Analysis of 480,000 Blood Genomes Reveals Bidirectional Selection in Mosaic Chromosomal Alterations
Background: Chromosomal alterations as a record of blood-cell expansion
A study analyzing blood genomes from 484,081 UK Biobank participants detected 43,617 autosomal mosaic chromosomal alterations (mCAs). Rare coding variants in 38 genes were associated with expansion of cell populations carrying copy-neutral loss of heterozygosity (CN-LOH) across the relevant gene regions. The findings showed that the variant allele could either remain in two copies or be eliminated.
Clonal hematopoiesis refers to the expansion in blood of some hematopoietic cells that gain a proliferative advantage. Unlike clonal hematopoiesis of indeterminate potential (CHIP), which is characterized by somatic single-nucleotide variants and small insertions or deletions in leukemia-associated genes, this study focused on mCAs involving deletions, gains, or allelic replacement across large chromosomal regions. Conventional single-nucleotide polymorphism array (SNP-array) analyses had difficulty detecting short alterations and alterations present in a low fraction of cells.
Key findings: Improved detection and two directions of allele selection
The researchers corrected read-depth measurements in whole-genome sequencing (WGS) data, filtered out germline copy-number variants, and reduced duplicate counting of observations derived from the same DNA fragment. Using the improved method, they detected 43,617 mCAs in 35,033 people, including cases previously detected by SNP-array. Thus, not all 43,617 were newly discovered alterations.
Compared with the prior SNP-array analysis of the same cohort, detection sensitivity was about twice as high, and the number of detected mCAs shorter than 1 megabase (Mb) was 10.5 times higher. This reflects improved detection of small chromosomal alterations; it is not a measure of cancer diagnostic accuracy.
CN-LOH replaces one allele with the other while maintaining the total DNA copy number in the region. Among 21,050 CN-LOH events analyzed, the researchers found statistical associations between rare protein-altering variants in 38 genes and CN-LOH involving those genes. The associated genes were found to be involved in DNA damage response, apoptosis, cytokine signaling, and protein ubiquitination, among other processes.
For most candidate genes, CN-LOH tended to duplicate the variant allele that conferred a proliferative advantage. In contrast, for four genes, including CFLAR and IL2RB, the direction was biased toward eliminating the allele carrying a damaging variant. The result suggests that clones may also expand by removing variants that impair cell survival or proliferation.
However, rare coding variants in candidate genes were estimated to explain about 9% of CN-LOH events. Other factors may contribute to the expansion of the remaining clones, and causal mechanisms have not been established for every candidate gene.
Implications and outlook: Details needed to interpret blood-cancer risk
A targeted analysis of 13q14 deletions associated with chronic lymphocytic leukemia (CLL) increased the number detected to 2,906. In a progression analysis of participants of European ancestry without a prior blood-cancer diagnosis, clone sizes were compared for deletions with confirmed breakpoints and no accompanying 13q CN-LOH. The 10-year CLL-free rates were 92% in the group with a cell fraction below 5% and 72% in the group above 10%; the respective 95% confidence intervals were 89–95% and 66–76%. This observational evidence indicates that clone size should be considered alongside the presence of an alteration.
Individual blood-cancer risk scores and the clinical performance of liquid biopsy have not been established. Most factors determining blood-cancer progression remain unresolved, and clinical management after detecting a 13q14 deletion remains a question for future work. The study also could not assess changes in mCAs over time, the specific cell populations in expanded clones, therapy-related clonal hematopoiesis, or sex-chromosome alterations. Detecting short mCAs at low cell fractions and improving representation of non-European ancestry groups are further challenges.
The findings and analytical conditions can be reviewed in the original paper published in Nature Genetics.
Nature Genetics, Published online: 19 May 2026. DOI: 10.1038/s41588-026-02592-0
Summary: Utilizing high-resolution pipelines on whole-genome sequencing (WGS) data from the UK Biobank, this study identified 43,617 novel mosaic chromosomal alterations (mCAs). The architecture establishes a robust genetic link between rare protein-coding variants and the clonal expansion of copy-neutral loss-of-heterozygosity (cnLOH) mutations. Mechanistically, somatic duplication of these rare unmasked variants confers a selective fitness advantage to hematopoietic stem cells, mapping out predictable evolutionary trajectories of clonal hematopoiesis and highlighting high-fidelity targets for liquid biopsies.
Researchers who detect CN-LOH in blood WGS can examine not only the chromosomal region but also which allele remains in two copies and which is lost. Homozygosity of damaging variants and their removal suggest different selection processes; distinguishing between them can help narrow down candidate causes of clonal expansion. The CLL progression results by clone size for 13q14 deletions also provide a basis for follow-up risk-stratification research. However, separate clinical validation is needed before these findings can be applied to patient screening or monitoring criteria. The anonymized mCA detection results and analysis code for which the paper provides a public location offer a starting point for revisiting these interpretations and designing replication studies.