Spotlight Review

Introduction

The classic myeloproliferative disorders (MPDs)—chronic myelogenous leukemia (CML), primary myelofibrosis (PMF), polycythemia vera (PV) and essential thrombocythemia (ET)—were described in the nineteenth and the early twentieth century.^{1, 2, 3, 4, 5, 6, 7} In 1951, William Dameshek grouped these four clinicopathologic entities, along with DiGuglielmo's syndrome (erythroleukemia), under the rubric of 'MPDs'.⁸ Dameshek recognized the bone marrow-derived generalized myeloproliferation in these disorders and underscored their overlapping clinical and laboratory features. The following lines taken from his seminal editorial reflect the eternal nature of Dameshek's concept—'It is possible that these various conditions—'myeloproliferative disorders'—are all somewhat variable manifestations of proliferative activity of the bone marrow cells, perhaps due to a hitherto undiscovered stimulus. This may affect the marrow cells diffusely or irregularly with the result that various syndromes, either clear-cut or transitional, result.' Dameshek initially suspected steroid-like hormones as the 'myelostimulatory' factor in these disorders; we now believe this to be mutant tyrosine kinases, for the most part.^{9, 10, 11, 12, 13, 14, 15} The history of MPDs antedates Dameshek by more than a century and substantial progress has been made since his time, including the discoveries of BCR–ABL,^{9, 16, 17, 18} imatinib¹⁹ and JAK2V617F.^{10, 11, 12, 13} Herein, I summarize some of the key events in the history of the classic MPDs before and after Dameshek.

The early times: humors, plethora and bloodletting

According to Hippocrates, The Father of Medicine, (460–370 BC) and Galen, the most prominent physician after Hippocrates, (129–200 AD), blood was recognized as one of the four 'humors' (the other three being phlegm, black and yellow bile) and plethora (Greek—'fullness') as an imbalance of the humors, where blood dominated over the others.^{20, 21, 22} The Hippocratic school of thought reigned for many centuries and phlebotomy (bloodletting) was often practiced in an attempt to maintain humoral equilibrium.²³ Robin Fahreus (1888–1968), a Swedish scientist, suggested that clotted blood, which separates into four distinct layers in a test tube, formed the basis for the 'humoral theory';²⁴ yellow bile represented the serum that separated from the blood clot, whereas the other three layers from top to bottom were thought to represent phlegm (white blood cells), blood (oxygenated red blood cells) and black bile (deoxygenated red blood cells).²⁴

The ancient practice of phlebotomy, as universal treatment for a variety of unrelated conditions, was heavily promoted by Galen²² and endorsed by great philosophers of the medieval period, including Avicenna (980–1037)²⁵ and Maimonides (1135–1204).^{26, 27} Thus, in spite of opposing views from Galen's contemporaries, including Erasistratus of Alexandria (304–250 BC), the practice of indiscriminate bloodletting continued until the nineteenth century;²² aggressive phlebotomy is believed to have contributed to the death of George Washington (1732–1799), the first president of the United States.²⁸ In 1628, William Harvey (1578–1657), an English physician, demonstrated the circulatory nature of blood, and in the process disproved many of Galen's fallacies regarding blood and the heart, including the belief that blood was created to be consumed and sometimes stagnated to cause illness.^{29, 30} This was the beginning of the end of phlebotomy as a remedy for all diseases.

1665: microscopy and the description of myeloid cells

Description of blood cells followed the development and scientific application of improved microscopes (1665–1673), pioneered by the English scientist Robert Hooke (1635–1703)³¹ and the Dutch naturalist Anton van Leeuwenhoek (1632–1723).³² Jan Swammerdam (1637–1680; a Dutch biologist)³³ and Joseph Lieutaud (1703–1780; a French anatomist)³⁴ were the first to respectively describe red and white blood cells, in 1668 and 1749. However, a more convincing description of the red blood cell was made by van Leeuwenhoek and of white blood cells by the English physicians William Hewson (1739–1774; the first to be called 'Father of Hematology') and William Addison (1802–1881).^{32, 35, 36, 37} Alfred Donne (1801–1878), a French histologist, was the first to recognize platelets, in 1842, identifying them as 'globulins of chyle'.³⁸ However, both Max Schultze (1825–1874; a German cytologist) and Giulio Bizzozero (1846–1901; an Italian pathologist) are also credited with the earliest description of platelets and their role in hemostasis.³⁹

Detailed morphological evaluation of blood cells became possible, thanks to the famous English scientist, Paul Ehrlich (1854–1915), who in 1877 pioneered the use of chemical dyes as selective biological stains.^{40, 41} Based on the specific affinities of certain blood cells for either basophilic or acidophilic dyes, Ehrlich defined and named several aniline-reactive leukocytes and distinguished granulocytes from lymphocytes.

