How radiotherapy induces fibrosis in cancer patients

How radiotherapy induces fibrosis in cancer patients

Radiation therapy is a common cancer treatment but the doses applied are often limited by the onset of adverse effects in the co-irradiated normal tissue. They can occur even months to years after radiotherapy and susceptibility differs widely among patients. In breast cancer patients, fibrosis in the irradiated tissue is a frequent late reaction occurring in ~20% of treated women. Risk factors for radiation-induced fibrosis are not well-understood, but include genetic predisposition, mode of application and dose-related factors.

Radiation-induced fibrosis is characterized by increased connective tissue stiffness and loss of tissue function at the irradiated site. While its clinical features depend on the affected organ it was shown that common disease patterns exist on the molecular and cellular level.

Fibrotic tissue shows aberrant signalling by cytokines like transforming growth factor-beta 1 (TGFB1) and generation of permanently activated fibroblasts as key inducers of fibrogenesis. Further fibrosis-associated signalling cascades include diacylglycerols (DAGs) that are known regulators of pleiotropic downstream signalling via activation of DAG-binding proteins such as protein kinase C (PKC). Cellular DAG levels are tightly regulated by ubiquitously expressed DAG kinases that limit DAG production via metabolism of DAG to phosphatidic acid (PA). However, the role of DAG kinases in fibrotic processes has remained unexplored.

Given the irreversible nature of fibrotic disease much effort has been made to identify risk factors for radiation fibrosis to adjust radiotherapy to individual patient susceptibility. Genetic variants defined by single nucleotide polymorphisms have been studied as predictors for radiation toxicity, but these markers cannot fully explain the high incidence of radiation-induced fibrosis.

Epigenetic regulation has emerged as a potential mechanism of various diseases including fibrosis. The field of epigenetics comprises pathways and regulatory features that control genomic activity without changes in the DNA sequence. Epigenetic modifications include DNA methylation, histone modifications, non-coding RNAs and three-dimensional chromatin organization. The links between epigenetic alterations and downstream events in fibrosis remain, however, often unknown.

Comprehensive genome-wide DNA methylation analyses of epigenetic patterns in radiation-induced fibrosis have not been done. Therefore, researchers based the present study on the hypothesis that pre-existing DNA methylation differences in unirradiated primary human fibroblasts influence the risk of later fibrosis development after radiotherapy.

They identified DGKA as an epigenetically deregulated kinase involved in fibroblast activation after irradiation and stress exposure which has potential to serve as a therapeutic target for preventing radiation-induced fibrosis.

DNA methylation profiling of dermal fibroblasts obtained from breast cancer patients prior to irradiation identifies differences associated with fibrosis. One region is characterized as a differentially methylated enhancer of diacylglycerol kinase alpha (DGKA).

Decreased DNA methylation at this enhancer enables recruitment of the profibrotic transcription factor early growth response 1 (EGR1) and facilitates radiation-induced DGKA transcription in cells from patients later developing fibrosis. Conversely, inhibition of DGKA has pronounced effects on diacylglycerol-mediated lipid homeostasis and reduces profibrotic fibroblast activation.

Collectively, DGKA is an epigenetically deregulated kinase involved in radiation response and may serve as a marker and therapeutic target for personalized radiotherapy.