HPRT mutations in vivo in human CD 34+ hematopoietic stem cells

The rate of spontaneous mutations in human myeloid cells

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.

Evidence of genetic instability in tumors and normal nearby tissues

BACKGROUND

Comprehensive analyses have recently been performed on many human cancer tissues, leading to the identification of a number of mutated genes but providing no information on the variety of mutations present in each of them. This information is of interest to understand the possible origin of gene mutations that cause tumors.

METHODOLOGY/PRINCIPAL FINDINGS

We have analyzed the sequence heterogeneity of the transcripts of the human HPRT and G6PD single copy genes that are not considered tumor markers. Analyses have been performed on different colon cancers and on the nearby histologically normal tissues of two male patients. Several copies of each cDNA, which were produced by cloning the RT-PCR-amplified fragments of the specific mRNA, have been sequenced. Similar analyses have been performed on blood samples of two ostensibly healthy males as reference controls. The sequence heterogeneity of the HPRT and G6PD genes was also determined on DNA from tumor tissues. The employed analytical approach revealed the presence of low-frequency mutations not detectable by other procedures. The results show that genetic heterogeneity is detectable in HPRT and G6PD transcripts in both tumors and nearby healthy tissues of the two studied colon tumors. Similar frequencies of mutations are observed in patient genomic DNA, indicating that mutations have a somatic origin. HPRT transcripts show genetic heterogeneity also in healthy individuals, in agreement with previous results on human T-cells, while G6PD transcript heterogeneity is a characteristic of the patient tissues. Interestingly, data on TP53 show little, if any, heterogeneity in the same tissues.

CONCLUSIONS/SIGNIFICANCE

These findings show that genetic heterogeneity is a peculiarity not only of cancer cells but also of the normal tissue where a tumor arises.

PIG-A mutations in normal hematopoiesis

Paroxysmal nocturnal hemoglobinuria (PNH) is caused by phosphatidylinositol glycan-class A (PIG-A) mutations in hematopoietic stem cells (HSCs). PIG-A mutations have been found in granulocytes from most healthy individuals, suggesting that these spontaneous PIG-A mutations are important in the pathogenesis of PNH. It remains unclear if these PIG-A mutations have relevance to those found in PNH. We isolated CD34+ progenitors from 4 patients with PNH and 27 controls. The frequency of PIG-A mutant progenitors was determined by assaying for colony-forming cells (CFCs) in methylcellulose containing toxic doses of aerolysin (1 x 10(-9) M). Glycosylphosphatidylinositol (GPI)-anchored proteins serve as receptors for aerolysin; thus, PNH cells are resistant to aerolysin. The frequency of aerolysin resistant CFC was 14.7 +/- 4.0 x 10(-6) in the bone marrow of healthy donors and was 57.0 +/- 6.7 x 10(-6) from mobilized peripheral blood. DNA was extracted from individual day-14 aerolysin-resistant CFCs and the PIG-A gene was sequenced to determine clonality. Aerolysin-resistant CFCs from patients with PNH exhibited clonal PIG-A mutations. In contrast, PIG-A mutations in the CFCs from controls were polyclonal, and did not involve T cells. Our data confirm the finding that PIG-A mutations are relatively common in normal hematopoiesis; however, the finding suggests that these mutations occur in differentiated progenitors rather than HSCs.

The fundamental prevalence of chronic myeloid leukemia-generating clonogenic cells in the light of the neutrality theory of evolution

A variety of normal human tissues have been reported to harbor small cell populations carrying potentially oncogenic gene rearrangements. This backdrop of mutant cells may be present in the majority of healthy individuals and is apparently weakly selected against. This may provide empirical support for the concept of global neutrality, or near-neutrality (very weak selection), of many somatic mutations. Many healthy individuals, as well as patients with chronic myeloid leukemia, manifest the BCR-ABL fusion gene in blood cells. The presumed neutrality of the BCR-ABL rearrangement-carrying pluripotential hematopoietic stem cells and the relative uniformity of the incidence rate of CML worldwide were used to estimate the extent of the background of BCR-ABL-positive stem cells and the numerical size of the human pluripotential hematopoietic stem cell pool. Three different approaches (molecular-epidemiological, statistical, and population genetical) were employed. Each resulted in very similar estimates of the size of the stem cells carrying the BCR-ABL allele fusions (1.4 x 10(4) cells) and the size of the total human stem cell pool (1.6 x 10(9) cells per individual). The implication of these estimates in the context of the hierarchical nature of the stem cell pool is also considered. The presumptive smaller-sized population of CD34(-) stem cells could not be characterized by any of the approaches used as a "founding" population, representing an ultimate source of all hematopoietic progenitors, or as a subset of stem cells comprising a deeper "kinetic" segment of the total (10(9)-sized) stem cell compartment.