Bone-aging study links R-loop buildup to osteoblast DNA damage

Aging bones lose more than mineral density; they lose the cells that build bone in the first place. A new study published in Cell Death Discovery reports that a signaling chain running from the enzyme PRMT6 through the mitochondrial protein SIRT5 to the fatty-acid oxidation protein HADHA helps protect osteoblasts from a specific form of genome stress: the accumulation of R-loops, RNA–DNA hybrids that can collide with DNA replication and trigger DNA damage. In mouse models and cultured primary osteoblasts, disrupting this pathway increased R-loop signals, raised markers of DNA damage and cellular senescence, and worsened trabecular bone loss, while restoring SIRT5 partially reversed these effects.

The analysis showed that the proportion of osteoblasts declines during aging, and that expression of Sirt5, which encodes a mitochondrial desuccinylase, drops alongside it. SIRT5 is known to regulate metabolic adaptation by removing succinyl groups from lysine residues on proteins, and it has been implicated in cellular stress responses, but its role in R-loop homeostasis and skeletal aging had not been established.

Compared with wild-type controls, Sirt5-deficient mice showed more pronounced trabecular bone loss and impaired osteogenesis. Molecular markers told a consistent story: levels of the osteogenic factors Runx2, ALPL and OCN were reduced, while the senescence markers p16 and p21 were elevated. Alkaline phosphatase activity decreased, and more cells stained positive for senescence-associated beta-galactosidase.

Transcriptomic profiling of the deficient cells revealed activation of stress-related gene programs and suppression of osteogenic and metabolic pathways. The mechanistic core of the study, however, came from lysine succinylation proteomics. When Sirt5 was absent, succinylation accumulated globally across the proteome, and the succinylation landscape of metabolic proteins was reshaped. Among the affected proteins was HADHA, a component of the mitochondrial trifunctional enzyme complex involved in fatty-acid beta-oxidation. Sirt5 deficiency led to elevated multi-site succinylation of HADHA and a corresponding reduction in HADHA protein abundance, linking the loss of a desuccinylase to a specific metabolic protein target.

In Sirt5-deficient osteoblasts, R-loops were broadly increased around transcription start site regions. Immunofluorescence and Western blotting showed elevated signals for S9.6, the antibody that recognizes RNA–DNA hybrids, and for gamma-H2AX, a marker of DNA double-strand breaks. The picture that emerged was of transcription–replication conflicts arising from unresolved R-loops, producing DNA damage accumulation that would normally be kept in check.

The upstream regulator came into view through the Prmt6-deficient model. PRMT6, an arginine methyltransferase, was found to sit above SIRT5 in the pathway: Prmt6 deficiency downregulated Sirt5 and produced a phenotype that mirrored Sirt5 loss, including trabecular bone deterioration and increased R-loop signals. This placed PRMT6 upstream of the succinylation remodeling that drives the downstream damage, defining what the authors describe as the PRMT6–SIRT5–HADHA axis.

Restoring SIRT5 increased HADHA expression, attenuated R-loop accumulation, and reduced the senescence phenotypes induced either by oxidative stress, modeled in primary osteoblasts with hydrogen peroxide treatment, or by the loss of Prmt6. In aged or deficient mice, Sirt5 overexpression alleviated trabecular bone loss. The rescue experiments support the interpretation that SIRT5 acts as a functional node whose activity can be manipulated to protect osteoblasts from R-loop-driven damage.

The convergence of evidence from naturally aged mice, two genetic knockout models, cell culture stress models and viral rescue experiments strengthens the case that the PRMT6–SIRT5–HADHA pathway is a genuine regulator of osteoblast aging rather than an artifact of any single experimental system.

Age-related osteoporosis is characterized by impaired bone formation and progressive deterioration of trabecular microarchitecture, and declining osteoblast abundance and function together with the accumulation of cellular senescence are recognized as key contributors to skeletal aging. During aging, increased genomic instability and persistent DNA damage responses further impair osteogenic capacity. The new findings add a specific molecular mechanism to this framework: aberrant accumulation of R-loops, which can induce transcription–replication conflicts and DNA damage, appears to be kept in check in osteoblasts by a PRMT6-dependent supply of SIRT5, whose desuccinylase activity maintains HADHA abundance and, by extension, R-loop homeostasis.

The authors suggest that targeting the PRMT6–SIRT5 signaling pathway may represent a potential therapeutic strategy for preventing osteoporosis associated with aging. As with all preclinical work, the findings rest on mouse models and cultured cells, and the study is an early-access, peer-reviewed accepted version that remains subject to further edits before the final Version of Record. Whether modulating PRMT6 or SIRT5 activity in humans can safely preserve bone-forming capacity in older adults remains an open question, but the identification of a succinylation-dependent route from nuclear arginine methylation to mitochondrial metabolic protein stability, and from there to R-loop control and genome integrity in osteoblasts, offers a concrete molecular target for future investigation into the biology of skeletal aging.

Subject of Research: How the PRMT6–SIRT5–HADHA signaling pathway protects osteoblasts from R-loop-induced DNA damage during age-related osteoporosis.

Article Title: PRMT6–SIRT5–HADHA signaling safeguards osteoblasts from R-loop-induced DNA damage during age-related osteoporosis

Article References: Song, X., Huang, S., Guo, X., Gao, C., Yue, G., Chen, L., Du, Z., Xu, J., Jiang, X., Liu, X., & Wang, S. (2026). PRMT6–SIRT5–HADHA signaling safeguards osteoblasts from R-loop-induced DNA damage during age-related osteoporosis. Cell Death Discovery. https://doi.org/10.1038/s41420-026-03354-x

Image Credits: AI Generated

DOI: 10.1038/s41420-026-03354-x

Keywords: osteoporosis, osteoblasts, SIRT5, PRMT6, HADHA, R-loops, DNA damage, succinylation, cellular senescence, bone aging, trabecular bone, single-cell RNA-seq