Jumping Genes and Cancer: How DNA Parasites Fuel Tumor Evolution
A modern study published in the journal Science reveals how mobile fragments of human DNA, often referred to as “jumping genes,” contribute to genomic instability in cancer. This instability provides a fertile environment for cancer cells to grow, adapt, and resist treatment.
The Role of LINE-1 Elements
Researchers analyzed genome sequences from tumors exhibiting high activity of LINE-1 (L1) elements – DNA fragments capable of copying themselves and inserting those copies into new locations within the genome. Previously, L1 activity was thought to cause only localized mutations when inserted into genes. However, this research demonstrates that L1 activity can likewise drive large-scale architectural changes in the genome, creating widespread genomic chaos.
“Cancer genomes are more influenced by these jumping fragments of DNA parasites than we previously thought,” explains Professor José Tubio, researcher at the Centro de Investigación en Medicina Molecular y Enfermedades Crónicas (CiMUS) at the Universidade de Santiago de Compostela (USC) and coordinator of the study.
Challenging Previous Assumptions
The findings challenge the long-held belief that L1 activity is merely a consequence of an already unstable cancer genome. The study found that in two-thirds (65%) of cases, L1 events occurred during the early stages of tumor evolution, suggesting a causative role rather than a byproduct.
This discovery could lead to new strategies for early cancer detection and treatment by helping scientists understand how cancer reshapes the genome in its initial stages. As Dr. Bernardo Rodriguez-Martin, Independent Fellow at the Centre for Genomic Regulation in Barcelona and one of the study’s main authors, notes, understanding “when and where L1 activity tips the balance” is crucial for therapeutic targeting.
The Legacy of Ancient ‘DNA Parasites’
L1 elements are ancient genetic sequences considered “parasitic” because they primarily exist to replicate themselves through a process called retrotransposition, often with neutral or detrimental effects to the host organism. 1
Over millions of years of mammalian evolution, L1 elements have amplified within the genome. They comprise approximately 17% of the human genome, with roughly 500,000 copies. However, most are inactive. On average, each individual has a smaller fraction – between 150 and 200 – that retain the ability to “jump” and insert themselves into new genomic locations.
L1 retrotransposition is frequently observed in various cancers, including head and neck, lung, and colorectal tumors. Early evidence suggests these events contribute to tumor growth and adaptation by inducing genomic aberrations affecting cancer-related genes.
Long-Read Sequencing Reveals Genomic Rearrangements
Precisely how L1 elements disrupt genomes, and the extent of that disruption, has been unclear due to limitations of traditional short-read DNA sequencing technologies. Short-read sequencing struggles to reconstruct the architectural alterations caused by L1 elements.
To overcome this, the researchers employed long-read sequencing, which allowed them to visualize the full spectrum of changes L1 elements induce in cancer genome structure, including deletions, translocations, and other rearrangements.
One in 40 Jumps Rewire the Genome
The researchers analyzed ten tumors with high L1 activity – five head & neck squamous carcinomas, four lung squamous carcinomas, and one colorectal adenoma – identifying a total of 6,418 retrotransposition events. The majority of these events were insertions. However, they also identified 152 instances where L1 elements created large-scale structural rearrangements, occurring at a rate of 1 in 40 tumors with high L1 activity and 1 in 60 with lower activity.
“On paper, 152 might not sound like a huge number. But when you’re looking at just ten tumors, that’s extraordinarily high,” says Rodriguez-Martin.
This finding supports the leverage of long-read sequencing in cases where standard tests fail to explain tumor behavior, as short-read sequencing would miss these structural changes. The cost of long-read sequencing is also expected to decrease significantly, making this type of analysis more accessible.
A Novel Mechanism: Reciprocal Translocations
The structural rearrangements exhibited diverse mechanisms, including a previously unknown DNA exchange between chromosomes. The researchers hypothesize this occurs through a “reciprocal translocation,” where two L1 events happen simultaneously on different chromosomes, swapping equivalent amounts of DNA. As Sonia Zumalave, first author of the study, explains, “It’s as if two different pages of a book were torn simultaneously and fragments exchanged with each other. L1 elements behave like glue that sticks both pages together.”
Early Events in Tumor Formation
A common early event in tumor formation is whole genome doubling, where a cancer cell duplicates its entire set of chromosomes. The study found that most L1 activity preceded whole genome doubling, suggesting that retrotransposition is an early mutational process and a significant contributor to the genomic chaos that precedes cancer development.
The study also found that L1 event promoters are typically less methylated in tumors than in surrounding non-tumor tissue, suggesting that epigenetic changes can activate dormant L1 elements.
Study Limitations
The study focused on a deliberately selected set of cancers with extreme L1 activity to detect rare mechanisms. The findings may not be generalizable to all types of tumors.
The research was a collaborative effort involving the Centro de Investigación en Medicina Molecular y Enfermedades Crónicas (CiMUS) at the Universidade de Santiago de Compostela, the Centre for Genomic Regulation (CRG) in Barcelona, Université Côte d’Azur in France, the Francis Crick Institute in the United Kingdom, and the University of Texas MD Anderson Cancer Center in the United States.