DNA Repair Protein RAD54L Protects Developing Egg and Sperm Precursors from Toxic Enzyme Traps

Every embryo begins with a race against time. Primordial germ cells, the rapidly dividing precursors that ultimately give rise to eggs and sperm, must copy and safeguard their entire genome over and over while laying down the reproductive reserve that an individual will depend on for life. Any failure to keep that genetic material intact can shrink the reserve and compromise fertility long before anyone notices. A new study published in Cellular and Molecular Life Sciences has now identified an unexpected guardian of these fragile cells: RAD54L, a protein better known for its role in DNA repair, turns out to protect the germline by helping to dispose of a particularly stubborn form of DNA damage.

The research, led by Chenxi Li, Yingying Qin and Yajuan Yang of Shandong University and collaborating institutions in China, focused on what happens when mouse primordial germ cells lose RAD54 like, or RAD54L. The team found that without this factor, the cells proliferate poorly, leaving an insufficient pool of germ cells and ultimately impairing fertility. The defect traces back to a specific molecular hazard: the accumulation of topoisomerase I cleavage complexes, abbreviated TOP1ccs, which are among the most common and most dangerous lesions that arise during DNA replication.

To appreciate why TOP1ccs matter, it helps to understand what topoisomerase I normally does. As the two strands of the double helix are prised apart for copying, the DNA ahead of the replication machinery becomes overwound, like a rope twisted too tightly. Topoisomerase I relieves this torsional stress by nicking one strand of the DNA, allowing it to swivel freely, and then sealing the break again. The enzyme normally completes this cut-and-paste cycle in a fraction of a second. Occasionally, however, the enzyme becomes trapped mid-cycle, covalently bonded to the DNA at the nick it just created. That trapped enzyme-DNA adduct is the TOP1 cleavage complex, and if it is not removed promptly, the approaching replication fork collides with it, converting a transient intermediate into a double-strand break and stalling DNA synthesis.

Cells possess dedicated machinery to clear these protein-DNA adducts, and the new study adds a surprising player to that list. The researchers showed that RAD54L promotes the degradation of TOP1ccs through the ubiquitin-proteasome pathway, the cellular recycling system that tags unwanted proteins with chains of ubiquitin and delivers them to the proteasome for destruction. The key to this activity lies in a partnership between RAD54L and tripartite motif-containing protein 21, or TRIM21, which the authors identify as a newly recognized E3 ubiquitin ligase responsible for clearing TOP1ccs. E3 ligases are the enzymes that confer specificity on the ubiquitin system, deciding which targets get marked for degradation. In this case, TRIM21 appears to be the enzyme that flags trapped topoisomerase I for removal.

The mechanistic twist is that RAD54L does not itself ligate ubiquitin. Instead, it interacts with TRIM21 and stabilizes the protein, keeping sufficient TRIM21 available to do its clearing work. When RAD54L is absent, TRIM21 is not maintained properly, TOP1ccs accumulate, and the resulting DNA damage escalates. In RAD54L-deficient primordial germ cells, this cascade of events exacerbates DNA damage, undermines proliferation and erodes the reproductive reserve. The finding reframes RAD54L, historically viewed as a homologous recombination factor that helps search for and invade homologous DNA templates during double-strand break repair, as also acting upstream of repair, at the stage of removing the lesions that would otherwise create breaks in the first place.

Primordial germ cells are an especially revealing setting for this kind of analysis. These cells undergo rapid mitotic divisions during embryonic development, and their genome must be transmitted faithfully across generations. High levels of replication, combined with the metabolic demands of a growing embryo, make TOP1ccs a constant threat. The DNA damage response mechanisms that protect somatic cells have been studied extensively, but the regulatory networks that safeguard the germline during this early mitotic phase have remained largely unexplored. By showing that a defect in a single DNA damage response factor can compromise the founding population of the germline, the study connects genome maintenance at the molecular level to fertility at the organismal level.

The experimental logic of the work follows a path familiar to genome stability researchers but with distinctive results. Mice lacking RAD54L in their germ cells show proliferation defects in the primordial germ cell population. The DNA damage observed in these cells is not random; it is specifically tied to TOP1cc accumulation, which means the damage is preventable if the trapped complexes are cleared. The demonstration that RAD54L stabilizes TRIM21 provides a coherent causal chain: loss of RAD54L destabilizes the E3 ligase, the ligase can no longer ubiquitinate trapped topoisomerase I efficiently, the adducts persist, replication forks collide with them, and DNA damage mounts until cells falter or die.

The discovery also resonates with a broader and clinically important theme in cancer medicine. Drugs called topoisomerase I poisons, including camptothecin and its clinical derivatives irinotecan and topotecan, work precisely by stabilizing the TOP1 cleavage complex, trapping the enzyme on DNA and forcing tumor cells into lethal collisions between replication forks and the drug-stabilized adducts. Understanding how healthy cells, and particularly vulnerable populations like germ cells, clear TOP1ccs has implications for how such treatments affect fertility and for how resistance to these drugs emerges. A pathway involving RAD54L and TRIM21 that governs the lifespan of TOP1ccs could, in principle, influence both the toxicity and the efficacy of these widely used chemotherapeutics, although the new study addresses physiology rather than treatment outcomes.

TRIM21 itself carries an interesting scientific history. Long studied in immunology as an antibody receptor inside cells, it has more recently been appreciated as a versatile quality-control factor that recognizes and ubiquitinates a range of intracellular targets. Its identification here as the E3 ligase for TOP1cc clearance in germ cells extends that repertoire into genome maintenance and adds a new dimension to how the ubiquitin system participates in the DNA damage response. The RAD54L-TRIM21 axis suggests that repair factors may do more than mend breaks after they occur; some may actively manage the burden of endogenous lesions so that breaks are less likely to arise at all.

For the field of reproductive biology, the study fills in a piece of a larger puzzle: how the mitotic phase of gametogenesis, before meiosis begins, is protected against the wear and tear of rapid proliferation. The authors frame their results as extending understanding of the regulatory mechanisms that safeguard genome integrity during this process, and the practical significance is clear. A diminished primordial germ cell pool is a diminished reproductive reserve, and the work points to TOP1cc accumulation as one preventable driver of that loss. As with any mouse study, translating the findings to human fertility will require further work, but the core machinery of topoisomerase I, the ubiquitin-proteasome system and the DNA damage response is conserved across mammals, making the pathway a plausible target for future investigations into reproductive health and the side effects of topoisomerase-targeting therapies.

Subject of Research: The role of RAD54L and TRIM21 in clearing topoisomerase I cleavage complexes to maintain genome stability in primordial germ cells

Article Title: RAD54L counteracts topoisomerase I cleavage complexes by stabilizing E3 ligase TRIM21 to maintain genome stability

Article References: Li, C., Wang, S., Xu, W., Kong, Z., Wen, C., Zhao, S., Cao, L., Chen, Z.-J., Zhao, S., Qin, Y., & Yang, Y. (2026). RAD54L counteracts topoisomerase I cleavage complexes by stabilizing E3 ligase TRIM21 to maintain genome stability. Cellular and Molecular Life Sciences. https://doi.org/10.1007/s00018-026-06458-w

Image Credits: AI Generated

DOI: 10.1007/s00018-026-06458-w

Keywords: RAD54L, TRIM21, topoisomerase I cleavage complexes, primordial germ cells, genome stability, DNA damage response, ubiquitin-proteasome pathway, replication stress, fertility, gametogenesis, E3 ubiquitin ligase, DNA repair