Effect of Cytotoxic Chemotherapy on Markers of Molecular Age in Patients With Breast Cancer

With the aging of the American population, the incidence of new cancer diagnoses is projected to increase 45% from 2010 to 2030 (1). Coupled with the growing proportion of cancer patients who are cured (2), we face a new challenge: a large population of aging cancer survivors (3). Long-term survivors of childhood and adult cancer can exhibit substantial late sequelae, including endocrine dysfunction, cognitive impairment, cardiovascular morbidity, secondary neoplasms, and neuromuscular impairment (4–8). Little is known about how chemotherapy causes long-term adverse effects and whether it alters the pace of physiologic aging. Human aging is characterized by a steady decline in organ function, which leads to loss of physiologic reserve and frailty (9). This loss of function is characterized by a decline in the replicative capacity of certain self-renewing cells and the accumulation of cells that have undergone cellular senescence (10,11). Cellular senescence is triggered by the activation of tumor-suppressor mechanisms in response to varied cellular stresses such as oncogene activation, tissue injury, telomere dysfunction, and persistent DNA damage. Senescence is strongly associated with activation of the INK4/ARF (CDKN2a) locus on human chromosome 9p21.3, which encodes the p16INK4a and ARF tumor suppressor proteins. Several lines of evidence suggest senescence influences mammalian aging: 1) expression of p16 INK4a increases exponentially with chronological aging (12–14) and causes reduced replicative capacity of some cell types (15–19); 2) regulatory polymorphisms of senescence regulators (eg, CDKN2a and TERT) have been linked through unbiased genome-wide studies with many age-associated conditions such as cancer, pulmonary fibrosis, atherosclerosis, and type II diabetes (20); and 3) therapies to decrease the production of or increase the clearance of senescent cells in mice ameliorate certain age-associated phenotypes (21–23). Because of the intimate links between senescence and aging, markers of cellular senescence, including leukocyte telomere length (LTL), expression of senescence-associated (SA) cytokines such as interleukin 6 (IL-6), and expression of INK4a/ARF transcripts, have been tested as potential biomarkers of molecular aging. Decreased LTL has been linked with chronological age and age-promoting stressors such as cigarette smoking in several studies in human populations (24–26). Senescent cells elaborate a host of potent cytokines (ie, the senescence-associated secretory phenotype) (27), which promote a proinflammatory tissue microenvironment. Expression of SA-cytokines, such as IL-6, has been reported to increase with aging and to predict age-associated morbidities and mortality (28–32). More recently, expression of p16 INK4a, and to a lesser extent ARF, in defined tissues such as peripheral blood T cells (PBTLs) have been described as biomarkers of aging. Using the p16 INK4a assay, we have shown that smoking, physical inactivity, and chronic human immunodeficiency virus infection accelerate expression of this biomarker of molecular age in the PBTL compartment (14,33). Given the apparent long-term toxicities of DNA-damaging agents, we sought to determine whether cytotoxic chemotherapy given with curative intent accelerates molecular aging in humans.