When zebrafish have scoliosis

“Don’t slouch, sit up straight, or you’ll get scoliosis!”

Getting this kind of comment from adults used to infuriate me when I was a teenager, since 1) I did sit up straight, and 2) I already had scoliosis (despite all the sitting-up straight). In my mind, it ranked somewhere up there with, say, telling kids not to make a face because, if the wind happened to turn at that same time, their face would remain stuck that way, (I’m not the only one who’s ever heard that, am I?)

Scoliosis is a deformation of the spine. To be precise, it is defined as a lateral curvature of the spine of more than 10° (measured with the Cobb’s method), accompanied by a rotation of the vertebrae. In lay terms: one’s spine is not straight – in my case, it looks like a vertically elongated S.

Often, there is no known reason why the scoliosis developed. These cases are called “idiopathic scoliosis”. Somehow, somewhere along the way as one is growing up, the spine decides to take a little detour instead of growing straight.

The fact that idiopathic scoliosis tends to be found in multiple members of the same family indicates that there is a genetic component to it. However, genome-wide association studies have not led to conclusive results: although they identified some regions of the genome associated with susceptibility to scoliosis, the main finding was actually that there was a large genetic heterogeneity in scoliosis. In other words, there may be gene variants that raise one’s risk of developing scoliosis (if ever so slightly), but these variants are many and vary a lot from one individual to the other.

So, when earlier this year researchers reported having identified one single gene variant that not only associated with idiopathic scoliosis in humans, but also caused a deformation of the spine in zebrafish, it was breaking news (well, ok, maybe not normal-people-breaking-news, but certainly of great interest within particular circles). For the first time, here was a gene variant that, on its own, seemed to cause a deformation of the spine, not just maybe-maybe raise the risk of scoliosis by a fraction of a percent.

How did the researchers identify this gene? How did they show that it could cause a deformation of the spine?

Step 1: identification of a gene variant associated with disease

The team of researchers started out by gathering 41 families including individuals with idiopathic scoliosis. By drawing family trees and recording who had scoliosis and who had not, they could then calculate the frequency of transmission of disease. Out of these 41 families, 6 matched the criteria for a possible monogenic transmission, that is, fitted with a scenario in which scoliosis would be due to the transmission of one single gene variant. The scientists then analyzed the genomes of the members of these 6 families and identified two chromosomic regions linked to scoliosis in one family. They finally zeroed in on a gene called POC5 by sequencing these two genomic regions and finding that all 11 individuals who had scoliosis in the family had the same mutation in POC5, namely, a single nucleotide change (nucleotides are the building blocks of DNA) that resulted in a single amino acid change in the protein encoded by POC5.

Going back to all the other 40 families with cases of idiopathic scoliosis, the researchers found that the same POC5 variant was in fact present in 3 more families and associated with the occurrence of scoliosis in family members. In addition, they discovered two other variants of POC5 in other individuals with idiopathic scoliosis (also single nucleotide changes, but the mutations were occurring at other positions in the DNA sequence of the gene).

Step 2: can the gene variant cause disease?

Having identified POC5 as a gene associated with idiopathic scoliosis, the researchers next wanted to know whether the gene variants they had found were functional variants that could cause disease. Indeed, it could be that the gene variants simply associated with the presence of scoliosis by chance (although the probability of this was low – for example, in the case of the 3 families mentioned previously, the probability of both the gene variant and scoliosis always being found together in individuals just by chance was about 1.6%). It could also be that the association between the POC5 variants and scoliosis was not due to POC5 variants causing disease, but rather to POC5 variants being associated with nearby mutations in other genes that were actually the ones causing disease.

Therefore, to see if mutations in POC5 could directly cause scoliosis, the researchers injected zebrafish embryos with any one of four versions of the POC5 gene: either the “normal” version, or one of the three mutated versions present in patients with scoliosis. (Zebrafish is a tiny fish commonly used in research; the POC5 gene is highly conserved between species – meaning the DNA sequences are very similar, and the researchers also verified that the human gene could indeed replace the zebrafish one and work just as well before going on with further experiments.)

As it turns out, about half of the zebrafish embryos that had been injected with the mutated forms of POC5, but none of the ones injected with the normal POC5 gene, showed mild to severe curvature of the body axis. When the fish reached the juvenile stage, curvature of the vertebral column was still present and was in some cases accompanied by rotation of the vertebrae, similar to what is seen in humans with scoliosis. Notably, it seems that the effects of the mutated forms of POC5 were limited to deformation of the spine, as no other skeletal or body malformations were observed in the fish.

Step 3: well, it’s always a bit more complicated …

Based on the fact that mutations in POC5 were found in 4 families with multiple cases of idiopathic scoliosis out of a total of 41 studied, the researchers suggest that POC5 variants may be responsible for about 10% of familial forms of idiopathic scoliosis. [To give some context, these familial forms represent about 40% of all cases of idiopathic scoliosis, which is itself present in about 3% of the population.] However, as always in science, these findings need to be replicated in other and larger cohorts of individuals.

Given the relatively low penetrance of scoliosis in a control population that included individuals carrying POC5 mutations (that is, having a mutant POC5 did not necessarily equate with having scoliosis), the scientists suspect that, in the families they studied, there may be another gene variant involved that would modulate the risk of developing scoliosis when a mutated POC5 is present. To complicate things further, it is likely that this additional “risk-modifying” variant is different from one family to the next, making them difficult to identify.

Now, I don’t know if I’m a POC5 mutant or other. But wouldn’t it have been cool if, as a teenager, I could have told off those busybodies with a simple, yet impressive, “I don’t slouch, I’m a mutant”?

Reference
Functional variants of POC5 identified in patients with idiopathic scoliosis. Patten SA, et al. J Clin Invest. 2015 Mar 2;125(3):1124-8. doi: 10.1172/JCI77262
PMID: 25642776

ResearchBlogging.orgPatten SA, Margaritte-Jeannin P, Bernard JC, Alix E, Labalme A, Besson A, Girard SL, Fendri K, Fraisse N, Biot B, Poizat C, Campan-Fournier A, Abelin-Genevois K, Cunin V, Zaouter C, Liao M, Lamy R, Lesca G, Menassa R, Marcaillou C, Letexier M, Sanlaville D, Berard J, Rouleau GA, Clerget-Darpoux F, Drapeau P, Moldovan F, & Edery P (2015). Functional variants of POC5 identified in patients with idiopathic scoliosis. The Journal of clinical investigation, 125 (3), 1124-8 PMID: 25642776

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