Some 5.7 million Americans are living with Alzheimer’s disease. For the vast majority of them, the disease is the result of an unpredictable perfect storm of lifestyle and genetic factors. But a small minority of people who have Alzheimer’s disease — fewer than three percent — inherit early-onset hereditary Alzheimer’s disease (eHAD) from a parent through a single gene mutation. Some also refer to this condition as early-onset familial Alzheimer’s disease.
If one of your parents has the gene mutation that causes eHAD, you have a 50 percent chance of having inherited the mutation yourself. People who inherit the mutation are virtually guaranteed to develop the condition, and symptoms start to appear in a person’s 30s, 40s or 50s.
The knowledge that you or your partner will develop familial Alzheimer’s disease at an early age will raise many questions about your future. Couples will have to consider whether and how to have a family. As Susan Hahn, a licensed genetic counselor and representative of the National Society of Genetic Counselors, remarks, “This isn’t something that people enter into lightly.”
Couples affected by eHAD who want to start a family must consider the realities and challenges of raising a child knowing that one parent will develop the condition, says Hahn. But if you and your partner do decide to have a family, she says, “There is a way to have a child that would not inherit this mutation and would not have to worry about having this condition.”
Asking the Hard Questions
Before exploring how to conceive a child that does not carry a mutation for eHAD and considering PGT, Hahn recommends genetic counseling to grapple with the tough questions that prospective parents in this situation must face.
“Regardless of whether you pass the gene on to your child, have you contemplated what it would be like to have a child and also know that, at some point, you or your partner will develop Alzheimer’s disease?” Hahn asks. Eventually, the unaffected parent will find him or herself both raising a child and caring for a partner with dementia. What’s more, the child will likely lose a parent, or have a parent that no longer remembers him or her before he or she is grown.
Weighing the Options
Many couples who are at risk of passing on eHAD to their offspring start their family by taking the 50% chance that they will pass on the gene mutation. These couples may do so with the hope that by the time their children develop eHAD, there may be better treatment available.
The only fully preventative option by which couples can have a biological child is through IVF with PGT for Alzheimer’s disease. Through this method, eggs are fertilized in vitro — that is, outside of the body — to create embryos. With this method, the embryos can be tested for the gene mutation before they are implanted in the mother. Only unaffected embryos are implanted.
How PGT for Alzheimer’s Disease Works
A single mutation in any one of three known genes — amyloid precursor protein (APP), presenilin-1 (PS1) and presenilin-2 (PS2) — can cause eHAD. “These genes are thousands of base-pairs long,” says Hahn. “You can have a mutation anywhere along the way in that gene that can cause an issue.”
In order for doctors to test the embryo for one of these mutations, they must first identify the prospective parent’s specific mutation. A laboratory then uses the affected parent’s DNA — that is, his or her genetic material — to create a genetic test for the embryo. Sometimes the laboratory must obtain DNA from other family members to create this customized test.
After the egg is fertilized, the embryos develop for about five days until they’ve grown to the size of 100 to 150 cells. Then, a few cells are removed to test for a mutation that matches the one in the affected parent. If there is an unaffected embryo, it will be implanted in the mother. Other unaffected embryos can be frozen and stored.
Prospective parents should be prepared for setbacks. For one, all embryos could carry the gene mutation. Also, because PGT is not 100 percent accurate, parents-to-be may choose to undergo prenatal testing during pregnancy as an added safeguard to verify that the child will not be born with the gene mutation. Finally, not every implantation results in a pregnancy, though many do.
“The hopeful part is that if you want to have a child, it is possible,” says Hahn. “You do have options.”