inactivate a cell, a wild-type bacterial cell can repair and therefore recover from 53 to 71 psoralen crosslinks. Eukaryotic yeast cells are also inactivated by one remaining crosslink, but wild type yeast cells can recover from 120 to 200 crosslinks. In yeast, three pathways have a role in repair or toleration of crosslinks: homologous recombinational repair, nucleotide excision repair and translesion synthesis.
Recombinational repair of DNA crosslinks also likely occurs in plants where it depends on gene rad51, a recA ortholog. In the plant Arabidopsis thaliana, mutants defective in a gene rad51 paralog XRCC3 are hypersensitive to mitomycin C, a crosslinking agent. In rice (Oryza sativa), mutants with a defective RAD51C gene have increased sensitivity in somatic cells to mitomycin C.
In humans, the leading cause of cancer deaths worldwide is lung cancer, including non small cell lung carcinoma (NSLC) which accounts for 85% of all lung cancer cases in the United States. Individuals with NSLC are often treated with therapeutic platinum compounds (e.g. cisplatin, carboplatin or oxaliplatin) (see Lung cancer chemotherapy) that cause inter-strand DNA crosslinks. Among individuals with NSLC, low expression of BRCA1 in the primary tumor correlated with improved survival after platinum-containing chemotherapy. This correlation implies that low BRCA1 in the cancer, and the consequent low level of DNA repair, causes vulnerability of the cancer to treatment by the DNA crosslinking agents. High BRCA1 may protect cancer cells by acting in the homologous recombinational repair pathway that removes the damages in DNA introduced by the platinum drugs. Taron et al. and Papadaki et al. concluded that the level of BRCA1 expression is a potentially important tool for tailoring chemotherapy in lung cancer management.