The rate of spontaneous mutations in human myeloid cells

• We provide the first measurement of the mutation rate (μ) in human myeloid cells.
• μ is measured to be 3.6–23 × 10−7 per cell division.
• The AML-ETO and MLL-AF9 fusions do not seem to increase μ.
• Cooperating mutations in NRAS, FLT3 and p53 not seem to increase μ.
• Hypermutability may be required to explain leukemogenesis.

The mutation rate (μ) is likely to be a key parameter in leukemogenesis, but historically, it has been difficult to measure in humans. The PIG-A gene has some advantages for the detection of spontaneous mutations because it is X-linked, and therefore only one mutation is required to disrupt its function. Furthermore, the PIG-A-null phenotype is readily detected by flow cytometry. Using PIG-A, we have now provided the first in vitro measurement of μ in myeloid cells, using cultures of CD34+ cells that are transduced with either the AML-ETO or the MLL-AF9 fusion genes and expanded with cytokines. For the AML-ETO cultures, the median μ value was ∼9.4 × 10−7 (range ∼3.6–23 × 10−7) per cell division. In contrast, few spontaneous mutations were observed in the MLL-AF9 cultures. Knockdown of p53 or introduction of mutant NRAS or FLT3 alleles did not have much of an effect on μ. Based on these data, we provide a model to predict whether hypermutability must occur in the process of leukemogenesis.