Robustness in eye development: addressing the mechanisms behind eye formation and growth compensation

The eye is made of diverse structures and cell types that enable light to be transformed into a nerve impulse that travels to the brain. Despite their complexity, eyes develop from the eye primordium, a single group of cells established during the third week of human gestation. These cells will bud apart laterally to form the two optic vesicles, which will keep growing independently and differentiate into the distinct cell types that make an eye.
There are still many unanswered questions regarding eye formation, for example, why are some people born with smaller or no eyes? This project aims to answer this particular question by combining genetic analysis of patients that have developmental eye globe defects with research in zebrafish, this project aims to answer this particular question.
Genes control embryo development, and when specific genes are impaired by mutations, people are born with morphological defects. Therefore, studying what goes wrong when genes are mutated provides information about their function. Knowing which genes are mutated in patients can help us understand how organs are formed and are also necessary for the development of therapies to prevent or cure disease. Currently, only 10% of patients born with anophthalmia (A, no eyes) or microphthalmia (M, very small eyes) can be genetically diagnosed with a single pathogenic mutation. For the rest, we do not know the responsible gene mutation, which limits both the possibility of appropriate medical care and of developing therapies.
Humans and zebrafish eyes develop in a similar way, even using the same genes. Therefore, studying the genes that work in fish eye development is applicable to humans. In zebrafish, we can also mimic mutations found in patients and study their effects upon eye formation.
I recently discovered that zebrafish mutants that develop an eye primordium with half the number of cells can continue growing until they reach the appropriate size due to a growth compensation mechanism. Hence, growth compensation can mask the effect of mutations, which could explain why it is difficult to find genes involved in eye defects. However, I also found that certain combinations of mutations can further impair eye formation leading to embryos with no eyes or with small underdeveloped eyes, like in human patients. The important question is: how can we identify more gene mutations that lead to eye formation defects?

Building on my recent findings, and using tools I have generated, this project aims to:

1. Identify new gene mutations in patients with anophthalmia and microphthalmia.

2. Validate mutations identified in anophthalmia and microphthalmia patients in zebrafish.

3. Molecular and cell biological characterisation of validated genes and mutations.


Overall, this project aims to identify new diagnostic genes for anophthalmic and microphthalmic patients, to enable appropriate genetic counselling, enhance our understanding of eye development, and set the grounds to generate new therapies.

Addressing the function of the genes that control organ development is a fundamental goal in biology. However, compensatory mechanisms can mask the phenotype of mutations in many genes, showing no effect and limiting our ability to understand their true function. Therefore, relating a phenotype to a gene represents a major challenge in the study of developmental genetics. For example, only ~10% of anophthalmic (A, no eyes) or microphthalmic (M, small eyes) patients can be genetically diagnosed. This suggests that there are many A/M related genes remaining to be discovered, making research in this area fundamental to our understanding of eye development, enhancing the chance to improve genetic diagnosis and patient care in the future.
We recently discovered that because of eye growth compensation, zebrafish tcf7l1a mutant embryos can form normal-sized eyes despite developing from an eye primordium with half the number of cells. Building on this finding, I performed a forward genetic modifier screen over the tcf7l1a mutant background and, by which we identified genes involved in zebrafish eye specification and growth compensation which would have otherwise remained undiscovered.
We will exploit this genetic approach to identify, validate and study new genes required for eye formation and growth compensation using zebrafish as an animal model system.
We hypothesise that genes required for eye formation remain undiscovered because of compensatory mechanisms and that new genes involved in eye formation can be validated and studied by using zebrafish mutant tcf7l1a as a genetically sensitised modifier background.
The overall objective of this project will be to identify and functionally characterise new genes required for eye specification and growth compensation.
By combining research in zebrafish and the genetic analysis of patients with eye globe defects, this project will identify new eye genes and unravel their function in early development.