Loss of the BRCA1-PALB2 interaction accelerates p53-associated tumor development in mice.

The BRCA1-PALB2-BRCA2 axis, or the BRCA pathway, plays key roles in genome stability maintenance and suppression of breast and several other cancers. Due to frequent p53 mutations in human BRCA1 breast cancers and mouse mammary tumors from Brca1, Brca2 and Palb2 conditional knockout models, it is often thought that p53 inactivation accelerates BRCA1/2 and PALB2-associated tumorigenesis. Here, we studied tumor development in mice with a mutation in Palb2 that disengages the PALB2-BRCA1 interaction in different Trp53 backgrounds. Rather than mammary tumors, Palb2 and Trp53 compound mutant mice developed, with greatly reduced latencies, lymphomas and sarcomas that are typically associated with germline Trp53 inactivation. Whole exome sequencing failed to identify any significant differences in genomic features between the same tumor types of Trp53 single mutant and Palb2;Trp53 compound mutant mice. These results suggest that loss of the BRCA pathway accelerates p53-associated tumor development, possibly without altering the fundamental tumorigenic processes.

Tumor suppressor PALB2 maintains redox and mitochondrial homeostasis in the brain and cooperates with ATG7/autophagy to suppress neurodegeneration.

The PALB2 tumor suppressor plays key roles in DNA repair and has been implicated in redox homeostasis. Autophagy maintains mitochondrial quality, mitigates oxidative stress and suppresses neurodegeneration. Here we show that Palb2 deletion in the mouse brain leads to mild motor deficits and that co-deletion of Palb2 with the essential autophagy gene Atg7 accelerates and exacerbates neurodegeneration induced by ATG7 loss. Palb2 deletion leads to elevated DNA damage, oxidative stress and mitochondrial markers, especially in Purkinje cells, and co-deletion of Palb2 and Atg7 results in accelerated Purkinje cell loss. Further analyses suggest that the accelerated Purkinje cell loss and severe neurodegeneration in the double deletion mice are due to excessive oxidative stress and mitochondrial dysfunction, rather than DNA damage, and partially dependent on p53 activity. Our studies uncover a role of PALB2 in mitochondrial homeostasis and a cooperation between PALB2 and ATG7/autophagy in maintaining redox and mitochondrial homeostasis essential for neuronal survival.

Genetic interactions among Brca1, Brca2, Palb2, and Trp53 in mammary tumor development.

Inherited mutations in BRCA1, BRCA2, and PALB2 cause a high risk of breast cancer. Here, we conducted parallel conditional knockout (CKO) of Brca1, Palb2, and Brca2, individually and in combination, along with one copy of Trp53, in the mammary gland of nulliparous female mice. We observed a functional equivalence of the three genes in their basic tumor-suppressive activity, a linear epistasis of Palb2 and Brca2, but complementary roles of Brca1 and Palb2 in mammary tumor suppression, as combined ablation of either Palb2 or Brca2 with Brca1 led to delayed tumor formation. Whole-exome sequencing (WES) revealed both similarities and differences between Brca1 and Palb2 or Brca2 null tumors. Analyses of mouse mammary glands and cultured human cells showed that combined loss of BRCA1 and PALB2 led to high levels of reactive oxygen species (ROS) and increased apoptosis, implicating oxidative stress in the delayed tumor development in Brca1;Palb2 double CKO mice. The functional complementarity between BRCA1 and PALB2/BRCA2 and the role of ROS in tumorigenesis require further investigation.