A FUNCTIONAL ASSAY FOR BRCA1

In the majority of families that show hereditary predisposition to breast and ovarian cancer, the increased cancer risk is attributable to inherited mutations in the BRCA1 gene.  One of the principal benefits expected to ensue from the discovery of this gene has been the ability to identify, in individual patients, deleterious mutations that could indicate an increased risk of cancer before there is any disease.  This allows increased vigilance, and preventive measures, to be targeted to those most at risk.  For other members of a high-risk family who have not inherited the mutation, such gene testing can reveal that their BRCA1 gene is fully functional and so reassure the individual that their risk of breast or ovarian cancer is no higher than that of the general population.  While analysis of a patient’s BRCA1 sequence has been informative and useful in many cases, in others the results have been frustratingly ambiguous.  The results depend on the nature of the mutation found.  If the alteration results in a truncated form of the BRCA1 protein, this typically results in a defective protein and correlates with increased risk.  However, if there is a missense mutation, which simply changes a single amino acid, it is often unknown whether this is a detrimental change or a harmless variant.  For this reason, an assay to evaluate the functional consequences of BRCA1 mutations, whatever their DNA sequence, will be a valuable diagnostic tool.

Dr. Alvaro Monteiro of Strang Cancer Prevention Center and Weill Medical College of Cornell University has been developing just such a functional assay for BRCA1 mutations.  The assay is based on Dr. Monteiro’s discovery that normal BRCA1 protein has an ability to activate the transcription of other genes (1).  This property is lost in mutant forms of BRCA1 associated with cancer risk and so may be a crucial feature of the normal protein.  To simplify the methodology for testing individual BRCA1 sequences, Dr. Monteiro has incorporated the transcriptional activation function of human BRCA1 into a gene expression assay in yeast, in which the read-out is the extent of a blue color developed in the yeast cells (2).  The assay is currently in an experimental phase and has recently been put to the test.  In a collaborative study with Dr. Ake Borg and colleagues in Sweden, Iceland, Denmark, and Norway, Dr. Monteiro’s transcription-based assay was evaluated using missense mutations found in Scandinavian breast and ovarian cancer families (3).  The ability of this functional assay to distinguish harmful mutations from benign variants was extremely encouraging and the accuracy of the method now warrants further evaluation.  It is likely that assays such as this, based on an understanding of BRCA1’s normal functions, will be critically important in the interpretation of BRCA1 gene sequencing data.  In some cases, women who have a significant family history of breast and ovarian cancer have their DNA tested to determine whether they are at increased risk.  Functional tests of the BRCA1 proteins encoded by that individual’s genes may provide crucial information that will assist Genetic Counselors in the interpretation of those DNA tests.  This will lead to greater accuracy in the identification of women at high risk and so assist in the prevention of breast and ovarian cancer.

1.  Monteiro, A.N.A., August, A., and Hanafusa, H.  Evidence for a transcriptional activation function of BRCA1 C-terminal region.  Proc Natl Acad Sci U S A. 1996, 93:13595-9.

2.  Hayes, F., Cayanan, C., Barilla, D., and Monteiro, A.N.A.  Functional assay for BRCA1: mutagenesis of the COOH-terminal region reveals critical residues for transcription activation.  Cancer Res. 2000, 60:2411-8.

3.  Vallon-Christersson, J., Cayanan, C., Haraldsson, K., Loman, N., Bergthorsson, J.T., Brondum-Nielsen, K., Gerdes, A.M., Moller, P., Kristoffersson, U., Olsson, H., Borg, A., Monteiro, A.N.A.  Functional analysis of BRCA1 C-terminal missense mutations identified in breast and ovarian cancer families.  Hum Mol Genet. 2001, 10:353-60.

Alvaro N. Monteiro, Ph.D., is a Research Scientist at Strang Cancer Prevention Center and Assistant Professor of Cell Biology at Weill Medical College of Cornell University.