Clonal Evolution

Clonal Sweep

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Clonal Sweep

Definition

A clonal sweep (also called a selective sweep) is the process by which a subclone harboring a fitness-conferring driver-mutation expands to dominate the tumor cell population, reducing genetic diversity by driving competing lineages to low frequency or extinction. The term is borrowed directly from population genetics, where a selective sweep describes the reduction of genetic variation near a positively selected allele.

Mechanism

In Nowell’s (1976) model, when a variant subpopulation acquires an additional selective advantage, “this mutant becomes the precursor of a new predominant subpopulation” (p. 24). As this new dominant clone expands, the previously dominant clone is displaced. The sweep may be complete (all cells in the population carry the adaptive mutation) or incomplete (the adaptive clone dominates but does not fully eliminate competitors).

When Sweeps Can Occur

A clonal sweep requires a simple mathematical condition to be met (Greaves & Maley, 2012):

Time to next driver mutation > Time required for the current clone to sweep through the population.

flowchart TD
    N[N = tumor size] --> TauK[τ_k ≈ T/ks × log 2ks/u<br/>waiting time for next driver]
    N --> SweepT[Sweep time ∝ N / ks<br/>time for clone to reach fixation]
    TauK --> Cond{"τ_k > sweep time?"}
    SweepT --> Cond
    Cond -->|"YES: early tumors N ~ 10³−10⁵"| Sweep[Clean sequential sweeps]
    Cond -->|"NO: late tumors N ~ 10⁸−10¹¹"| Interference[Clonal interference]
    Sweep --> Result1["Nowell's sequential sublines<br/>one clone dominates at a time"]
    Interference --> Result2["Branching evolution<br/>multiple clones coexist and compete"]
    Therapy[Therapy] -.->|"kills sensitive cells<br/>artificially shortens sweep time"| Sweep

If the next driver mutation appears before the previous clone has reached fixation, the two clones compete. The first never completes its sweep — the result is clonal-interference, not a clean succession.

This condition explains why sweeps dominate early tumor evolution but become rare later:

Early tumors are small (N ~ 10³–10⁵). The sweep time — the number of generations for a clone to expand from one cell to dominance — is short. Meanwhile, the waiting time for the next driver is long because the target population (the N cells that could mutate) is small. The condition holds: τ_k > sweep time. Clean sequential sweeps are the expected pattern. This is the regime Nowell (1976) described: “sequential selection by an evolutionary process of sublines.”

Late tumors are large (N ~ 10⁸–10¹¹). The sweep time is long — a clone must traverse an enormous population. The waiting time for the next driver is short because the target population is vast. The condition fails: a new driver mutation appears before the previous clone can sweep. Multiple clones expand simultaneously, competing for resources and space. The result is branching evolution with clonal interference, not successive sweeps. Greaves & Maley (2012) emphasized this shift: clonal evolution is “not always successive selective sweeps.”

Mutation rate matters. Higher mutation rates (e.g., from mutator-phenotypes or chromosomal-instability) shorten τ_k, making sweeps less likely at any given tumor size. Lower rates extend τ_k, preserving the sweep regime further into tumor growth. The Bozic et al. (2010) waiting-time formula quantifies this: τ_k ≈ (T/ks) × log(2ks/u). As u increases, τ_k decreases — the next driver arrives sooner.

Fitness advantage matters. A larger s shortens both sweep time (the clone grows faster) and τ_k (more divisions = more mutation opportunities), but the effects are not symmetric. The sweep time scales as 1/(ks); τ_k scales as 1/k × log(1/u). For fixed u, variation in s affects sweep time more strongly, meaning strongly advantageous drivers are more likely to complete sweeps.

Therapy removes competition. Cytotoxic treatment eliminates sensitive cells, clearing the field for any resistant clone. This artificially shortens sweep time while τ_k remains unchanged — the condition for a sweep is imposed by treatment. This is why therapy-induced sweeps are the central mechanism of treatment failure (see therapy-resistance).

Complete vs. Incomplete Sweeps

When a clonal sweep is complete, the within-tumor evolution reverts to neutral-evolution with respect to that selective event — the population is now homogeneous for the adaptive mutation, and further selection acts on new variants arising within the new dominant clone (Turajlic et al., 2019).

When a sweep is incomplete, multiple lineages coexist, and evolution continues to be shaped by both selection and clonal-interference.

Detection

Detecting past clonal sweeps from genomic data is challenging. Once a selected clone has taken over and reached fixation, the population is homogeneous with respect to the selective alteration, and “dense longitudinal sampling is necessary to accurately detect selection” (Turajlic et al., 2019, p. 415). Without longitudinal data, sweeps that occurred before sampling are invisible — evolution appears neutral even though selection was the driving force.

Metastatic Timing

Al Bakir et al. (2023) used the last clonal sweep in the primary tumour as the reference point for timing metastatic divergence in NSCLC. Primary clonal mutations that arose during or before the last sweep are present in all primary tumour cells. If the metastasis shares these mutations, divergence occurred after the last sweep (late divergence, ~75% of cases). If the metastasis lacks them, divergence occurred before the last sweep (early divergence, ~25%), and the primary subsequently underwent a complete clonal sweep that the metastatic clone missed. This framework makes clonal sweeps the fundamental clock for timing metastatic events in tumour evolution.

Punctuated Sweeps

In punctuated-evolution, multiple driver events are acquired in a short burst, producing a rapid and comprehensive clonal sweep that results in a functionally homogeneous tumor mass. Such tumors are characterized by low driver intratumour heterogeneity and high levels of clonal aneuploidy that became fixed early in evolution (Turajlic et al., 2019). In TRACERx Renal, tumors with this pattern grew rapidly, metastasized widely, and had worse outcomes than those with ongoing subclonal diversification.

Clinical Significance

Clonal sweeps under therapy are a primary mechanism of treatment failure. Chemotherapy and targeted therapy exert strong selective pressure that can trigger sweeps of pre-existing resistant subclones. As noted by Nowell (1976), “the same capacity for variation and selection which permitted the evolution of a malignant population from the original aberrant cell also provides the opportunity for the tumor to adapt successfully to the inimical environment of therapy” (p. 27).

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