Clonal Evolution

Punctuated Evolution

4 min read

Punctuated Evolution

Definition

Punctuated evolution in cancer refers to rapid bursts of adaptive genomic change — often involving large-scale chromosomal alterations — rather than the gradual, stepwise accumulation of mutations. The result is a rapid clonal-sweep and a functionally homogeneous tumor mass with low intratumor-heterogeneity. This contrasts with gradual (Darwinian) evolution, which produces ongoing subclonal diversification.

The concept is borrowed from Eldredge and Gould’s (1972) theory of punctuated equilibrium in species evolution, which proposed that speciation occurs in rapid bursts separated by long periods of stasis (Turajlic et al., 2019).

Punctuated Equilibrium

In the cancer context, punctuated equilibrium describes a specific evolutionary pattern: adaptation occurs within a small, spatially isolated niche until the newly adapted clone rapidly expands through the wider population. Because the niche is small, the gradually adapting population is unlikely to be sampled before expansion, and therefore the evolutionary dynamics appear punctuated. “Equilibrium refers to long periods of apparent clonal stasis during which the adapted clone persists at low, likely undetectable, frequency in the population” (Turajlic et al., 2019, p. 405).

The Hopeful Monster

When the adaptive mutation is a large-scale alteration of the genome — loss, gain, translocation, or fusion of chromosomes — the adapted clone is termed a hopeful-monster (Turajlic et al., 2019). The term originates from Goldschmidt’s (1940) theory that major evolutionary transitions in species could occur through single macromutations rather than incremental changes. In cancer, CIN provides the mechanism: “CIN allows for the generation of true hopeful monsters — grossly altered clones that may be adaptive — a phenomena thought to be very rare in species evolution” (Turajlic et al., 2019, p. 404).

Mechanisms

Several genomic catastrophes can produce punctuated change:

  • chromothripsis: shattering of one or two chromosomes followed by error-prone repair (Stephens et al., 2011)
  • chromoplexy: complex rearrangements involving multiple chromosomes (Baca et al., 2013)
  • whole-genome-duplication: tetraploidization followed by chromosomal losses
  • Mitotic crises: simultaneous gains of multiple chromosomal segments (Gerstung et al., 2020)

The Illusion of Punctured Gradualism

The punctuated appearance of some tumor evolutionary trajectories may partly be an observational artifact. Turajlic et al. (2019) describe a scenario in which adaptation occurs within a small, spatially isolated niche until the newly adapted clone rapidly expands through the wider population. Because the niche is small, the gradually adapting population is unlikely to be sampled before expansion, and the evolutionary dynamics appear punctuated. “Equilibrium refers to long periods of apparent clonal stasis during which the adapted clone persists at low, likely undetectable, frequency in the population” (p. 405).

The intermediate-clones that bridge the pre-adaptation and post-sweep states are real but invisible — they exist below the cancer-cell-fraction detection floor (~0.05–0.10 at standard ~100× sequencing), in a spatially restricted niche, and are erased by the sweep itself before the tumor reaches a clinically detectable size.

A further complication: even after a sweep, not all clonal mutations are truncal. Bozic et al. (2016) showed that when the death-birth ratio δ = d/b is close to 1, neutral passenger-mutations can reach fixation during clonal expansion — they become clonal (present in all cells) without ever being present in the founding cell. A tumor that appears homogeneous after a sweep may harbor multiple clonal passengers acquired during the expansion phase, each representing a bottleneck where intermediate lineages went extinct. The apparent homogeneity thus masks a more complex history of competition and extinction among intermediate clones (see intermediate-clones).

Clinical Consequences

In TRACERx Renal, punctuated evolution was associated with rapid tumor growth, widespread early metastasis, and poor outcomes. These tumors are characterized by low driver ITH and high levels of clonal aneuploidy that became fixed early in tumor evolution (Turajlic et al., 2019). In TRACERx Lung, CIN conferred increased risk of recurrence and death independently of known predictive markers (Jamal-Hanjani et al., 2017, cited in Turajlic et al., 2019).

The clinical challenge is acute: tumors that undergo punctuated evolution may acquire metastatic competency at the earliest stages of cancer evolution — they are “born to be bad.” The window for cancer-early-detection may be very limited, as “the latency between the emergence of the invasive clone and metastatic spread can be short” (Turajlic et al., 2019, p. 413).

Graph View

Backlinks