Is there any disorder (genetic or otherwise) that causes an abnormally dark skin tone and eye color, regardless of race? Seems that if there’s such a thing as having too little melanin, there should be a corresponding syndrome that is signaled by having too much. A Wiki search on “hyperpigmentation” revealed nothing. Nothing!
The term is hypermelanism, like albinism a genetically transmitted trait. (“Melanism” is synonymous with hypermelanism.) AFAIK it is not a significant gene in the human population, but a web search on it indicates that it is very common in Squamate (snake/lizard) genetics.
But a search for melanism would have been a little better.
Melanism is fairly common in many kinds of animals. The most famous is the “black panther,” which is nothing more than a melanistic form of the ordinary leopard. Melanistic individuals are quite common in Asia but very rare in Africa.
My Google for “human melanism” was fruitless.
The melanistic squirrels in Toronto think they own the place.
They look just like grey squirrels but they’re solid black. Tourists love 'em - until their first close encounter, that is …
Melanistic mosquitofish and killifish are fairly common in certain places, like the Everglades. They aren’t black all over, though. They have blotches.
In humans, the condition is called “melanosis.”
The other term for albinism is hypopigmentation, which is why I searched on that other term. Interesting, but it doesn’t mention humans or even if humans can be affected. I am mostly curious about the social aspects of such a disorder after reading about black albinos on the Internet.
The OMIM results for melanosis and skin hyperpigmentation.
Generally, it would be easier to imagine albinism arising, as all you have to do is break one link in a chain of a biosynthetic pathway (tyrosine -> melanin), or kill the melanocytes. The first seems to happen more, and the link that seems to be broken most is tyrosinase, although there are probably a dozen kinds of albinism out there. Melanocytes migrate from the neural crest (groups of cells that develop near the spine), and there are many neural crest disorders and other disorders that can cause hypopigmentation by affecting the melanocyte (like Prader Willi). To imagine a hyperpigmentation syndrome you would need to ramp up the level of melanin production or increase the number of melanocytes. While I’m sure this happens at low levels in the population, generally breaking something or decreasing its levels is easier than increasing them. That means that I would guess that generalized hyperpigmentation is not a common finding. That said, there are plenty of diseases associated with nevi (moles), hyperpigmented skin patches, or other localized hyperpigmented spots (off the top of my head Peutz-Jeghers, McCune-Albright, Incontinentia Pigmenti, Basal Cell Nevus Syndrome, Neurofibromatosis type I, and of course the congenital melanoma syndromes and this is far from a complete list).
But, if you tell someone that you have a disorder associated with hyperpigmentation, the first things that should come to mind are Addison disease (adrenal insufficiency leads to a feedback derepression/upregulation of ACTH from the pituitary and CRH from the hypothalamus; CRH is synthesized in a prepeptide called POMC that includes a melanocyte stimulating hormone). This causes a “bronzed” appearance. In the setting of adrenalectomy performed due to intractable Cushing syndrome (hypercortisolism), a ACTH-secreting pituitary adenoma can form, and this is called Nelson syndrome. This too is associated with hyperpigmentation due to derepression of CRH and increased melanocyte stimulating hormone.
Fer completeness, before bed, I should mention the most common causes of hyperpigmentation that I see from day to day are acanthosis nigricans, atopic dermatitis and lichenification, lentigo and various benign and malignant moles, actinic keratoses, and skin cancers especially melanoma. There are lots of other things that can make the skin look darker without increased melanin as well, notably jaundice, cyanosis, hemochromatosis, and carotenemia in the kids eating too many carrots and sweet potatoes.
I have a form of hyperpigmentation. I don’t have the name of the condition handy, but its basically like freckles in my digestive tract, which is apparently a rare thing.
I found this out when a physician saw what he thought was a melanoma in my mouth, it was tested and turned out to be a freckle. Big deal, right? Well, maybe – it turns out that someone who pigments in this fashion – interior freckles – has a heightened risk of colon cancer. So I get to have colonoscopies every 3 years for the rest of my life. Whee.
Having said all that… in my limited experience hyperpigmentation does not appear to be the “opposite” of albinism. In my case, the pigment seems to collect at defined places. I’d bet this reflects most other cases as well, but I’d be guessing.
Oops, I should have read edwino’s post more carefully. My condition was diagnosed as Peutz-Jeghers syndrome. No big deal, hopefully, as long as I keep a colonoscope on things.
Isn’t that why JFK was said to have such a “healthy” appearence? Because Addison’s disease kept him looking like he had a healthy “tan?”
It appears to be correct that JFK had Addison disease as caused by an autoimmune process. Apparently, so did Jane Austen.
I’ve read theories that JFK’s Addison’s (which I’m told likely would have killed him before his term in office ended, had he not been shot) was iatrogenic; long-term treatment with steroids for an old football injury destroyed his adrenals.