22 exons spanning 76 kb

Two isoforms are expressed, isoform 1 at 4772nt; isoform 2, 4688 nt, transcribed from an alternative first (noncoding) exon and lacking exon 5.

Both isoforms are approximately 80 kDa. Isoform 1 includes 708 amino acids; isoform 2 includes 680. Molecular studies of Mre11 typically do not distinguish between the different isoforms. Mre11 is a subunit of the Rad50/Mre11/NBS1 (R/M/N) complex and serves as a single-strand DNA endonuclease, a 3 to 5 DNA exonuclease, and to open hairpin DNA structures.

All tissues examined, with higher levels in proliferating tissues.

Nuclear.

Mre11 participates in the repair of DNA double-strand breaks and replication errors as well as in meiotic homologous recombination. The R/M/N complex is part of the BRCA1-associated genome surveillance complex (BASC). The phosphorylation of Mre11 and NBS1 by another member of this super-complex, ATM, is essential for an early step in the response to DNA double-strand breaks (DSBs) and for their repair by either non-homologous end joining (NHEJ) or homologous recombination (HR). The interaction of DNA end-bound Mre11 with Ku70 may direct the break to rejoining by NHEJ, while the absense of Ku70 favors repair by HR. Current models propose DSB detection by R/M/N is required for the activation of ATM, which in turn phosphorylates Mre11 and NBS1, thus placing Mre11 both upstream and downstream of ATM in the DNA damage response signal transduction cascade.

A mechanism has been proposed in which each end of a DNA DSB is bound by an R/M/N dimer, the two dimers being held to each other via the Zinc-hook domain of each Rad50 unit. As the Zinc-hook of Rad50 is located at the end of a long coiled-coil domain, this provides a flexible structure in which each DNA end is accessible to additional repair enzymes while being held in close proximity to each other in preparation for re-ligation.

Cells lacking Mre11 are deficient in DSB repair, and exhibit hypersensitivity to DNA damaging agents such as ionizing radiation and radiomimetic drugs. Such cells also have abnormal DNA replication and high levels of chromosomal instability.

The gene is conserved throughout eukaryotes, with 70% nucleic acid homology to S. cerevisiea Mre11.

The hypomorphic arg633ter, asn117ser and arg571ter alleles have been described in ATLD patients. Homozygosity for null alleles is thought to be lethal in embryogenesis, as is the case in Mre11 knockout mice. Germline mutations have also been found in sporadic hematopoetic malignancies, with loss of the wild-type allele in the malignant cells.

Rare mutations have been found in breast cancer and lymphoma. In colon cancers not expressing Mre11, the mutation of a poly-T tract in intron 4 has been shown to induce a splicing error that truncates the protein. Seven of 20 gastric tumors failed to express Mre11, although the cause of this was not demonstrated.