1845: description of chronic myelogenous leukemia

In October, 1845, John Hughes Bennett (1812–1875), an English pathologist at the Royal Infirmary in Edinburgh, Scotland, published the first report of CML entitled 'Case of hypertrophy of the spleen and liver in which death took place from suppuration of the blood'.¹ The clinical description of the patient was similar to other case reports, published around the same time, by David Craigie (1793–1866), a physician from Edinburgh who was Bennet's mentor, and Rudolph Virchow (1821–1902), a German anatomist.^{2, 3} Virchow wanted to name the newly described entity as 'Leukhemia', a term he coined in 1847, while Bennet argued that 'leucocythaemia' was a more appropriate term. Regardless, Virchow ultimately ceded priority to Bennett, regarding the first description of the new disease.⁴²

Virchow recognized different types of leukemia and distinguished between 'lymphatic' (lymphocytic) and 'splenic' (granulocytic) variants. However, in 1868, Ernst Christian Neumann (1834–1918), a German pathologist, proposed the concept that blood cells are formed in the bone marrow, and that some cases of splenic leukemia originated in the bone marrow, instead of the spleen.⁴³ Subsequently, Neumann published an extensive account of the different cellular components of the bone marrow, which he considered to be the site of the 'ancestral cell' that gave rise to circulating red blood cells. Paul Ehrlich later (1880) classified leukemias into myeloid and lymphoid subtypes and endorsed the notion of a common stem cell that gave rise to distinct cell lineages.⁴⁴

1879: description of primary myelofibrosis

A German surgeon, Gustav Heuck (1854–1940) was the first to describe PMF, in 1879, under the title of 'Two cases of leukemia with peculiar blood and bone marrow findings'.⁶ Heuck described two young patients with massive splenomegaly, circulating nucleated red blood cells, and increased number of morphologically abnormal leukocytes; he referred to the two cases as 'splenic-medullary leukemia' and 'pure splenic leukemia', respectively.⁶ Heuck recognized that the findings in his two patients differed from those described for CML because of the presence of marrow fibrosis and extensive extramedullary hematopoiesis (EMH). Osteosclerosis was also noted by Heuck in an autopsy report.⁶

Additional case reports with a PMF phenotype started appearing in the beginning of the twentieth century;^{45, 46, 47, 48, 49} in 1904, Max Askanazy (1865–1940), a German pathologist, reported a case with substantial EMH of the liver and diffuse bone marrow fibrosis;⁴⁵ in 1907, Herbert Assmann (1882–1950), a German internist, described a similar case, which he called 'osteosclerotic anemia'⁴⁶—subsequently referred to as 'Heuck–Assmann syndrome'. Other pseudonyms of PMF include agnogenic myeloid metaplasia (a term first used in 1940),⁵⁰ chronic idiopathic myelofibrosis (a term used in the 2001 World Health Organization monograph on the classification of tumors of hematopoietic and lymphoid tissues)⁵¹ and myelofibrosis with myeloid metaplasia. In 2006, the International Working Group for Myelofibrosis Research and Treatment reached a consensus to exclusively use the term PMF.⁵²

The paraneoplastic nature of EMH was first suggested by Donhauser in 1908⁴⁸ and splenic pathology in PMF was further elaborated by Hirschfeld in 1914.⁴⁹ In 1935, post-PV myelofibrosis was described by Hirsch,⁵³ and in 1939,⁵⁴ Vaughan and Harrison underscored the relationship between PMF, PV and ET, in terms of their origination from a common ancestral cell. This view was supported by others^{55, 56} and ultimately led to the formal description of the MPD concept by Dameshek in 1951.⁸ Murray N Silverstein (1928–1998), an American hematologist from the Mayo Clinic, Rochester, MN, defined most of the contemporary natural history and treatment in PMF, outlined in his classic monograph published in 1975.⁵⁷

1892: description of polycythemia vera

Louis Henri Vaquez (1860–1936), a French physician, was the first to describe PV in 1892 in a 40-year-old man with chronic 'cyanosis' distended veins, vertigo, dyspnea, palpitations, hepatosplenomegaly and marked erythrocytosis.⁴ It should be noted that the term 'cyanosis' was used at the time to describe congestion or ruddiness. Vaquez considered his case to be congenital heart disease at first but post-mortem examination revealed no abnormalities of the heart and instead showed marked enlargement of the spleen and liver. Accordingly, Vaquez speculated that the increased red blood cell count was due to hematopoietic hyperactivity. The constellation of symptoms and signs described by Vaquez was later named 'maladie de Vaquez (Vaquez's disease)'.

