Alzheimer’s Disease and the genetic study of twins
Dr. Bill Beckwith, Phd., a noted memory expert in Naples, told me the story of one of his patients, a terrified woman in her 60’s. She had come to him for assessment of her mental capacity because her identical twin was in a nursing home with Alzheimer’s disease. She was afraid that this was to be her fate, as well. He tested her. He found that she had an excellent memory and excellent mental skills.
A Study of Twins
The study of identical twins would be the perfect population to discover the degree to which Alzheimer’s disease is determined by genetics because genetics is completely controlled for in this population. Fortunately, studies with twins have been done. The Swedish Twins Registry was established in the early 1960s and includes data on more than 15,00 fraternal and identical twins.
A study using data from this registry was published by Margaret Gatz, Nancy Pederson and other researchers in the april 2005 issue of Neurobiological Aging. This study is entitled, “Complete ascertainment of dementia in the Swedish Twin Registry: The Harmony Study.” The study found that the “concordance rate” for Alzheimer’s disease among identical twins – meaning both twins either have dementia or both do not – runs about 60 percent.
The concordance rate was 32 percent for fraternal twins of the same sex and 24 percent for fraternal twins of the opposite sex. The authors concluded that the results of the study mean that about 60 percent of Alzheimer’s can be attributed to genes, but 40 percent can be traced to environment.
However, it could be argued that even more than 40 percent of Alzheimer’s disease can be attributed to environment because twins usually share the same environment, at least during the formative years of their lives. Therefore, part of the concordance rate for Alzheimer’s among identical twins could be the result of their shared environment as well as their shared genetics.
Josephine and Una
In a front-page article in The New York Times of August 31, 2006, there appeared the second in a series of articles looking at the science of aging. The article began with the story of identical twin sisters, Josephine and Una, who were 92 years old.
Josephine is straight backed, firm of jaw and vibrantly healthy. She lives alone in an immaculate brick ranch house. She still drives. She works part-time in a hospital gift shop and belongs to four bridge groups. Una is incontinent; she has had a hip replacement; she has a degenerative disorder that destroyed most of her vision; she also has dementia.
They were always different. Josephine worked her way through college, earned a master’s degree in education and pursued a career as a high school teacher.
Una was not much of a student. She failed a grade in high school and barely graduated.
Josephine had always been active. She was a self-described tomboy growing up and played tennis until she was 85. Una was the “frilly” type.
Although both sisters married and had children, they lived different lives.
The article did not go into the lifestyles of the two sisters. However, the difference in their fates can only be explained by a difference in their environments, which includes lifestyle, since their genetics are identical. Their lifestyles probably were different since they were so different in their interests and personalities.
If sporadic Alzheimer’s disease is not completely genetic, but influenced by environmental factors, its onset can be modified by finding out what the environmental risk factors for Alzheimer’s disease are and avoiding them where possible. These risk factors will be the subject of subsequent articles.
Mary Lou Williams, M. Ed., is a lecturer and writer in the field of nutrition. She welcomes inquiries. She can be reached at 267-6480.