Making Sense of Biology: Evolutionary Medicine and the TNT phenotype
Nothing in Biology Makes Sense Except in the Light of Evolution, Theodosius Dobzhansky. (1973). The American Biology Teacher, 35(3), 125-129.
Black in America 2 aired on CNN in late July of 2009. It featured a compelling story concerning Triple Negative Breast Cancer and the fact that African American women are twice as likely to suffer from this particularly aggressive form of the disease1. According to the American Cancer Society, 192,370 new cases of invasive breast cancer will be diagnosed among women in the United States in 2009. Triple negative breast cancer (TNT phenotype) represents approximately 15 percent of breast cancer cases in the United States. Lund et al. 2009 reports that the prevalence of TNT may range between 10 – 30% of invasive breast cancers; again stressing that African American women are twice as likely to suffer from this disease2. The CNN segment focused on Dr. Lisa Newman, a surgeon and director of the Breast Care Center at the University of Michigan. Dr. Newman has embarked on a project to collect genetic samples from Ghanaian women to test the hypothesis that African ancestry may be playing a role in predisposing women to this particular form of the disease. The notion that African American ancestry is the culprit in this disparity raises specters of “racial medicine.” That is the idea that races of humans exist, and that they can be differentiated by genetic predispositions to particular diseases. An evolutionary perspective on cancer can resolve this confusion and at the same time offer fruitful avenues for further research on health disparity.
TNT phenotype of breast cancer
Cancer is defined as approximately one hundred complex diseases that behave differently depending upon the cell types in which they originate. Cancer cells show higher than normal rates of mutation, chromosomal abnormalities, and genomic instability. For this reason, even within cancers of a particular tissue (e.g. breast) there are several distinct molecular subtypes with distinct etiologies3. These include the luminal subtypes which typically express hormone receptor-related genes, and two hormone receptor subtypes—the human epidermal growth factor receptor 2 (HER2) positive/oestrogen negative (ER) negative subtype, and basal-like subtype4. Triple negative breast cancer is a basal-like phenotype that lacks expression of estrogen receptor (ER-negative), progesterone receptor (PR-negative) and lacks HER2 overexpression. Other relevant features of the TNT phenotype include:
|Increased5||Decreased or not found|
|p53 protein/TP53 mutations||PR|
|Basal cytokeratins 5, 14, 17|
It is of interest in that this phenotype is particularly aggressive, is highly likely to reoccur, and is resistant to the current HER-2 targeted therapies such as trastuzumab, and hormone therapies such as tamoxifen and aromatase inhibiters. This phenotype has been associated with mutations that occur in the BRCA1 locus (chromosome 17q21-q24). One mutation of interest (rs79916) has a G to T polymorphism has been associated with increased risk of developing breast cancer but not necessarily the TNT phenotype6. FST for this locus worldwide is 0.150 based on 37 populations, with the G variant of higher frequency in Sub-Saharan Africans and the T variant in higher frequency outside of Africa. The G variant has been associated with the higher risk of breast cancer in persons of European descent attending Australian family clinics. Other risk factors that have been associated with elevated risk of developing breast cancer with the TNT phenotype are younger age and African American ancestry. It is yet to be determined what is it about African American ancestry that contributes the risk factor. Implicitly Dr. Newman’s collecting of genetic samples from Ghana assumes that there may be genetic factors associated with population history. However, there is no reason to believe that and I shall explain why that is true below.
Evolutionary Explanation of Cancer
Cancers show a clear age-specific pattern. Yet not all biomedical scientists understand why. For example, Klug et al. write in their popular genetics textbook that:
“The phenomenon of age-related cancer is another indication that cancer develops from the accumulation of several mutagenic events in a single cell. The incidence of most cancers rises exponentially with age. If a single mutation were sufficient to convert a normal cell to a malignant one, then cancer incidence would appear independent of age7.”
