RNF8
E3 ubiquitin-protein ligase RNF8 is an enzyme that in humans is encoded by the RNF8 gene. RNF8 has activity both in immune system functions and in DNA repair.
Function
The protein encoded by this gene contains a RING finger motif and an FHA domain. This protein has been shown to interact with several class II ubiquitin-conjugating enzymes, including UBE2E1/UBCH6, UBE2E2, and UBE2E3, and may act as a ubiquitin ligase in the ubiquitination of certain nuclear proteins. Alternatively spliced transcript variants encoding distinct isoforms have been reported.RNF8 promotes repair of DNA damage through three DNA repair pathways: homologous recombinational repair, end joining">Sticky and blunt ends">end joining, and nucleotide excision repair. DNA damage is considered to be the primary cause of cancer, and deficiency in DNA repair can cause mutations leading to cancer. A deficiency in RNF8 predisposes mice to cancer.
Chromatin remodeling
After the occurrence of a double-strand break in DNA, the chromatin needs to be relaxed to allow DNA repair, either by HRR or by NHEJ. There are two pathways that result in chromatin relaxation, one initiated by PARP1 and one initiated by γH2AX . Chromatin remodeling initiated by γH2AX depends on RNF8, as described below.The histone variant H2AX constitutes about 10% of the H2A histones in human chromatin. At the site of a DNA double-strand break, the extent of chromatin with phosphorylated γH2AX is about two million base pairs.
γH2AX does not, by itself, cause chromatin decondensation, but within seconds of irradiation the protein "Mediator of the DNA damage checkpoint 1" specifically attaches to γH2AX. This is accompanied by simultaneous accumulation of RNF8 protein and the DNA repair protein NBS1 which bind to MDC1. RNF8 mediates extensive chromatin decondensation through its subsequent interaction with CHD4 protein, a component of the nucleosome remodeling and deacetylase complex NuRD.
RNF8 in Homologous Recombinational Repair
DNA end resection is a pivotal step in HRR repair that produces 3' overhangs that provide a platform to recruit proteins involved in HRR repair. The MRN complex, consisting of Mre11, Rad50 and NBS1, carries out the initial steps of this end resection. RNF8 ubiquitinates NBS1, and this ubiquitination is required for effective homologous recombinational repair. Ubiquitination of NBS1 by RNF8 is, however, not required for the role of NBS1 in another DNA repair process, the error-prone microhomology-mediated end joining DNA repair.RNF8 appears to have other roles in HRR as well. RNF8, acting as a ubiquitin ligase, mono-ubiquitinates γH2AX to tether DNA repair molecules at DNA lesions. In particular, RNF8 activity is required to recruit BRCA1 for homologous recombination repair.