This article by Dr. Gerard M. Nadal was originally posted on February 25, 2010. Reposting here with permission.
In the past lesson we saw that the hormones estrogen and progesterone are responsible for normal breast development in an adolescent girl’s menstrual cycles. Birth control pills have extremely high levels of these hormones which mimic the elevated levels of the hormones during pregnancy. In either case, the breasts are stimulated to develop numerous extra cancer-prone Type-1 and Type-2 lobules with doubling of the number of ductules on each lobule. Today we consider the mechanism (we love that word in biology), or how, these situations give rise to cancers.
In past lessons we covered mitosis. During mitosis, DNA is replicated. When this happens, errors in DNA synthesis occur. Sometimes the errors escape the notice of a proofreading molecule that corrects these errors. When the repair crews come in they can make mistakes which lead to permanent mutation. If such a mutation occurs in certain cancer-causing genes, as illustrated in the diagram to he left, that cell is in danger of becoming cancerous.
Type-1 and Type-2 cells have greater numbers of receptors to bind estrogen and progesterone, which precipitates the DNA synthesis. The more these cells are stimulated, the greater the chance that the requisite number of mutations in a cell will be reached leading to cancer, as shown below right.
The conversion of 85% of these cells to cancer-resistant Type-4 cells at the end of a first full term pregnancy (FFTP) is the good news of pregnancy. The younger a woman is when she has a FFTP, the less cancer-prone cells she has developed over time. One Harvard study puts the risk of breast cancer at 3.5% elevation per year after the age of 24 until a woman has a FFTP. After a FFTP, the number of Type-4 cells produced from the remaining 15% of Type-1 and Type-2 cells increases.
Women add substantially to their risks when they delay FFTP, take oral contraceptives and/or have induced abortions, as these leave the breasts with an ever-increasing number of cancer-prone cells over a greater period of time in which cancer can develop.
Again, miscarriages are the exception to this rule, as it is precisely the lack of elevated estrogen and progesterone that led to the miscarriage.
Illustrations from the Breast Cancer Prevention Institute