This analysis fails to take into account the fitness related impacts of disease. If the accumulation of mutations in somatic cells were enough to explain cancer prevalence than why do we see such disparate life spans and cancer prevalence amongst Metazoans (e.g. Birds, Reptiles, and Mammals?) The giant tortoise can live more than 100 years while the changeable lizard lives less than 4; in mammals the maximum life span for humans is greater than 110 years, but the mouse lives between 4 – 5 years. Cancer is a common disease that occurs in mice as they age and some endogenous mouse mammary tumor like viral sequences have been found in human breast cancers8. Thus, organisms have the same cellular structure and many molecular mechanisms in common show widely different life spans. Therefore the explanation that cancer mortality increases with age as a result of increases frequency of somatic mutation cannot be sufficient.
The evolutionary theory of aging is premised on that notion that natural selection cannot act against genetic (or epigenetic) phenomena that occur after organisms cannot contribute to the future gene pool. Thus, what is of real interest is the relationship between the inflection point at which the exponential increase in mortality begins and its position relative to the graph of an organism’s reproductive fitness. For example, Darwinian fitness of US women derived from 1996 census data is zero by age 50. At the same time, the age-specific mortality plot of American men and women for several complex causes in the same time period (cancer, cerebrovascular disease, and accidents) changes slope from close to zero before 50 and increases rapidly afterward.
These results are consistent with the evolutionary theory of aging, which results from the declining force of natural selection (NS) with age. In other words this states that NS is uncompromising in its action against alleles that negatively impact fitness early in life (during the reproductive period); while it has no power against genes whose negative impacts occur once net-reproductive output is zero. The evolutionary theory of aging was first outlined theoretically by Peter Medawar, George C. Williams, and William D. Hamilton. It experimental validation was conducted by Brian Charlesworth, Michael Rose, Leo Luckinbill, me and a host of others9.
The evolutionary theory of aging demonstrates that selection acts in the following way to mold the genetic architecture of all metazoans organism relative to their age-specific fitness:
|Mechanism||Early Life||Late Life||Disease Example|
|Mutation/selection balance||( – )||( – )||Progeria|
|Mutation accumulation||( 0 )||( – )||Alzheimer’s Disease, Huntington’s Chorea|
|Antagonistic pleiotropy||( + )||( – )||Various Cancers|
Thus alleles that negatively impact early-life fitness are governed by the mutation/selection balance mechanism and will be extremely rare. They are observed at the frequency of their mutation (barring modern medical intervention.) Progeria is a disease that mimics aging and progeriacs almost never reproduce. Its frequency is around 1 in 22 million. Mutation accumulation refers to genes that have no impact on early life fitness, but do have negative impact on late-life fitness. Alzheimer’s disease is an example, and its frequency is determined by chance population events. Antagonistic pleiotropy refers to genes that have a positive impact on early life fitness, but a negative impact on late life fitness. Due to their positive impact, natural selection increases their frequency, despite the fact that they have negative late-life effects. These alleles are therefore widespread. The genetic loci that are involved in the formation of cancer all have essential functions in early life, thus any genetic variants that impede their function will be rare. New research suggests that hereditary cancers are actually very rare. Thus the process that is responsible for creating cancer in individuals is actually epigenetic, that is factors that influence the expression of genes in a heritable way but do not alter the nucleotide sequence. Examples of this are DNA methylation and histone modification. The evolutionary theory of aging will act in the same way with regard to epigenetic factors, any occurring at early life would be strongly selected against, those that are neutral in early life may accumulate, and those that are beneficial should spread.
Life History Trade-offs and Survival
The field of life history evolution exams how evolutionary mechanisms shape survival and reproductive patterns of organisms. A consistent theme in life history evolution is the existence of trade-offs. Some central tradeoffs observed in life history are shown below:
|Central Life History Tradeoffs|
|Current reproduction and survival|
|Current reproduction and future reproduction of offspring|
|Number, size, and sex of offspring|
Experimental work concerning the evolutionary theory of aging in invertebrate organisms (Drosophila) demonstrated that there were trade-offs in life history features that resulted from selection for delayed reproduction. It was further demonstrated that these differences in life history resulted from genetic variation. These experiments showed that there was a genetically based trade-off between early fecundity and late-life survival. One could utilize results like this to argue for genetic variation in human populations that impact life history trade-offs. If such genetic differences existed they could be relevant to specific disease systems operating within the evolutionary mechanisms of aging. J.P. Rushton utilized just such as argument to claim that intelligence differences between blacks, whites, and yellows were genetically based and resulted from life history differences. I debunked these claims in Graves 200210.
