BRCA1 Gene Sequence Variation in Centenarians

of Gerontological Research, Italian National Research Center on Aging (INRCA), Ancona, Italy

ABSTRACT: With the ample gene sequence information that has become avail- able with the human genome project virtually completed, it has become possi- ble to identify functional gene variants and their frequencies in elderly populations with different aging-related characteristics. Such a genetic epide- miological approach could lead to new insights with respect to the basic mech- anisms of aging and longevity as well as the identification of new targets to prevent or retard some of the late-age adverse effects. Using our recently devel- oped two-dimensional gene scanning (TDGS) technology platform we demon- strate the feasibility of this approach by screening two different populations of centenarians for polymorphic variation in the BRCA1 breast cancer suscepti- bility gene, one of the many genes involved in genome maintenance. The initial results obtained with this approach suggest differences in BRCA1 genotype fre- quencies between the centenarian populations and controls.

KEYWORDS: 2-D gene scanning; BRCA1; Centenarians; Allelic variants; Ge- nome instability

The success of the human genome project combined with new technologies of ge- netic analysis have greatly facilitated the identification of genes with inherited mu- tations that predispose to various age-related disorders, including specific cancers. Such genes are often involved in genome stability pathways, to maintain DNA integ- rity. Examples are the DNA mismatch repair genes, MLH1 and MSH2, heritable mu- tations in which predispose to colon cancer,1 and the breast cancer susceptibility genes BRCA1 and BRCA2.2,3 It is assumed that increased genomic instability and mutation accumulation is causally related to the increased risk of cancer observed in individuals with heritable defects in these genes.

Address for correspondence: Dr. Jan Vijg, University of Texas Health Science Center, Research Park, South Texas Centers for Biology in Medicine, 15355 Lambda Drive, San nio, TX 78245. Voice: 210-562-5027; fax: 210-562-5028. vijg@uthscsa.edu

ANNALS NEW YORK ACADEMY OF SCIENCES

In a more general sense, genomic instability in organs and tissues has been con- sidered as a major cause of aging. In this respect, increased mutation accumulation could lead to tissue atrophy and neoplasms, as well as increased loss of cellular func- tion.4–6 This process could be driven by oxygen free radicals and other sources of spontaneous DNA damage, although unambiguous evidence for such a causal rela- tionship is still lacking.7,8 Oxidative stress is known to induce genome rearrange- ment mutations, such as large deletions, inversions or translocations.9,10 These types of mutation have been demonstrated to accumulate during aging in various tissues of normal mice.11 In this respect, it has been speculated that, in addition to causing cell death and cell transformation, a relatively small number of such large mutations could destabilize the genome, leading to dysregulation of normal patterns of gene expression.6

Ultimately, the quality of genome stability systems determines the rate and spec- trum of mutation accumulation during aging. In this respect, evidence is accumulat- ing that defects in those systems that control the frequency of large DNA rearrangement mutations are associated with symptoms of accelerated senescence, in both mice and humans. This is the case for mice with defects in double-strand break repair genes, which can be expected to greatly accelerate the rate of illegiti- mate recombinations. Recent evidence indicates that mice with inactivated genes in- volved in non-homologous end joining, i.e., Ku86, prematurely exhibit symptoms of age-related degeneration in various tissues, including liver, skin and bone.12 Disrup- tion of the mouse Ercc1 gene, which is involved in both nucleotide excision repair and a mitotic recombination pathway, also shows symptoms of accelerated senes- cence, including premature replicative senescence of Ercc1-mutant cells.13 Prema- ture replicative senescence, which could be an important marker for in vivo aging,14 has also been observed for murine cells deficient for other genes involved in recom- binational repair and non-homologous end joining, including Atm, Brca2, Ku70 and DNA ligase IV.12 Cellular proliferation defects have also been observed in murine Brca1 mutant cells and a mammary-specific conditional Brca1 knock-out mouse model revealed genetic instability and tumorigenesis in combination with growth in- hibition in vivo.15

The possible role of genomic instability in aging is further exemplified by recent work investigating telomere maintenance in the preservation of tissue functioning during aging. In a study of third generation telomerase-deficient mice, shortened life span was found to be accompanied by reduced capacity to respond to stresses such as wound healing, hematopoietic ablation and by an increased incidence of sponta- neous malignancies.16 The results of these studies underscore that in proliferative or- gans the early initiation of genetic instability due to telomere erosion can greatly accelerate age-related loss of cell viability and increase tumor formation.

An important body of evidence that large genomic mutations are causally related to the degenerative aspects of the aging process has been derived from human syn- dromes of accelerated aging, such as Werner syndrome. Werner syndrome, which is caused by a heritable mutation in a human homologue of the bacterial RecQ gene, termed WRN in humans,17 is characterized by the accelerated occurrence of certain aspects of the senescent phenotype, including cancer.18 The WRN gene encodes both a helicase and an exonuclease19 and is thought to play a role in suppressing genomic instability.20 Indeed, cultured somatic cells from patients with Werner syndrome dis-