Why use Zebrafish to study human diseases

May Lee (OHS), Izabella Polgar-Wiseman (WHS), Millie Yates (WHS)

The use of zebrafish in scientific research has evolved since the 1960s because 84% of disease related genes in humans are similar to genes found in the zebrafish genome. Therefore, they can be used as a model to explore treatment in human diseases, such as muscular dystrophy, neurodegenerative and congenital diseases.

Muscular dystrophy is a group of muscular diseases that causes the weakening and breakdown of skeletal muscles over time, which can lead to loss of movement. The human disease Duchenne muscular dystrophy is caused by mutations to the DMD gene. This is the largest known human gene which provides instructions for the formation of the protein dystrophin, located mainly in muscles and used for movement in the skeletal muscles and in the cardiac muscle. When this error in the genetic instructions occurs, cells cannot make the protein dystrophin, therefore the associated muscles cannot work properly.

Dystrophin is also necessary to form stable muscle attachments in the zebrafish embryo. In both humans and the zebrafish model, the loss of dystrophin gradually causes muscle cells to become damaged and, in time, they are replaced by scar tissue and fat in a process called fibrosis. The decline of the dystrophin gene is very similar in zebrafish and humans and in both cases leads to the progression of muscular dystrophy. In both organisms, over time, the loss of dystrophin leads to necrotic muscle fibres, which have been irreversibly damaged by abnormal cell death. These are then supplanted by inflammatory cells (those that enter tissue during inflammation), fibrosis, and irregularly sized muscle fibres. Titin, also known as connectin, is another protein, the largest known protein in humans, which is responsible for the passive elasticity of muscle. It is coded for by the TTN gene. Studies of the zebrafish genome have suggested that they contain two adjacent TTN genes, so we also have the muscular protein titin in common with them. Scientists can alter the genetic code of zebrafish to replicate health conditions, which has helped them to understand developmental impairments in humans, as well as many other diseases and infections. Therefore, the zebrafish has appeared as an auspicious organism to help with the study of vertebrate muscle development and disease and research into treatments and cures.

Dementia is a syndrome, which describes different neurological disorders that cause a progressive decline in cognitive ability. Although there are over 200 types of dementia, Alzheimer’s disease (AD) is the most common form to be diagnosed in the UK. AD is a physical, degenerative disease that causes chronic inflammation in the brain. The disease is caused by the formation of excessive deposits of the proteins amyloid beta-peptide and tau inside and around cells within the brain. These protein deposits block the pathway of neurotransmitters between neurons and, as a result, also prevent the passing of messages between neurons. It has also been shown that in the brain of those suffering from AD, the number of neurons decrease with the progression of the disease. Recent studies suggest that the reason for this might be that AD prevents neurogenesis, i.e. stops the brain from producing new neurons. However, other scientists disagree and maintain that there is no sufficient evidence for the existence of neuron development in the human brain post toddlerhood.

Meanwhile, strong resemblance in neurological behaviour has been discovered between humans and zebrafish, which include neuroanatomical, neurochemical, behavioural as well as pathophysiological similarities.

However, unlike mammals, zebrafish also have a unique ability to regenerate their neurons in their adulthood, producing 12,000 new cells per hour. These cells are formed in around 16 proliferating regions within the brain of adult zebrafish. In 2014, a team of scientists studying zebrafish discovered a previously unknown regulatory process for the development of nerve cells. This research has prompted significant interest, since it has the potential to aid in the understanding of how zebrafish can restore lost neurons, and eventually apply this knowledge to modulate the behaviour of cells in the injured human brain.

Zebrafish therefore present some unique characteristics that make them ideal candidates for not only modelling neurological diseases of the human brain, such as AD, but also for the study of neurodevelopment, as well as pharmacological screening of new drugs.

Moreover, using zebrafish, we can find out the genes that are responsible for a phenotypic trait which embryo zebrafish inherit from their parents. Traits include physical features, for example, pigmentation, size, shape or physiological characteristics like a heartbeat or food metabolism. Behavioural studies have also been carried out on aggressiveness, sensory perception and even cocaine addiction. Zebrafish are very suitable organisms for genetic analysis as they can produce thousands of varieties of offspring in a short length of time. Traits can be mapped to a gene by first matching the phenotype with a known genetic polymorphism. This is a difference in a specific region of DNA sequence in the chromosomes of different embryos that can be used to distinguish the gene responsible for a trait. For example, the gene responsible for a curly tail can be identified: Can you find a difference in the genotype of a curly tail that is not present in the genotypes of wild type embryos?

You may have discovered that the A allele is not present in the genotype for a curly tail. Therefore, this shows that the curly tail gene maps to the chromosomal region neighbouring the A/a polymorphism. This helps locate the gene responsible.

If the particular polymorphism is always identified with a phenotypic trait, it suggests that the trait gene neighbours the polymorphism. Fine mapping is particularly successful with zebrafish rather than with for example other models such as mice as a great number of offspring are able to be tested. However, they become more important in helping identify genes responsible for congenital abnormalities in human embryos for example heart and craniofacial abnormalities. Phenotype variation is induced by natural mutations with the cases looked at so far. However, in order to observe a big phenotypic difference, a mutagen such as ethylnitrosourea (ENU) is used on zebrafish which increases errors during DNA synthesis to cause mutations, increasing mutation rate. The embryos of treated zebrafish are then screened for any abnormalities. They are examined for deformities with a microscope or changed patterns of gene expression. As a result, these screens (mutagenesis-based screens) have enabled the identification of hundreds of genes that are essential for normal development of vertebrate embryos. Identifying the genes contributing to abnormalities can enable carriers to take genetic tests and receive early diagnosis and therapy.

Sources:

https://www.nature.com/articles/s41420-018-0109-7

http://zebrafishucl.org https://wellcome.ac.uk/news/zebrafish-genome-yieldssignificant-similarity-human-genome https://irp.nih.gov/blog/post/2016/08/why-usezebrafish-to-study-human-diseases https://www.yourgenome.org/facts/why-use-thezebrafish-in-research

https://www.ncbi.nlm.nih.gov/pmc/articles/ PMC3470424/