However not all life history tradeoffs result from genetic differences in organisms. For example, caloric restriction can produce life history trade-offs such as depressed early reproduction and prolonged survival in variety of organisms including primates11. The Colman et al. 2009 report is particularly important to this argument because it caloric restriction (without malnutrition) which is an environmental intervention, delays the onset of complex diseases (including cancer) in a primate. With this idea in mind we can utilize evolutionary perspectives and re-examine the TNT disparity between African American and European American women to address whether it is reasonable to be looking for causality in the disparity within supposed genetic differences that might exist between these groups.
Drilling into the TNT phenotype
Recent work on the prevalence of the TNT phenotype shows that it is related to a number of life history attributes and behaviors that are not shared equivalently by African- and European American women. For example, life history variables that are associated with a greater risk of basal-like breast cancers include: lower age at menarche, increased risk for parity, younger age at first full-term pregnancy, lower duration of breast feeding, lower number of children breast-fed, decreased number of months of breast-feeding, multiple-live births and not breast feeding, and use of lactation medication. In addition, body mass index and elevated waist to hip ratio was associated with greater risk of basal-like breast cancer in pre- and post-menopausal women. Other variables that are associated with greater risk of basal-like breast cancers include diet, duration of smoking, and poverty12. Both the Lund et al. 2009 (Atlanta GA study) and Millikan et al. 2008 (Carolina Breast Cancer Study) found self-identified race (African American) as a statistically significant risk factor for TNT phenotype with many of these variables controlled between populations. However this result in and of itself does not indicate that the important causal factor in self-identified African American race is genetic difference. Indeed, given the large number of environmental variables that are associated with this phenotype and the large number of environmental variables that differ between African American and European American women there is no reason to believe that genetic variation uncovered in Ghana will have any use in redressing the health disparity.
Environmental versus Genetic Interventions
Even if some underlying genetic factor were contributing to the African-/European- American difference in TNT prevalence it is clear that many environmental and behavioral interventions could help reduce this disparity. For example, the differences between African and European-American women in age of first full term pregnancy cries out for intervention. The table below shows the ratio of European American to African American births by age in 2004.
Source: US Bureau of the Census 2004
This table shows very clearly that African American women are having many more children at younger age than European American women. This life-history feature, especially the teen-age pregnancies is detrimental to the social and economic well-being of the African community as well as possibly predisposing African American women to TNT breast tumor phenotype at later age. Indeed the fact that more African American women are having children at younger age may be associated with less breast feeding by those women. This may occur simply because many of these women are unmarried and therefore must work or attend school as part of caring for their children. From the standpoint of evolutionary mechanism the greater prevalence of the TNT phenotype in African American women results from them experiencing a novel environment. It is likely that our female ancestors were more likely to breast feed their children and maintained much lower BWI and WHR due to greater activity and less fatty diets. Again, these social issues may be responsible for later epigenetic events which predispose these women to the TNT phenotype. Ironically, even if genes are examined in Ghanaian women, the fact that they do share the same environment with African American women means that it will difficult if not impossible to correlate genetic differences with epigenetic mechanisms in the US.
Finally, it is important to evaluate that usefulness of genetic or intrinsic mechanisms of disease versus environmental/behavioral ones. Genetic/intrinsic approaches suggest that an individual’s illness is the result of something that is wrong with them. This is the thinking that resides behind all of the discussion of “racial medicine.” The racial medicine paradigm continues to find inferior genes in persons of African descent. This notion is absurd at face value, since there is much greater genetic variation in sub-Saharan Africans than in all the rest of the world’s population combined13. How can it be that all the inferior alleles should be found in Africans? Indeed, recent studies shown that there are more deleterious alleles in European populations compared to Africans. This begs the question why do European-derived populations in the US enjoy greater health if their genes are so bad?
