Stephen Crohn, an artist from New York, lost a lot to the AIDS epidemic of the 1980s. For more than a decade, he watched his boyfriend, Jerry Green, as well as nearly all of his friends slowly grow sicker and perish from this mysterious illness. His passion for his art dried up, and survivors’ guilt consumed him.
But the one thing he didn’t lose during this scourge was his own life. The reason he survived was not luck, however. Rather, he made it through because he carried a rare genetic mutation that made him virtually immune to HIV.You may have heard people call your genome the “blueprint” for your body. But your genome doesn’t just dictate what color your eyes will be or whether you can curl your tongue; it also influences when, where, and how you will come down with an illness. Whether it’s a genetic mutation that directly causes an illness, one that makes someone more likely to develop a disease down the road, or one that protects someone from health problems (like Mr. Crohn’s mutation), genetics plays a role in virtually every facet of our health.
Since scientists first sequenced the human genome 15 years ago, they have come a long way in linking genetic mutations to various diseases. Doctors can now attribute conditions like hemophilia and cystic fibrosis to mutations in single genes, but just as importantly, researchers can also point to a mutation in people like Mr. Crohn and thank it for protecting him from HIV. Scientists are also finding that groups of genetic mutations contribute to more complex diseases, such as breast cancer and Alzheimer’s disease, which you can learn about in Part II of this series. For now, I will reveal how and why your life can either be upended or saved by mutations in single genes.
What’s a mutation?
First, let’s get something straight – a genetic mutation is not necessarily a bad thing. We often associate mutations with disease — or superpowers that make you stick out — but actually, a genetic mutation is merely any change in your genome that differentiates you from most other people. We’re all mutants – if our DNA never changed, we wouldn’t possess the unique characteristics that make us who we are today.
Mutations happen rather frequently. Your body is making new cells all the time by splitting your old cells into two, and every time this happens, your old cells need to replicate their DNA so each new cell gets a copy. But the copy machines in your cells aren’t perfect; your machinery can make errors and introduce new mutations into your DNA. (Give your cells a break – try typing out War and Peace on your computer without a backspace button and see how well you do). Besides errors in copying DNA, though, mutations can be caused by the sun, nuclear radiation, or toxins.
If errors occur in your sperm or egg cells (your “germline” or reproductive cells), and a child is borne from one of those cells, your child will carry these errors in every cell in their body. If these errors occur in any other cell in your body (the “somatic” cells, such as in the liver or kidney), they can cause a specific disease, but in you only — you won’t pass these mutations onto their kids.
But as I said, mutations aren’t all bad. Our genome mutates all the time without us noticing. Genetic mutations led us to become who we are as humans, and at the same time, they make us all different from each other. Mutations create diversity in a species, which makes life healthier for our children. (For an example of what a lack of genetic diversity does to a family, look at the health problems purebred dogs deal with, or at what happened to the royal families of Europe and Egypt).
How Do Genetic Mutations Affect Our Health?
Genes are the instruction manual for building proteins, the molecules responsible for giving you strong teeth, digesting your food, creating your memories, clotting your blood, growing your hair, replicating your DNA, and pretty much every other thing that happens in your body at any moment. Since proteins come from instructions in your DNA, genetic mutations can directly influence the structure, number, mobility, and activity of your proteins, which, in turn, determines the way your body develops and functions. And sometimes, these changes cause–or prevent–disease.
A Mutation in A Single Gene Can Make you Sick
The diseases we traditionally call genetic diseases are monogenic (mono = one) — they are caused by mutations in single genes. We have two copies (alleles) of each gene (one from our mother and one from our father), and mutations in one or both of these alleles can make you sick, depending on the effect they have on your body. Dominant disease mutations impact the body so greatly that they only have to come from one parent to make you ill. Recessive mutations have a lesser impact; therefore, someone has to inherit the mutation from both parents to see a difference in their own health.
Huntington’s disease is caused by a dominant mutation in a single gene: the HTT gene. A mutation in this gene causes progressive damage to the brain (learn more here). Since this mutation is dominant, someone only needs to inherit the disease-associated HTT allele from one of their parents to develop Huntington’s.
Sickle cell disease is also monogenic. People with sickle cell disease harbor a mutation in the hemoglobin gene (called HgbS) that causes red blood cells to turn sickle-shaped. This makes it difficult for red blood cells to carry oxygen, often leading to anemia (more here). But, unlike the mutation in HTT that causes Huntington’s disease, the HgbS mutation is recessive; a person needs to inherit the bad form of the gene from both parents to actually develop the disease. If someone carries just one copy of the HgbS mutation, only half of their red blood cells will develop a sickle shape, which is not enough to make someone feel sick.
Interestingly, the sickle cell gene can protect people from malaria! In fact, thee microbe that causes malaria grows in red blood cells and cannot survive in sickle-shaped cells. For individuals who carry one copy of the bad form of the hemoglobin gene, this is enough to stop malaria from infecting someone. In this situation, sickle cell carriers have the best of both worlds: they won’t develop sickle cell disease, and they’re protected from getting malaria. This genetic oddity might explain why sickle cell disease is so common among people of African descent, given the prevalence of malaria-carrying mosquitoes in Sub-Saharan Africa.
Genetic Mutations Can Be Protective
Carrying the HgbS version of the hemoglobin gene protects people from getting infected by the Plasmodium protozoa, which causes malaria. Are you surprised that your genes can protect you from infections? You shouldn’t be, because as you read at the top of this article, you already know there’s a mutation out there that protects people from HIV. Here’s how it works:
The reason HIV is so deadly in most people is that it infects the very cells that protect your body from infection: T-cells. But HIV cannot get into these cells by themselves; they need to find a passageway that lets them get through. Fortunately for HIV, T-cells produce a protein called CCR5, which floats on their surface like a buoy in a pool, that HIV latches onto to dive into the cell and wreak havoc on its contents.
Mr. Crohn, the man who was immune to HIV, carried a rare mutation in the CCR5 gene called Delta23. This mutation makes T-cells produce a defective CCR5 protein that HIV doesn’t recognize. Since HIV has nothing to grab on to, it has no passageway into T-cells, and it can’t cause harm.
About 1% of northern Europeans are fortunate enough to harbor this mutation in both of their CCR5 alleles, which makes them effectively immune to HIV. Does this mean gene therapy for HIV is on the horizon? Maybe, but it’s too soon to tell.
Single Genes Make a Big Difference
With so many genes in our DNA, it’s amazing that a defect in only one of them can make someone chronically ill. At the same time, it’s pretty incredible that a single genetic mutation can protect someone from something as deadly as HIV. But while mutations in single genes can have a major impact on our lives, mutations don’t always directly cause disease. Most of the time, they have no effect, and sometimes it takes a combination of mutations to make someone ill. In Part II of this series, I discuss diseases that only develop in the presence of several mutations in multiple genes. If you think this article helps you wrap your head around the causes of monogenic disorders, just wait: Polygenic disorders, ones caused by mutations in multiple genes, are whole new ballgame.