Stochastic modeling reveals an evolutionary mechanism underlying elevated rates of childhood leukemia

Abstract

Young children have higher rates of leukemia than young adults. This fact represents a fundamental conundrum, because hematopoietic cells in young children should have fewer mutations (including oncogenic ones) than such cells in adults. Here, we present the results of stochastic modeling of hematopoietic stem cell (HSC) clonal dynamics, which demonstrated that early HSC pools were permissive to clonal evolution driven by drift. We show that drift-driven clonal expansions cooperate with faster HSC cycling in young children to produce conditions that are permissive for accumulation of multiple driver mutations in a single cell. Later in life, clonal evolution was suppressed by stabilizing selection in the larger young adult pools, and it was driven by positive selection at advanced ages in the presence of microenvironmental decline. Overall, our results indicate that leukemogenesis is driven by distinct evolutionary forces in children and adults.childhood leukemia | somatic evolution | cancer | stochastic modeling | aging T he incidence of leukemia, like most cancers in humans, increases exponentially with age. However, most types of leukemia have an early peak of incidence (at 0-7 y of age), which subsequently decreases before rising again later in life (Fig. S1). Cancer development is generally thought to result from a sequence of cancer driver mutations that promote selection for recipient cells by conferring a positive fitness advantage within competing stem cell (SC) and progenitor cell pools (1-4). The acquisition of oncogenic mutations is thus thought to be rate-limiting for cancer development, leading to increased cancer incidence with age. Within this paradigm, the higher incidence of leukemia in young children compared with young adults is puzzling, because younger tissues should have accumulated fewer mutations.Evolution is driven by multiple forces, including mutation, selection, and drift. Although mutation is necessary for cancer development, a large body of evidence has accumulated indicating that the ability of oncogenic mutations to drive clonal evolution is not universal and depends on external factors (5-12). Carcinogenesis may therefore be driven or suppressed by non-cell-autonomous processes. One factor capable of limiting the ability of selection to influence population dynamics is drift. In evolutionary biology, the power of drift is known to be inversely related to population size (13). This relationship also holds true for mammalian tissues, as shown for intestinal SC pools, which are segregated into small groups within intestinal crypts (10,14,15). The number of hematopoietic stem cells (HSCs) per individual has been reported to be conserved across mammals at 11,000-22,000 cells in adults (16,17), with an initial pool size of ∼300 HSCs at birth (17) (Fig. S2A). Although higher estimates of the pool size exist (18), it is clear that during prenatal development, and perhaps the early postnatal period of life, the number of HSCs is substantially smaller than the number in the adult pool. Be...

Clonal hematopoiesis, with and without candidate driver mutations, is common in the elderly

“…Estimates for the size of this pool vary widely. [43][44][45] The model was also sensitive to the value of p, Hazard ratio (95% Cl)…”

The Evolution of Lifespan and Age-Dependent Cancer Risk

“…To address this, we suggest that selection for oncogenic clones is highly dependent on tissue context, which changes with age, and necessarily implies that clonal evolution is differentially regulated throughout life. As the probability of sequential mutation accumulation in one cell depends both on cell division rates and the number of dividing cells, early in life when cells cycle rapidly, the lower observed risk of cancer should require that those cells that acquire mutations do not expand into multi-cell clones (which would increase the target size for the next oncogenic mutation) [10]. Late in life, the slow cell division program needs to be compensated by more profound clonal expansions of pre-malignant cells to provide for the elevated risk of cancers that require multiple mutations.…”

Aging of hematopoietic stem cells: DNA damage and mutations?

“…It could already be shown that aging of the HSCs niche and environment plays an important role in selecting and expanding normal and pre-leukemic HSC and HPC clones upon aging (Vas, Senger, et al 2012; Vas, Wandhoff, et al 2012). Thus the concept of adaptive landscapes has been recently developed (Rozhok, Salstrom, and DeGregori 2016). In this concept, the niche environment of HSCs changes upon aging, influencing the functionality of HSCs.…”