Another feature of the genetic/intrinsic mechanism paradigm is its association with and utility for the biomedical/pharmaceutical complex. If it is really something intrinsically wrong with African American women that predisposes them to the TNT phenotype, than the obligation is there to find a drug or other medical intervention that cures them. If true, this does not present and ethical or moral dilemma. Drug companies should make products that help us cure disease. However if we consider the notion that social and environmental causes play more important role than genetic differences than the dominance of research by the intrinsic/genetic paradigm is more problematic. Here the expenditure of dollars in biomedical research and pharmacological developments may actually be causing harm. That is, if we spent the same amount of money on social interventions, such as education, preventive care, or job creation for the unemployed, might we not narrow the health disparity gap sooner? My reading of the entire health disparity conundrum (TNT phenotype and many others) suggests that we have legitimate reasons to question the moral and ethical standing of the intrinsic/genetic program and its actions.
1. CNN Black in America 2, http://www.cnn.com/SPECIALS/2009/black.in.america/.
2. Lund, MJ et al., Race and triple negative threats to breast cancer survival: a population based study in Atlanta, GA, Breast Cancer Research and Treatment 113: 357-370, 2009.
3. Troester, M.A. and Swift-Scanlan, T, Challenges in studying the etiology of breast cancer subtypes, Breast Cancer Research, 11:104, 2009; http://breast-cancer-research.com/content/11/3/104
4. Irvin, W. and Carey, L, What is triple-negative breast cancer? European Journal of Cancer 44: 2799-2805, 2008.
5. Definitions: c-kit is a cytokine receptor, p53 is a tumor suppressor gene involved in cell cycle checkpoints and apoptosis, TP53 is gene coding tumor protein 53, cyclins are involved in cell cycle control and checkpoints, p16 is a tumor suppressor protein, EFGR is epidermal growth factor receptor, cytokeratins are proteins of keratin-containing intermediate filaments found in the intracytoplasmic cytoskeleton of epithelial tissue, ab crystallin is a heat shock protein, and Rb is a tumor suppressor gene involved in cell-cycle check points.
6. Honrado, E, Benı´tez, J and Palacios, J, The molecular pathology of hereditary breast cancer: genetic testing and therapeutic implications, Modern Pathology 18, 1305–1320, 2005.
7. Klug, W.S, Cummings, M.R, Spencer, C.A, and Palladino, M.A, Concepts of Genetics 9th Edition, (San Francisco, CA: Benjamin Cummings), 2009.
8. Wang, Y, Pelisson, I, Melana, S.M, Go1,V, Holland, J.F, and -T. Pogo, G.T, MMTV-like env gene sequences in human breast cancer, Arch Virol. 146: 171–180, 2001.
9. Hamilton, W.D, The moulding of senescence by natural selection. J. Theor. Biol. 12:12‑45, 1966; Rose, M.R., The Evolutionary Biology of Aging, (New York, NY: Oxford University Press), 1994; Graves, J.L., General Theories of Aging Unification and Synthesis, in Principles of Neural Aging, Dani, Hori, and Walter, eds., Elsevier, 1997; M.R. Rose, H. Passananti, & Margarida Matos, eds., Methuselah Flies, (New York, NY: World Scientific Publishing),2004.
10. Graves, J.L., What a tangled web he weaves: Race, reproductive strategies, and Rushton’s life history theory, Anthropological Theory, Sage Publishers, vol. 2(2): 131-154, 2002.
11. Graves, J.L., The costs of reproduction and dietary restriction in mammals, Growth, Development, and Aging 57(4):233-249, 1993 and Colman, R, Caloric restriction delays disease onset and mortality in Rhesus monkeys, Science 325: 201-204, 2009.
12. Lund, MJ et al., Race and triple negative threats to breast cancer survival: a population based study in Atlanta, GA, Breast Cancer Research and Treatment113: 357-370, 2009 and Millikan, R.C. et al, Epidemiology of basal-like breast cancer, Breast Cancer Research and Treatment 109: 123-139, 2008.
13. Lohmueller K, Indap A, et al. Proportionally more deleterious genetic variation in European than in African populations. Nature 2008; 451: 994-98, 2008.