Who doesn’t like to eat a nice tasty orange? With their tangy juice and bright colour, oranges and other citrus fruit are probably the most well-known source of vitamin C. Humans need this nutrient to survive and without it will suffer from scurvy. Interestingly, this is not the case for many other organisms. Fish, amphibians, reptiles, birds, and most mammals can all make their own vitamin C in a series of chemical reactions.
These animals start the process with glucose, a sugar, that is digested. The glucose is absorbed by the bloodstream and taken to the liver or kidneys where an enzyme will convert it into another compound. In a series of steps, this process is repeated as enzymes work on each new compound, converting it into the next compound to be used in the process. Eventually, the chemical compounds are converted into ascorbic acid (AKA vitamin C) by the enzyme gulonolactone oxidase. It is this gulonolactone oxidase, the enzyme used in the final step of vitamin C synthesis, that is missing in humans. Some other primates, guinea pigs, and certain bats are also without this interesting ability due to a lack of gulonolactone oxidase.
Genes code for different proteins, such as enzymes, and the gene for gulonolactone oxidase is still present in animals that cannot manufacture their own vitamin C. At some point in the evolution of primates (approx. 63 million years go), this gene, known as GULOP, experienced mutations that left it inactive. However, mutated genes that become inactive do not simply drop out of their chromosomes to be lost forever. Instead, they are often retained within the DNA and passed to offspring. If you checked, you could still see the remnants of the GULOP gene in your own DNA sequence. If it were to suddenly become functional, your body would be able to make its own vitamin C.
For a moment, lets enter the realm of speculation. Much has been written about gene editing technologies and their applications in the world of medicine. You may have heard about research into fighting cancer by targeting the faulty genes causing tumour growth. Currently, gene editing works best by targeting point mutations, which are when you have a specific change at a single point in a gene’s DNA sequence that has the overall affect of inactivating the gene. Gene editing could allow you to go into the DNA sequence, restore it, and thus restore function to the gene. This has been done in many different research labs around the world for many different genes. However, it is still premature to jump onto the gene-editing-cure-for-cancer bandwagon. Cancer is complicated and not just one disease. There are many different types of cancer and even one cancer cell may have multiple faulty genes and multiple errors within the sequence of those genes. Better examples of diseases that gene editing could help include sickle cell anemia, Tay-Sachs disease or neurofibromatosis where the disease is caused by a single specific mutation. Eventually, once the technology has been honed, you could address more complicated genetic issues. For example, imagine using gene editing to restore the function of the GULOP gene so humans would be able to manufacture their own vitamin C. Scurvy is still a problem in some parts of the world, and many lives could be saved. Imagine humans on long voyages, such as sailors in the Age of Discovery, or perhaps spaceships to Mars or the moons of Jupiter, not being susceptible to the lethal effects of scurvy. But ask yourself: is this a reasonable course of action to address this problem? Its always better to first think about how best to solve a problem and ask questions. On the issue of getting enough vitamin C, I’d rather just sit back and take a bite from a nice tasty orange.