Is the primary event in radiation-induced chronic myelogenous leukemia the induction of the t(9;22) translocation?

Preleukemic Fusion Genes Induced via Ionizing Radiation

Although the prevalence of leukemia is increasing, the agents responsible for this increase are not definitely known. While ionizing radiation (IR) was classified as a group one carcinogen by the IARC, the IR-induced cancers, including leukemia, are indistinguishable from those that are caused by other factors, so the risk estimation relies on epidemiological data. Several epidemiological studies on atomic bomb survivors and persons undergoing IR exposure during medical investigations or radiotherapy showed an association between radiation and leukemia. IR is also known to induce chromosomal translocations. Specific chromosomal translocations resulting in preleukemic fusion genes (PFGs) are generally accepted to be the first hit in the onset of many leukemias. Several studies indicated that incidence of PFGs in healthy newborns is up to 100-times higher than childhood leukemia with the same chromosomal aberrations. Because of this fact, it has been suggested that PFGs are not able to induce leukemia alone, but secondary mutations are necessary. PFGs also have to occur in specific cell populations of hematopoetic stem cells with higher leukemogenic potential. In this review, we describe the connection between IR, PFGs, and cancer, focusing on recurrent PFGs where an association with IR has been established.

Chronic Myeloid Leukemia, from Pathophysiology to Treatment-Free Remission: A Narrative Literature Review

Chronic myeloid leukemia (CML) is one of the most common leukemias occurring in the adult population. The course of CML is divided into three phases: the chronic phase, the acceleration phase, and the blast phase. Pathophysiology of CML revolves around Philadelphia chromosome that constitutively activate tyrosine kinase through BCR-ABL1 oncoprotein. In the era of tyrosine kinase inhibitors (TKIs), CML patients now have a similar life expectancy to people without CML, and it is now very rare for CML patients to progress to the blast phase. Only a small proportion of CML patients have resistance to TKI, caused by BCR-ABL1 point mutations. CML patients with TKI resistance should be treated with second or third generation TKI, depending on the BCR-ABL1 mutation. Recently, many studies have shown that it is possible for CML patients who achieve a long-term deep molecular response to stop TKIs treatment and maintain remission. This review aimed to provide an overview of CML, including its pathophysiology, clinical manifestations, the role of stem cells, CML treatments, and treatment-free remission.

Unusual case of simultaneous presentation of plasma cell myeloma, chronic myelogenous leukemia, and a jak2 positive myeloproliferative disorder

Background. Multiple articles discuss the rare incidence and potential causes of second hematologic disorders arising after treatment of Chronic Myelogenous Leukemia (CML), leading to the theory of imatinib, the current treatment regimen for CML, as a possible trigger for the development of secondary neoplasms. Our case eliminates the possibility of imatinib as the sole cause since our patient received a diagnosis of simultaneous plasma cell myeloma, CML, and a Jak2 mutation positive myeloproliferative disorder (MPD) arising de novo, prior to any treatment. We will further investigate into alternative theories as potential causes for multiple hematopathologic disorders. Case Report. There are currently no reported cases with the diagnosis of simultaneous plasma cell myeloma, chronic myelogenous leukemia, and Jak2 positive myeloproliferative disorder. We present a case of a 77-year-old male who was discovered to have these three concurring hematopathologic diagnoses. Our review of the literature includes a look at potential associations linking the three coexisting hematologic entities. Conclusion. The mechanism resulting in simultaneous malignancies is most likely multifactorial and potentially includes factors specific to the host, continuous stimulation of the immune system, previous chemotherapy or radiation, a potential common pluripotent stem cell, or, lastly, preexisting myeloma which may increase the susceptibility of additional malignancies.

Quantitative modeling of chronic myeloid leukemia: insights from radiobiology

Mathematical models of chronic myeloid leukemia (CML) cell population dynamics are being developed to improve CML understanding and treatment. We review such models in light of relevant findings from radiobiology, emphasizing 3 points. First, the CML models almost all assert that the latency time, from CML initiation to diagnosis, is at most ∼10 years. Meanwhile, current radiobiologic estimates, based on Japanese atomic bomb survivor data, indicate a substantially higher maximum, suggesting longer-term relapses and extra resistance mutations. Second, different CML models assume different numbers, between 400 and 10(6), of normal HSCs. Radiobiologic estimates favor values>10(6) for the number of normal cells (often assumed to be the HSCs) that are at risk for a CML-initiating BCR-ABL translocation. Moreover, there is some evidence for an HSC dead-band hypothesis, consistent with HSC numbers being very different across different healthy adults. Third, radiobiologists have found that sporadic (background, age-driven) chromosome translocation incidence increases with age during adulthood. BCR-ABL translocation incidence increasing with age would provide a hitherto underanalyzed contribution to observed background adult-onset CML incidence acceleration with age, and would cast some doubt on stage-number inferences from multistage carcinogenesis models in general.

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.

