Читать книгу Bipolar Disorder For Dummies - Joe Kraynak - Страница 13

Now that scientists are able to map human genes fairly easily and inexpensively, genetic research is exploding. Understanding the genetics of an illness doesn’t just shed light on causes but also seeks to identify specific biological processes that contribute to the illness, which in turn helps to develop more effective treatments. But before even jumping into a search for genes, scientists need to know if they have any reason to suspect that genetics are partly to blame. Studies of families, particularly studies of twin siblings, have been the gold standard of this research. These genetic studies reveal the following:

✔ The identical twin of a person with bipolar I disorder has about a 50 percent risk of developing the disorder. Because identical twins share all their genes, this finding means genetics accounts for some, but not all, of the chance of developing bipolar disorder. (If the genetics were fully responsible and one identical twin had bipolar I, then the other twin would have a 100 percent chance of developing the disorder.)

✔ If someone has an immediate family member with bipolar I disorder, the person has about a 10 percent chance of developing the disorder, which is much higher than the 1 to 2 percent chance of having the disorder if for an individual with no family history of bipolar.

✔ The familial risks of bipolar II and cyclothymia – so called bipolar spectrum disorders – are less clear in the research. First-degree family members (parents, siblings, or offspring) of people with bipolar disorder are more likely to have other disorders as well, such as unipolar depression, schizophrenia, autism disorder, anxiety disorders, substance abuse, attention deficit hyperactivity disorder (ADHD), and personality disorders.

Putting together all the family and twin studies, researchers say that heritability – how much of bipolar disorder is due to genetics – accounts for between 60 and 70 percent of the risk of getting the illness.

Shaking the family tree

A history of bipolar disorder in an immediate family member is important to know, because it increases your risk of getting the disorder. However, families of people with bipolar disorder have higher rates of many other psychiatric conditions, as well. When considering your risk for bipolar disorder, a look at all major psychiatric disorders in close family members can help identify some genetic susceptibility to bipolar disorder. If you ask relatives whether anyone in your family has had bipolar disorder, they may not know the answer. Older relatives may have been misdiagnosed as having schizophrenia or may have never been diagnosed but had a history of self-treating with alcohol or drugs. Or a relative may have been considered eccentric, but for whatever reason was never diagnosed.

To obtain better answers, try rephrasing the question:

✔ Has anyone in the family had alcohol or substance abuse problems? Many people with bipolar disorder self-medicate with alcohol and drugs, which does significantly more harm than good but may provide temporary relief from the psychological pain. Some research also suggests that bipolar disorder and substance abuse may share some genetic risk factors.

✔ Have any family members been diagnosed with schizophrenia? In the not-so-distant past, doctors commonly misdiagnosed bipolar disorder as schizophrenia. Schizophrenia is more common in the relatives of people with bipolar disorder, likely because of some shared genetic risks.

✔ Has any family member been treated for any other mental illness? If a family member has received treatment for depression, psychosis, or other mental illnesses, he may not have received the correct diagnosis. Also, relatives of people with bipolar disorder are more likely to have many other types of mental illness, which may be partly genetic and may be due to other mechanisms.

✔ Has anyone in the family had to go away for a while to an institution, sanatorium, or rehab center? Families sometimes cover up memories of relatives who had to be hospitalized for mental illnesses by saying that they had to go away for a while.

✔ Have any family members been known to be particularly energetic or eccentric? In the past, people politely described relatives with various degrees of mental illness as eccentric.

✔ Has anyone in the family suffered from physical symptoms such as chronic exhaustion, pain, or digestive problems? These symptoms may be physical manifestations of mood and anxiety disorders.

Families can be particularly secretive, especially when protecting the reputation of the dead. People can become even more defensive if you confront them while you’re in the throes of mania. Choose a time when you’re levelheaded to explain how important an accurate and detailed family history is for your diagnosis.

Realizing that genetic susceptibility is only one risk factor

Having a genetic susceptibility to bipolar disorder doesn’t guarantee that you’ll experience symptoms. The most recent research, described in the next two sections, suggests that a variety of genetic changes can create a susceptibility to developing the disorder. But susceptibility isn’t destiny – bipolar disorder seems to occur due to a combination of genetic and nongenetic factors. Although you can’t control all the possible triggers, managing the ones you can control may reduce your risk of developing the disorder, and doing so certainly may help lessen the severity of your illness and improve your prognosis if it occurs.

Highly charged life events, both positive and negative, seem to relate to the development of symptoms, but other biological events – including the number of episodes that you experience – are likely to be involved, as the next section explains. The severity of your underlying susceptibility is also important. Some subtypes of bipolar found in some families that are more extreme and more likely to evolve into illness without major or well-defined triggers, whereas other forms may require higher doses of stress responses to fully develop.

Grasping the genetic complexities

Even though indisputable evidence proves that bipolar disorder has strong genetic components, science has more recently been able to show clearly that that there is no single bipolar gene. What’s called bipolar disorder, based on emotional and behavioral symptoms, actually seems to be a group of disorders that have different underlying biological stories. Because genetics play a big role in these stories, scientists are likely to find many different types of genetic changes that contribute to the various causes of bipolar disorder. Current research has started to put some pieces of these stories together and suggests the following main points:

✔ Bipolar disorder seems to be the result of small changes in a high number of genes rather than big changes in just a few genes; high-volume studies that look at thousands of genomes are needed to pin down the specific genes involved in bipolar. (A genome is an organism’s complete set of DNA, its genetic makeup.) The small changes that have been found aren’t usually unique to bipolar disorder, but have a higher rate of occurrence in people with bipolar disorder compared to those without bipolar.

✔ Studies show a number of genetic changes that overlap between bipolar disorder and other conditions, especially schizophrenia and unipolar depression. Recently, a large and much publicized study published by the Cross-Disorder Group of the Psychiatric Genomics Consortium entitled “Genetic relationship between five psychiatric disorders estimated from genome-wide SNPs, published in the journal Nature Genetics, uncovered a shared genetic risk with autism spectrum and ADHD, in addition to depression and schizophrenia. However, researchers have also identified a number of genetic changes that seem to be specific to bipolar disorder.

✔ The numerous genetic factors uncovered so far affect a wide variety of structures and functions throughout the body. Many other studies are looking at how the brains and bodies of people with bipolar disorder differ from those without (see the later section, “Examining the Circuitry of Bipolar”). The integration of the genetic studies and this other research is where the big picture of bipolar disorder is unfolding.

✔ Because bipolar isn’t 100 percent genetic, another important goal is to understand how nongenetic factors interact with the genetic risk factors to cause someone to actually develop bipolar disorder, as we explain in the next two sections.

Flipping the bipolar switch: Epigenetics

Epigenetics is the study of changes that affect how genes are expressed without affecting the genes themselves. These changes occur through a variety of chemical interactions with the DNA. Sometimes these changes in expression occur as part of typical development and function, but some changes can disrupt normal processes and healthy cell function.

The science of epigenetics helps to explain the interplay between nature and nurture – how an organism’s genotype (genetic makeup) interacts with the environment to produce the organism’s phenotype (observable characteristics). External events such as parental neglect or other trauma or stress may trigger chemical changes to DNA, which affect how a person’s genes are expressed. And these changes can pass to the next generation. So, for example, if an expectant mom has been exposed to chronic stress that has affected her mood-related genes, then the stress effect on the gene (not just the gene itself) can be passed on to her child. As a result, the child’s own genes as well as the effects of long-term stress on her mother’s genes affect the child’s likelihood of eventually exhibiting depression or other mood disorders.