Baltimore - It seems more likely than ever that certain forms of Alzheimer's disease are hereditary, passed directly from parent to child, says a statistician at The Johns Hopkins University School of Public Health.
Alzheimer's disease begins as a progressive loss of memory, usually by the time a victim reaches the 70s or early 80s, and cuts remaining life expectancy by about half.
Physicians have suspected that at least one form of Alzheimer's was hereditary but have had difficulty proving it because many people who might come down with the condition die of other causes before Alzheimer's symptoms overtake them, explains Gary A. Chase, PhD, professor of mental hygiene, biostatistics, and psychiatry.
To interpret data on Alzheimer's disease accurately and predict the likelihood of developing Alzheimer's, Chase has devised mathematical formulas to estimate how many of the people who died early would have come down with the condition had they lived long enough.
In particular, the form of Alzheimer's disease that severely affects language ability - with deterioration of memory, reading, and writing functions - appears to be hereditary, Chase notes.
In earlier work at Hopkins, School of Medicine researchers Marshal Folstein, MD, and John Breitner, MD, suggested that this form of the disease was transmitted through a single dominant gene. This means that Alzheimer's probably will be inherited by half of those persons who have one parent with a single Alzheimer's gene, and in three quarters of those with each of two parents having a single Alzheimer's gene.
Other forms of Alzheimer's disease, Folstein and Breitner say, do not appear to be hereditary. Moreover, most people who have inherited Alzheimer's probably will not come down with the disease simply because they will not live long enough.
"Many past formulas used for computing the risks of Alzheimer's are simply incorrect," Chase says. "They're derived by people who are not mathematically trained, who have no background in actuarial science, and who simply write something down because it has been used by others."
Traditionally, researchers have estimated the risk of inheriting Alzheimer's with a statistical calculation called the Abridged Census Method. But this method, Chase adds, underestimates the number of people who eventually would develop the condition. To arrive at a more accurate estimate, Chase's calculations involve life tables, standard estimates of the causes of death among a group of people, and a formula called the Kaplan-Meier estimator.