By Sashini Ranawana
The increasing feasibility of sequencing whole human genomes has enabled the identification of genetic risk factors for numerous cancers and illnesses. Sequence variations in the BRCA1 gene have been firmly linked to ovarian and breast cancer, mutations in the RUNX1 gene predispose individuals to develop acute myeloid leukaemia, while the presence of the E4 allele for the apolipoprotein E is associated with Alzheimer’s Disease.1 Osteosarcoma, similarly, has been known to affect a greater proportional of individuals with mutations in the RB1 and TP53 genes encoding the retinoblastoma and p53 tumour-suppressor proteins.2 Its predominance in a young population hints at a strong hereditary risk, which opens up the potential for more indicative predictive markers to be identified. In 2013, a Genome-Wide Association Study showed that a crucial polymorphism in the GRM4 gene, coding for the glutamate metabotropic receptor 4, significantly increases the chance of developing this particular type of cancer.3 In doing so, it highlighted an unlikely link between immune molecules and the onset of osteosarcoma.
Cancer is a disease that can be exacerbated by the immune system, through the release of cytokines, survival and growth factors by immune cells in the tumour microenvironment. By considering this, researchers at the Garvan Institute of Medical Research were able to hypothesise a possible link between the GRM4 polymorphisms and the immune response to osteosarcoma tumours. Their experiments importantly highlighted a tumourigenesis-promoting role for the cytokine IL-23 in GRM4-mutated patients.4
By generating GRM4-knockout (GRM4-/-) mice, Kansara et al. were able to assess the rate of tumour formation in bones upon injection with the ionising carcinogen 45Ca. Knockout mice had enhanced osteosarcoma progression, as monitored by survival times compared to unmodified wild type mice. The dendritic cells of these GRM4-/- mice also secreted more of the pro-inflammatory cytokine IL-23. When exposed to osteosarcoma cell-cultured growth media, dendritic cells had an increased expression of IL-23 and cyclic AMP (cAMP). The direct role of cAMP was elucidated by adding an agonist for it, which increased IL-23 production.4 The GRM4 protein is known to negatively regulate the cAMP pathway, implicating it in the regulation of IL-23 as well.3 This was confirmed when dendritic cells were exposed to a positive allosteric modulator of GRM4, PHCCC, which increased the function of the endogenous GRM4 protein. Treatment with PHCCC reduced the cAMP signaling pathway-modulated expression of IL-23, revealing a role for GRM4 in supressing pro-inflammatory cytokine production.4 Such results indicate that osteosarcoma cells induce biochemical changes that utilise the inflammatory response to facilitate proliferation. The mutation of GRM4 in some patients could reduce glutamate signaling and therefore prevent this exaggerated response from being controlled in the tumour environment.
Initial experiments identified IL-23 as the key molecular connection between GRM4 and the severity of osteosarcoma. When IL-23 was knocked-out in experimental mouse models, a significantly lower population developed bone tumours after exposure to 45Ca, compared to mice still expressing the IL-23 gene in their immune cells. After moving onto human tissue, Kansara et al. found that both in situ hybridisation experiments and quantitative reverse transcriptase PCR showed higher IL-23 levels in osteosarcoma tumour samples compared to healthy control bone samples. A further correlational study between IL-23 levels and patient survival rates revealed a strong negative association.4 Such results were clear evidence that the pro-inflammatory cytokine influences tumour progression in vivo.
Even more notably, IL-23 also helped explain the results of a previous study conducted by Jensen et al. on the increased incidence of sarcomas in Danish patients with the autoimmune condition psoriasis.5 In these individuals, epidermal cells are stimulated to proliferate by the interleukins IL-17 and IL-23 released from CD4+ Th17 and dendritic cells respectively.6 This ‘IL-23/IL-17 axis’ plays a significant role in forming the keratinocyte plaques that are characteristic of the disease. As a result, IL-23 antagonists targeting the p19 protein subunit have already been developed as effective treatments against psoriasis. Understanding that the susceptibility of psoriasis patients to cancers of the bone and cartilage was due to IL-23 opened up the possibility of using such drugs in the treatment of osteosarcoma. To investigate this, Kansara et al. injected mice with osteosarcoma cells and subjected them to treatment with an anti-IL-23 antibody. Tumour progression and death rates in these mice were considerably lower compared to control mice treated with a non-specific antibody. Gene expression profiles of the tumour cells also showed reduced IL-23 transcription. When administered along with doxorubicin, a current and effective drug for osteosarcoma, the IL-23 antibody reduced the rate of tumour growth even more than when doxorubicin was given alone.4,7 The mechanism of a drug that is currently being used against an inflammatory skin condition can therefore be altered to utilise against an aggressive type of cancer.
There is an increasing need for new cancer therapies due to the growing resistance of malignant cells to chemotherapeutic drugs. Doxorubicin, cisplatin and methotrexate, amongst other drugs, are being limited in their efficacy as primary or adjuvant treatments for osteosarcoma.8 Identifying new predictive markers enables the early detection and treatment of pre-metastatic tumours and highlights molecular pathways that contribute to dysregulated cellular dynamics. With osteosarcoma, the association between GRM4 mutations and IL-23 levels led to the identification of new drug targets and a possible immune molecule biomarker to monitor cancer progression. It also provided a rational for further research into the repurposing of IL-23 antibody treatments, originally used against psoriasis.4 Pre-existing treatments like this could be used immediately if they are found to be effective in clinical trials, thereby reducing the costs and time needed to develop new drugs. In addition to osteosarcoma, IL-23 has also been associated with conditions like rheumatoid arthritis and multiple sclerosis, raising the possibility that an antagonist might be effective against these diseases as well.9 Innovative treatments are needed to combat multiple drug resistance, and repurposed drugs are only the first step.
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