The Mre11 complex, composed of RAD50, NBS1 and MRE11, has an essential role in the maintenance of genomic integrity and preventing cells from malignancy. Here we report the association of three Mre11 complex mutations with hereditary breast cancer susceptibility, studied by using a case–control design with 317 consecutive, newly diagnosed Northern Finnish breast cancer patients and 1000 geographically matched healthy controls ( P = 0.0004). RAD50 687delT displayed significantly elevated frequency in the studied patients (8 out of 317, OR 4.3, 95% CI 1.5–12.5, P = 0.008), which indicates that it is a relatively common low-penetrance risk allele in this cohort. Haplotype analysis and the screening of altogether 512 additional breast cancer cases from Sweden, Norway and Iceland suggest that RAD50 687delT is a Finnish founder mutation, not present in the other Nordic cohorts. The RAD50 IVS3-1G>A splicing mutation leading to translational frameshift was observed in one patient, and the NBS1 Leu150Phe missense mutation affecting a conserved residue in the functionally important BRCA1 carboxy-terminal (BRCT) domain in two patients, both being absent from 1000 controls. Microsatellite marker analysis showed that loss of the wild-type allele was not involved in the tumorigenesis in any of the studied mutation carriers, but they all showed increased genomic instability assessed by cytogenetic analysis of peripheral blood T-lymphocytes ( P = 0.006). In particular, the total number of chromosomal rearrangements was significantly increased ( P = 0.002). These findings suggest an effect for RAD50 and NBS1 haploinsufficiency on genomic integrity and susceptibility to cancer.

BARD1 was originally identified through its interaction with BRCA1, with which it has a closely related domain structure.¹¹ Both proteins possess an N-terminal RING finger motif and two BRCA1 C-terminal (BRCT) domains present in numerous proteins involved in DNA repair and cell cycle regulation.¹¹ The functionally important BARD1/BRCA1 heterodimer formation is mediated by the RING finger motifs and has also been shown to markedly increase the stability of both proteins.^{11– 13} The finding of breast cancer associated mutations within the RING finger domain of BRCA1, disrupting BRCA1/BARD1 interaction,^{11, 14} and the occurrence of BARD1 missense mutations in breast cancer patients,^{15– 17} implies participation of BARD1 in BRCA1 mediated tumour suppression. BARD1, unlike BRCA1, also contains a centrally located sequence of three ankyrin repeats¹¹ that are found in many proteins involved in transcriptional regulation.¹⁸

Besides mutations in BRCA1/BRCA2, heterozygous defects in PALB2 are important in breast cancer predisposition. PALB2 heterozygosity increases the risk of malignancy about sixfold. PALB2 interacts with BRCA1 and BRCA2 to regulate homologous recombination and mediate DNA damage response. Here we show, by analysing lymphoblastoid cell lines from heterozygous female PALB2 mutation carriers, that PALB2 haploinsufficiency causes aberrant DNA replication/damage response. Mutation carrier cells show increased origin firing and shorter distance between consecutive replication forks. Carrier cell lines also show elevated ATR protein, but not phosphorylation levels, and a majority of them display aberrant Chk1-/Chk2-mediated DNA damage response. Elevated chromosome instability is observed in primary blood lymphocytes of PALB2 mutation carriers, indicating that the described mechanisms of genome destabilization operate also at the organism level. These findings provide a new mechanism for early stages of breast cancer development that may also apply to other heterozygous homologous recombination signalling pathway gene mutations in hereditary cancer predisposition.