by ecancer reporter Clare Sansom

The tumour suppressor gene BRCA1 is one of the most important single genes that determine cancer risk.

Women carrying mutations that affect the function of this gene have an 80% risk of developing breast cancer and a 55% risk of developing ovarian cancer by the age of 90.

The protein product of this gene, BRCA1, forms part of a complex that repairs double-strand breaks in DNA arising both from natural wear and tear and from external factors including ionising radiation.

Cells that divide rapidly, including embryonic and adult neural stem cells (NSCs), are particularly susceptible to this type of DNA damage, and BRCA1, which is ubiquitously expressed, is enriched in the nuclei of proliferative cells.

Embryonic neural stem cells are also particularly sensitive to ionising radiation.

These observations led a group of researchers led by Inder M. Verma at the Salk Institute for Biological Studies, La Jolla, CA, USA to propose that BRCA1 might play an important role in the development of the embryonic brain.

To test this hypothesis, they first confirmed the expression pattern of BRCA1 in the embryonic mouse brain using fluorescence in situ hybridisation (FISH) and observed it to be most highly expressed in the upper part of the ventricular zone (VZ), where highly proliferative NSCs are also common.

They generated a mouse model in which BRCA1 was deleted specifically in these neuronal progenitor cells.

The brains of these BRCA1 knockout mice were observed to be slightly smaller during embryonic development, with most of the reduction in brain volume occurring in the neocortex.

This size difference became more apparent after birth, and by postnatal day 7 (P7) it is particularly clearly visible in the neocortex, cerebellum and olfactory bulbs.

The newborn mice are smaller than the wild type, exhibit ataxia and become severely agitated when removed from their mothers; their abnormalities increase over time until they die 19 days after birth.

These differences are consistent with the known role of the neocortex and hippocampus in motor learning and cognition.

Microarray analysis showed differences between wild type and BRCA1 knockout mice in the expression of many genes involved in brain development and function, with roles in (for example) neurogenesis, cortical differentiation, oligodendrocyte function, oxidative stress and proliferation.

The mammalian neocortex consists of six layers labelled I-VI, with the neurons in the deep layers (V-VI) being laid down at an earlier stage of embryonic development than those in the upper ones.

The neocortex in the BRCA1 knockout phenotype was severely disorganised, with neurons only observable in the deepest layers V and VI.

An observed reduction in the expression of the gene Ngn2, which is a marker for neuronal proliferation, was consistent with this finding.

Neurons in the cerebellum were also observed to be fewer in number in the BRCA1 knockout mice than in the wild type, and to be distributed in a disorganised manner.

The researchers observed an increase in activity of the protease caspase-3, which plays a central role in apoptosis, in the brain structures that are severely affected in this phenotype, suggesting that the neuronal deficiency arises from an increase in apoptosis.

Furthermore, p53 expression is induced alongside caspase activity, indicating that apoptosis arises through a p53-dependent pathway; this is consistent with the known mechanism of BRCA1 in DNA repair.

BRCA1 deletion in isolated embryonic neuronal precursor cell lines also induced apoptosis, and differentiation of these cell lines showed that neurons were more susceptible to this deletion than astrocytes.

The phenotype arising from BRCA1 loss in embryonic precursor cells could be partially rescued by deletion of p53, although subtle abnormalities in the cerebellum were still observed.

The DNA damage sensor kinase ATM is known to activate the p53-dependent apoptosis pathway in response to sensing double-stranded breaks in DNA; it is also active in the centrosome.

Knocking out this kinase as well as BRCA1 produced mice with a phenotype that was more similar to wild type than the BRCA1/p53 double knockout phenotype.

Taken together, these results suggest that BRCA1 plays an important role in neuronal development, protecting embryonic neurons from apoptosis arising from DNA damage and centrosomal defects, and regulating brain size.

Reference Pao, G.M., Zhu, Q., Perez-Garcia,C.G., Chou, S.J., Suh, H., Gage, F.H., O'Leary, D.D. and Verma, I.M. (2014). Role of BRCA1 in brain development. Proc. Nat. Acad. Sci. USA, published online ahead of print March 17, 2014. doi: 10.1073/pnas.1400783111