How Mammals' Mucus-Producing Genes Evolved Through "Mucinization"

Genes that produce proteins responsible for the sticky qualities of mucus – known as mucins – have evolved from unrelated genes again and again in mammals, according to a study published in Science Advances. Studying 49 mammal species, the University at Buffalo team behind the research identified 15 examples of this “surprising” evolutionary process.

What does mucus do? And how did it evolve?

Mucins are a group of large proteins with important functions in the body – for example, one of their main functions is as part of our innate immune system, forming physical barriers against disease-causing bacteria. Mucins in found in saliva coat our teeth to protect them from the cavity-causing Streptococcus mutans, while others trap and clear allergens and debris on mucosal surfaces in the airways.

Mucins are heavily glycosylated, meaning they are coated with sugar molecules (glycans) that give them their gelatinous consistency. Many sites where these proteins are glycosylated feature repeated sequences of the amino acid building blocks known as proline (P), threonine (T) and serine (S). Together these “PTS” repeats are a defining feature of mucins.

The genes that provide the blueprints for mucins have sparked a lot of interest among geneticists, as they are not grouped by their ancestry (much like lions and tigers, which share a common big cat ancestor) – instead, they are grouped by their function (like dolphins and sharks – they are unrelated, but both are ocean predators and physically similar). Some of these mucin genes are “lineage-specific”. These share a common ancestry and are sorted into two groups – secreted gel-forming mucins and membrane-bound mucins. However, there is a third population of mucins that do not belong to either of these families, named “orphan” mucins as they have no known genetic relatives.

In the current study, the researchers aimed to answer the question: How did these orphan mucus genes evolve? Studying the genomes of a wide variety of mammal species, the research team put forward a new evolutionary model in which genes can gain new functions by adding on many short, repeated regions of their genetic code. Focusing on mucin evolution, the researchers propose that non-mucin genes were “mucinized” by an additive process of gaining PTS repeats, which occurred in parallel across many different mammal species.

Independent evolution of “orphan” mucins

To investigate how orphan mucin genes evolved, the researchers trawled through the genomes of different species to pick out genes with PTS repeats, a reliable indicator of mucin function. Initially using human, mouse, cow and ferret genomes, they found that most mucins were in fact shared between these mammals. However, at least one mucin gene was found unique to each species – including six mucin genes exclusive to ferrets. Most genes were found in the so-called secretory calcium-binding phosphoprotein (SCPP) locus, allowing researchers to narrow their search to this one promising area of the genome.

The team then explored the SCPP loci of 49 other mammal species, identifying mucin genes and looking for which of these might be related to one another and possess similar DNA sequences. From this conservative search, the researchers found 15 examples of the evolution of unique mucin genes. This is known as parallel or convergent evolution, whereby novel mucin genes evolved separately in different lineages and all serve a similar function.

For example, the salivary mucin MUC7 is found in humans and most other mammals, with the notable exception of rats and mice. Instead, these rodents express a similarly sized but genetically distinct gene called Muc10. From their examination, the researchers found that Muc10 bore a striking genetic similarity to the human gene PROL1 found in tears. PROL1 looks almost identical to Muc10 but lacks the mucin-defining PTS repeats. The researchers suggest that one of these “lineage-specific events” occurred, in which an ancestral Prol1 gene in these rodents underwent “mucinization”, gaining PTS repeats and bestowing the Muc10 gene as we know it today with its mucinous properties.

Petar Pajic, University at Buffalo PhD candidate and lead author of the study, explains, “We show that mucin genes, which give rise to mucus that interacts with pathogens, and coats our organs, have evolved over and over again (at least 15 times) in the same location in the genomes of different mammals. We think that these new genes evolve by gaining a block of repeat sequences within an already existing gene. These repeats then promote the attachment of branched sugar molecules and give the resulting protein its new slimy properties.”

Pajic continues, “The current paradigm for how a new gene evolves is that an entire gene duplicates itself and then it gains a new or different function through a process known as ‘neofunctionalization’. Our study shows that mucin function often evolves in a different, surprising way. It turns out a gene can gain a block of repeat sequences and this process can facilitate a new function, in our case example: mucin function.”

Did mucinization occur in other slimy critters?

Discussing how the team plans to take this research further, Pajic explains, “Next steps would involve looking into other genes that have repeats in some species but none in others, and by this process, segway into finding additional precursors that may promote evolution through gains of repeats in other functional domains.”

The team also plans to expand its mucin gene discovery techniques to see if unusually slimy animals like slugs, hagfish and swiftlets have more mucins compared to other closely related species.

“Last but not least, people are realizing the importance of mucins in a disease context, especially cancer. Thus, our insights may prove useful in the biomedical realm as well,” Pajic concludes.

Petar Pajic was speaking to Sarah Whelan, Science Writer for Technology Networks.

Reference: Pajic P, Shen S, Qu J, et al. A mechanism of gene evolution generating mucin function. Science Advances. 2022;8(34):eabm8757. doi: 10.1126/sciadv.abm8757