Hypertrichosis: Is It a Throwback to Prehistoric Times?

[aychamo]

For anyone familiar with hypertrichosis (that Mexican family has the gene, it makes them VERY hairy), it's been around for a while. Could this be any type of throwback to times before we became "hairless apes"?

 

[spuriousmonkey]

I doubt it. Even the big apes are less hairy than those people.

 

[Monique]

It is a very rare X-chromosome linked (to be precise, it has been mapped to Xq27-q28) dominant trade seen in a single Mexican family..

Basically this is a demonstration of an atavism, the reappearance of ancestral characteristics in individual members of a species. The best known examples of atavisms in natural populations are hind limbs in whales and extra toes in horses. What probably happened is that the genetic regulation of the gene was shut off during human history by mutation. The people probably got back the regulatory portion of the DNA that turns on the gene, although the regulation is not proper, that is why it looks different from say the growth of hair on apes.

There is also a similar disease called Ambras syndrome, where the hair can grow up to several decimeters, rather than the short curly hair in hypertrichosis. The Ambras syndrome has apparently been mapped to a paracentric inversion on chromosome 8 q12;q22.

 

[aychamo]

Do you think it was a mutation of some sort that caused them to start expressing the gene again?

Also, how you said that the gene was shut off, would you say that is similar or the same to what happened with our tails?

 

[Monique]

It was a mutation for sure, that is the only way you'd get such inheritance patterns. The question is what kind of a mutation it was. They published the mapping in Nature Genetics 1995. I don't think they have actually identified the gene yet, which I find surprising.

Actually researchers have recently recognized that the major differences between human and chimps are not in the genes, but in the way those genes are regulated. In order to express a gene, a complex assembly has to come together upstream of a gene. Here is where regulatory elements are located. You know that the genetic code is entirely made up of the letters A T G and C. Well, the sequence TATA infront of a gene is known to be a very important signal required for gene expression. So for instance if this TATA is mutated to TACA, the gene won't be expressed anymore. If then the sequence mutates back to TATA, it comes to expression again.

I'd suspect something similar is going on in this instance.

There are different mutations possible though, like in the gene itself. But in that case I would not expect such a phenotype going from virtually no hair to a lot of hair. UNLESS, unless the gene in question is actually actively repressing hairgrowth in normal humans, and that this repressing gene got shut down or mutated to an inactive form in these patients, leading to out of control hair growth.. in that case it would be possible for part of the gene to be deleted.

 

[aychamo]

Hey Monique, do you know of any other type of avatisms that are seen these days?

 

[Monique]

It's atavism :) well, you could think about the fact how similar embryos look to other species. The theory still remains a controversial one though, look on this website.

http://www.epub.org.br/cm/n09/fastfacts/atavismo_i.htm

There are some rare cases in medical literature which could be interpreted as ATAVISMS, that is, the reappearance of our ancestors' organs, in which morphological traits, already lost by human beings thoroughout evolution, are expressed again.

Such is the case of supranumerary mammal glands, with V-shaped nipples similar to pre-primates mammals, of sacral tails, and pilosity patterns covering the whole face or body, very chimp-like (see above the case of a Chinese child, who was born with his body covered with thick hair; the man with a sacral tail and another with several nipples).

However, genetic atavism is not accept by many scientists, who agree only to the extent that the theme is a controversial one. What has been interpreted as atavism, they argue, sometimes is just a particular example of abnormal embryonic development or a rare disease. The fact that a phenotypic trait resembles morphological characteristics of primates or other mamals in the evolutionary development does not necessarily mean that a "hidden" gene in our genome has been de-repressed. This is yet to be to be proven at molecular level.

 

[aychamo]

It's atavism :)

Ahh.. That explains why my web search resulted in basically nothing :) Thank you for the info in your post though!

 

[aychamo]

Hmm. It seems weird that they would think it was a disease, and yet they were inherited so well, like in that Mexican family.

To me, this is a fascinating topic!

 

[Monique]

Diseases can be genetic too. The question is whether the manifestation is the result of an ancient gene being turned on, or just the coincidence that the phenotype of the disease looks just like that of our ancestors.

Think for instance about the babies born with a tail. Is there a tail gene involved, or is it just a mistake in the programming of the organism that has nothing to do with a tail gene?

 

[aychamo]

That thing about the tail, I guess that goes to like you said, whether or not it's more likely for it to be a trait that had it's regulator fixed, or a mutation that arose suddenly. I don't know much about it, but I find it hard to convince myself the trait arose in one mutation, instead of like you said in fixing the TATA box.

Regarding tails, from what I understand is that we actually differentiate a tail during embryonic development, but then it recedes further into development. Is this accurate?

If so, would that mean we had/have a(n) (in)active gene that gave us a tail? I wonder what would cause it to recede, and I guess failure of it to un-differentiate would be why kids can sometimes be born with tails (god I want to see a picture of that!)