Some people with acute lymphocytic leukemia (ALL) have one or more of the known risk factors (see “What are the risk factors for acute lymphocytic leukemia?”), but most do not. The cause of their cancer remains unknown at this time. Even when a person has one or more risk factors, there is no way to tell whether it actually caused the cancer.
During the past few years, scientists have made great progress in understanding how certain changes in DNA can cause normal bone marrow cells to become leukemia cells. Normal human cells grow and function based mainly on the information contained in each cell’s chromosomes. Chromosomes are like bundles of long molecules of DNA in each cell. DNA is the chemical that makes up our genes – the instructions for how our cells function. We look like our parents because they are the source of our DNA. But our genes affect more than the way we look.
Some genes contain instructions for controlling when our cells grow and divide. Certain genes that help cells grow and divide are called oncogenes. Others that slow down cell growth and division or cause them to die at the right time are called tumor suppressor genes.
Each time a cell prepares to divide into 2 new cells, it must make a new copy of the DNA in its chromosomes. This process is not perfect, and errors can occur that may affect genes within the DNA. Cancers can be caused by DNA mutations (changes) that turn on oncogenes or turn off tumor suppressor genes.
Translocations are the most common type of DNA change that can lead to leukemia. Human DNA is packaged in 23 pairs of chromosomes. A translocation means that DNA from one chromosome breaks off and becomes attached to a different chromosome. The point on the chromosome where the break occurs can affect genes – for example, it can turn on oncogenes or turn off genes that would normally help a cell mature.
The most common translocation in ALL in adults is known as the Philadelphia chromosome, which is a swap of DNA between chromosomes 9 and 22, abbreviated as t(9;22). It occurs in about 1 out of 4 adult ALL cases. Other, less common translocations are those between chromosomes 4 and 11, t(4;11), or 8 and 14, t(8;14).
Other chromosome changes such as deletions (the loss of part of a chromosome) and inversions (the rearrangement of the DNA within part of a chromosome) can also affect the development of ALL, although they are less common. In many cases of ALL, the gene changes that lead to the leukemia are not known.
Doctors are trying to figure out why these changes occur and how each of them might lead to leukemia. Not all cases of ALL have the same chromosome changes. Some changes are more common than others, and some seem to have more of an effect on a person’s prognosis (outlook) than others.
Some people with certain types of cancer have inherited DNA mutations from a parent. These changes increase their risk for the disease. But ALL is very rarely caused by one of these inherited mutations.
Usually DNA mutations related to ALL occur during the person’s lifetime rather than having been inherited before birth. They may result from exposure to radiation or cancer-causing chemicals, but in most cases the reason they occur is not known.
Last Revised: 02/18/2016