Saturday, June 27, 2015

Armand Leroi's Mutants: where the Mutter Museum meets Geek Love

Mutants: On Genetic Variety and the Human Body by Armand Marie Leroi explores the genetic and developmental reasons for variations in the human form. I thought this book would be a good follow up from Inheritance, which focuses on rare genetic disorders that often have a relatively minor physical abnormality, such as eyes that are widely space (hypertelorism). Based on the cover and title, I expected the book to be a bit of a carnivalesque spectacle part Mutter Museum, part Geek Love. In fact, the text treated these "mutants" respectfully and with due scientific diligence. The focus was generally on the developmental pathways that were altered in the mutations observed, but there was some exploration of the toll these mutations took on their bearer.

The book begins with the historical observations of mutants and the explanations for why these mutations arose. In the beginning, "monsters" were thought to be caused by the wrath of God. Eventually, scientists began to understand the developmental causes for these variations. In a chapter called "A Perfect Join", Leroi writes about conjoined twins, specifically how they form. Interestingly, conjoined twins are more likely to have a condition called situs inversus, where the internal organs are inverted on the left-right axis. In fact, the rare singleton birth with this condition has been useful in teaching us why our organs are oriented the way they are. Situs inversus is one symptom of Kartagener's syndrome, which is caused by a mutation in one of the proteins (dynein's outer arm) that control the movement of cilia (tiny projections on the surface of many different cell types). The other symptoms are chronic bronchitis (cilia are important for clearing the lung and nasal passages) and sterility in males (the same proteins are required for the movement of sperm). I always find it amazing how a small mutation in one protein can have such profound consequences. My favorite lines from the book describe how scientists study gene function: "It is actually quite hard to prove that a gene...does what one supposes. One way...is to eliminate the gene and watch what happens. This is rather like removing a car part...to see why it's there. Sometimes only a rear-view mirror falls off, but sometimes the car dies."

A funny story arises in the explanation of the Hox genes, which essentially control the pattern of embryonic development such that Hox1 makes a head, Hox2 makes a torso, etc. Mutations in Hox genes can lead to abnormal body plans, even changes in the number of vertebrae and ribs. About one in every ten adults has an extra pair of ribs, an abnormality that was first observed in a woman, leading the doctors on the case to conclude that it was evidence of the truth of Adam and Eve. In reality, the physiological variation occurs at equal rates in men and women.

The chapter on skin color explores the mutations that lead to albinism, piebaldism, and even red hair. Dozens of different mutations in MC1R (melanocortin 1 receptor), a protein that helps control pigmentation, have been described in red heads. (Because my house has three different types of red heads, I really fell into an Internet rabbit hole on this subject. I think I will write something more about this later.) The genetics of skin color are also complex and have been difficult to study due to the complicated social issues surrounding the subject. Leroi tells the story of a white woman living in Apartheid-era South Africa. Following a diagnosis with Cushing's disease, her adrenal glands were removed, which led to hyperpigmentation due to an abundance of the hormone melanotrophin. As her skin darkened, her social status and living conditions quickly changed. To me, the story highlights the problem with thinking that we can biologically define race. (A Troublesome Inheritance was released recently and covers this very topic, so watch this space for my review.)

Leroi also discusses the genetic basis for aging; in the simplest terms, aging is due to the "inability of natural selection to act against the mutations that cause disease in the very old." This hypothesis explains why dominant mutations like Huntington's can fix in the population. An experiment in fruit flies explored what would happen if only aged flies were reproducing; the effect should be an increase in genetic changes that promote longevity and fertility at an advanced age. After ten generations, the longevity of these flies increased by 30%; after fifty generations, life expectancy doubled. The flies were generally hardier, but the increased life expectancy came at a cost: the fruit flies were less active in their youth as they needed to preserve themselves to ensure survival and mating. This experiment supports the idea that aging comes at the expense of vitality in youth.

Overall, Mutants was quite readable; it had a nice balance of the science behind the mutations and the descriptions of the lives of the people who were affected with these mutations. I should note that the book was released in 2003, so some of the science is a bit out of date. For me, a good read is one that teaches me new science and spurs me to read and write more. Mutants definitely fits the bill in that regard. 

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