Investigating the roles of endoglin in cardiovascular development, disease and repair

Objective: ENG or ALK1 heterozygous loss of function mutations lead to the human disease
hereditary haemorrhagic telangiectasia. HHT is characterised by the development of
arteriovenous malformations (AVMs). These form when arteries and veins directly connect
without an intermediary capillary system. Large AVMs may occur in the lungs, liver and brain,
leading to severe morbidity. There is growing evidence that targeting VEGFA signalling has a
benefit in HHT. However, inhibiting VEGFA can have serious adverse effects.

Aims: Zebrafish embryos have a number of advantages for drug evaluation, including rapid
cardiovascular development, ease of drug administration and transgenic models for imaging.
VEGFA signalling is complex and has numerous downstream pathways. I therefore
hypothesised that selectively targeting individual pathway(s) downstream of VEGFA may be
beneficial in a previously-generated zebrafish genetic model of HHT caused by homozygous
mutation of endoglin (engmu130). I first characterised the cardiovascular phenotype of
engmu130 embryos and adults in vascular territories not previously examined, including the
cerebral vessels, skin, heart and retina. I then tested my hypothesis by treating zebrafish
engmu130 mutant and wildtype embryos with drugs that inhibit global VEGFA signalling or
different pathways downstream of VEGFR2.

Results and conclusions: eng mutant embryos displayed a range of previously undescribed
vascular defects, including an aneurysmal basilar artery (BA), while adult mutants developed
skin AVMs, retinal vascular abnormalities, and cardiac enlargement. VEGF receptor tyrosine
kinase inhibition prevented the vascular abnormalities in eng mutant embryos. Combined
subtherapeutic TOR and MEK inhibition prevented the vascular abnormalities, confirming
synergy between TOR and MEK/ERK signalling pathways. These results indicate the HHT-like
phenotype in zebrafish eng mutants can be mitigated through modulation of VEGFA
signalling, and suggest combined low dose MEK and TOR pathway inhibitors may represent a
therapeutic strategy in HHT.