Modeling the low-LET dose-response of BCR-ABL formation: predicting stem cell numbers from A-bomb data

Formation of the BCR-ABL chromosomal translocation t(9;22)(q34;q11) is essential to the genesis of chronic myeloid leukemia (CML). An interest in the dose-response of radiation induced CML therefore leads naturally to an interest in the dose-response of BCR-ABL formation. To predict the BCR-ABL dose-response to low-linear energy transfer (LET) ionizing radiation, three models valid over three different dose ranges are examined: the first for doses greater than 80 Gy, the second for doses less than 5 Gy and the third for doses greater than 2 Gy. The first of the models, due to Holley and Chatterjee, ignores the accidental binary eurejoining of DNA double-strand break (DSB) free ends ('eurejoining' refers to the accidental restitution of DSB free ends with their own proper mates). As a result, the model is valid only in the limit of high doses. The second model is derived directly from cytogenetic data. This model has the attractive feature that it implicitly accounts for single-track effects at low doses. The third model, based on the Sax-Markov binary eurejoining/misrejoining (SMBE) algorithm, does not account for single-track effects and is therefore limited to moderate doses greater than approximately 2 Gy. Comparing the second model to lifetime excess CML risks expected after 1 Gy, estimates of the number of hematopoietic stem cells capable of causing CML were obtained for male and female atomic bomb survivors in Hiroshima and Nagasaki. The stem cell number estimates lie in the range of 5 x 10(7)-3 x 10(8) cells.

Approaches for studying radiation-induced leukemia

According to the conclusion of the International Programme on the Health Effects of the Chernobyl Accident (IPHECA) Haematology Pilot Project (1991-1995), there was no increase in the incidence of malignant disease in hematopoietic and lymphoid tissues after the Chernobyl accident. Nevertheless, since studies of A-bomb survivors indicate that the peak in morbidity may occur more than 10 years after radiation exposure, long-term studies of hemoblastoses and myelodysplastic syndromes are needed today. Study of these leukemias and lymphomas that are potentially induced by ionizing radiation must include both fundamental and applied approaches, i.e., A) epidemiological design; B) utilization of modern methods of diagnosis (cytomorphology, immunocytochemistry, cytogenetics); C) studies of gene mutations, mechanisms of apoptosis, and G1 delay; D) monitoring of oncogene and multidrug resistance gene expression, and E) tracking changes in cell-cell signaling in the bone marrow microenvironment.

Development of Ph positive chronic myeloid leukemia in a patient with chronic lymphocytic leukemia treated with total body irradiation: a rare association

Philadelphia (ph) chromosome positive chronic myeloid leukemia developed in a patient treated for chronic lymphocytic leukemia after treatment with total body irradiation. The role of radiation in the development of CML is discussed.

A greater incidence of complex translocations in myeloid leukemias than in lymphomas and lymphoid leukemias associated with IGH rearrangement

We have shown that the incidence of complex translocations is approximately the same in chronic myeloid leukemia, characterized by the t(9;22)(q34;q11), and in acute myeloid leukemias, characterized by the t(15;17)(q22;q11) or t(8;21)(q22;q22). This incidence is almost threefold greater than the incidence of complex translocations in lymphomas and lymphoid leukemias characterized by the t(8;14)(q24;q32) or t(14;18)(q32;q21). The genomic recombination, which gives rise to the translocations in lymphoid cells, results mostly from errors of IGH gene rearrangement. Genomic recombination underlying myeloid leukemias has a different cause, and a clue to this may lie in the greater incidence of complex chromosome rearrangements.

Chronic myelogenous leukemia and exposure to ionizing radiation--a retrospective study of 443 patients

Exposure to ionizing radiations (Rx) has been implicated as a causative factor of chronic myelogenous leukemia (CML). We performed a retrospective study of 443 consecutive CML patients, looking for a history of significant exposure to Rx, and evaluated the clinical and hematological characteristics in order to find any difference between radiation-related CML patients and those with de novo CML. We identified 406 patients without known exposure to mutagens (group I) and 37 patients with prior significant exposure to Rx (group II). In comparison to patients of group I, those of group II showed particular clinical and hematological features: significantly lower incidence of bulky splenomegaly (p < 0.05) and hyperleukocytosis (WBC > 100 x 10(9)/l; p < 0.05); significantly higher incidence of anemia (Hb < 10 g/dl; p < 0.01). Patients with radiation-related CML had a significantly better survival than those with de novo CML (median survival 61 months vs 42 months; p < 0.05). In conclusion, this study of a large cohort of CML patients indicates that the subgroup of patients with a history of significant exposure to ionizing radiation has particular clinical and hematological features at onset (lower tumor burden, higher frequency of anemia) and a better survival.

Chronic myelogenous leukemia after lymphoid irradiation and heart transplantation

We have previously reported a case of recurrent cardiac rejection in a heart transplant recipient successfully treated with total lymphoid irradiation. Five years after transplantation chronic myelogenous leukemia was diagnosed in this patient. The long-term effects of total lymphoid irradiation in the heart transplant population have not been reported. Based on the known relationships among irradiation, gene translocations, and leukemia, we postulate that this patient's chronic myelogenous leukemia may be a long-term sequela of previous total lymphoid irradiation.

Cytogenetic study in lymphocytes from children exposed to ionizing radiation after the Chernobyl accident

The present study concerns the monitoring of children from the Byelorussian, Ukrainian and Russian republics exposed to the fall-out of the Chernobyl accident. Cytogenetic analyses have been performed on 41 children coming from different areas and exhibiting varying amounts of 137Cs internal contamination, as evaluated by whole-body counter (WBC) analysis. On a total of 28,670 metaphases scored, radiation-induced chromosome damage is still present, although at a very low frequency. Due to the very low fraction of dicentrics, because of the time elapsed from the accident and the relatively low doses of exposure, radiobiological dosimetry is not possible for these children. However, considering that the WBC data indicate that the children are still exposed to 137Cs contamination, the observed occurrence of stable chromosome rearrangements and breaks may represent the persisting effect of continuous low doses of radiation. The present study also indicates that the parallel use of internal contamination dosimetry and cytogenetics could be usefully employed to monitor individual exposure to radiation and to define further management measures.