DNA aptamers targeting endothelial RAS mutants for vascular drug delivery in brain arteriovenous malformations.

Brain arteriovenous malformations – the clinical problem: The overall goal of our research is to develop new ways to treat a blood vessel disorder that occurs in the brain, called arteriovenous malformations (AVMs). AVMs present as tangled collections of abnormal blood vessels that are at increased risk of rupturing, causing stroke. Each stroke has a 50% chance of causing permanent disability or death. This form of stroke occurs primarily in children and young adults, rather than as a result of aging, thus the financial and quality of life costs to individuals and their families over a lifetime are significant. While small AVMs can be removed by surgery or induced to closure using radiotherapy, these approaches carry their own risks or can be ineffective. Radiotherapy can take 2-3 years to induce AVM closure, while 20% of patients will see no cure even after 5 years. For patients with large AVMs, surgery is generally not suitable, and the dose of radiation required to achieve cure would risk significant damage to surrounding brain tissue. As such, 90% of patients with large AVMs currently have no treatment options. There is an urgent need to develop new treatments for these young, vulnerable patients who face a high risk of sudden death or disability.

A vascular targeting approach to AVM treatment: The neurosurgery research team at Macquarie University has been working towards improving current treatments for patients with brain AVMs using an approach called “vascular targeting”. This involves delivering a drug through the bloodstream to induce localised clotting and blood vessel closure specifically within the diseased AVM blood vessels. Key to this approach is 1) identifying molecular targets, proteins that are unique to the surface of the diseased blood vessels but absent from the normal, healthy blood vessels; and 2) developing complementary targeting molecules for drug delivery, that can recognise and bind these targets when delivered through the blood. No suitable molecular markers on AVM vessels were identified, so we investigated delivery of a focused dose of radiation to the AVM vessels to generate markers on the AVM surface that could then be targeted. Results in this area are promising and continue to be examined, however recent discoveries in the field have provided a new avenue for exploration with regard to our vascular targeting approach.
Targeting RAS mutations in AVMs: The cause of brain AVMs has been poorly understood. Approximately 5% of cases occur in hereditary conditions with known gene mutations that alter cell signalling pathways regulating blood vessel development. The other 95% occur sporadically in the population and until recently the cause was unknown. In 2018, it was discovered that at least 50% of brain AVMs carry a mutation in a family of genes called RAS and that cells with this mutation play a role in abnormal vessel development. Members of the RAS family are commonly mutated in cancer cells and contribute to the uncontrolled cellular growth that leads to tumour formation. This finding has been a significant advancement in the AVM field and in our understanding of how brain AVMs form, but in addition, for the first time this provides a defined cell type with characteristics that can be easily modelled for study in the laboratory.

The aims of this study are: 1) to develop an AVM cell culture model in the laboratory that expresses the RAS-activating mutation; and 2) use it to generate novel targeting molecules (DNA aptamers) that can specifically recognise and bind molecular targets that only occur on the cell surface of these mutant cells in response to RAS activation. These novel targeting molecules could then be used to deliver occlusive drugs with high specificity to the RAS-mutated AVM vessels and provide a safe and effective new treatment approach for AVM patients.