In 1899 and 1900, Richard Clark Cabot (1868–1939), an American physician, described two additional cases of PV, both with erythrocytosis and leukocytosis and one with massive splenomegaly.^{58, 59} One of the patients died of cerebral hemorrhage. Two additional cases of PV characterized by erythrocytosis, splenomegaly and microvascular symptoms were communicated by McKeen (1901),⁶⁰ Saundby (1902)⁶¹ and Russell (1902)⁶¹ before Osler's systematic review of his own cases as well as those of the literature in 1903.⁵

1903: Osler's contribution to the description of polycythemia vera

William Osler (1849–1919) is regarded as one of the greatest physicians of all times and contributed to many areas of internal medicine including hematology.⁶² In 1903, Osler published a landmark, descriptive case series consisting of four patients with 'Chronic cyanosis, with polycythemia and enlarged spleen: a new clinical entity'.⁵ All four cases displayed erythrocytosis, two had palpable splenomegaly and one had leukocytosis. In his paper, Osler included the aforementioned five additional cases from the literature.^{4, 58, 59, 60, 61} Osler distinguished PV from both relative polycythemia and secondary polycythemia associated with heart and lung diseases. By 1908, Osler had reviewed 18 cases of what he referred to as 'Vaquez's disease' or 'polycythemia with cyanosis'.⁶³ In 1904, Turk and others complemented Osler's observations by recognizing the generalized nature of myeloproliferation in PV as evidenced by the occurrence of leukoerythroblastosis and increased granulocytic and megakaryocytic activity,^{64, 65} an observation that was further elaborated by Frederick Parkes-Weber (1863–1962) in 1908.⁶⁶

1917: Di Guglielmo's syndrome

In 1917, Giovanni Antonio Di Guglielmo (1886–1961), an Italian hematologist, described 'eritroleucemia' in a patient with circulating erythroid progenitors, myeloblasts and megakaryoblasts.⁶⁷ He recognized the leukemic nature of this trilineage proliferative disorder and underscored the primitive cell-origin of the disease; accordingly, he referred to his syndrome as 'eritro-leuco-piastrinaemia'; piastrinemia is Italian for thrombocythemia. In the years that followed, Di Guglielmo further elaborated on the different variants of the disease including acute ('acute erythraemia')⁶⁸ and chronic ('myelosi eritremica cronica').⁶⁹ He noticed that unlike PV, these disorders were characterized by anemia, substantial and abnormal (often bizarre, dysplastic and megaloblastoid) erythroid lineage proliferation in the bone marrow and the presence of circulating megakaryoblasts, myeloblasts and nucleated red blood cells. It is today not clear what exactly constituted Di Guglielmo's syndrome but it is believed that the cases described included a spectrum of erythroid-dominant myeloid neoplasms including acute erythroid/myeloid leukemia and pure erythroid leukemia.^{70, 71}

1934: description of essential thrombocythemia

Essential thrombocythemia is the last of the classic MPDs to be formally described (as 'hemorrhagic thrombocythemia') in 1934 by Emil Epstein (1875–1951) and Alfred Goedel, both Austrian pathologists.⁷ Their patient presented with extreme thrombocytosis, slight erythrocytosis and recurrent mucocutaneous bleeding. They noticed the absence of prominent panmyelosis, which was different from a previous case report with thrombocythemia and erythroleukemia.⁷² Similar cases of thrombocythemia associated with bone marrow megakaryocytic hyperplasia, splenomegaly, thrombosis or hemorrhage had subsequently appeared in the literature under different names: hemorrhagic thrombocythemia, idiopathic thrombocythemia, thrombocythemia, ET, megakaryocytic leukemia, thromboblasthemia and piastrinemia. In a 1954 review of these cases,⁷³ the authors made note of the distinction between idiopathic and secondary thrombocythemia and suggested the former to be a manifestation of a primary bone marrow proliferative disorder that is distinct from both PV and 'megakaryocytic leukemia'.

In 1960, two papers provided detailed analysis of reported cases with persistent thrombocythemia and defended the existence of an MPD characterized by thrombocytosis, bleeding diathesis and thrombosis.^{74, 75} In one of these papers,⁷⁵ the authors identified 21 cases, from the published literature, that fulfiled 'their' diagnostic criteria for 'primary hemorrhagic thrombocythemia'; (1) a history of thrombohemorrhagic phenomena, (2) the presence of a spleen of at least normal size, (3) a peripheral blood picture with marked thrombocytosis but without either erythrocytosis or marked leukocytosis, (4) a bone marrow showing panmyelosis with prominent megakaryocytic hyperplasia and (5) and absence of 'leukemic infiltration'.

1951: William Dameshek and the coining of the term 'myeloproliferative disorders'

William Dameshek (1900–1969) was born on 22 May 1900 in Russia and moved to the United States with his parents in 1903.⁷⁶ He graduated from Harvard Medical School in 1923 and interned at Boston City Hospital where he began his hematology career. In 1928, Dameshek assumed a position as a hematologist in the new Beth Israel Hospital, Boston and served as chief of the blood clinic until he moved, in 1939, to Pratt Diagnostic Hospital, which later (1946) became part of the New England Medical Center, the main hospital for Tufts University School of Medicine. There, he set up and directed the Blood Research Laboratory. In 1946, together with Henry M Stratton, he founded BLOOD—The Journal of Hematology and became the first editor-in-chief.⁷⁷ Dameshek also played a major part in the creation of both the International Society of Hematology in 1946 and the American Society of Hematology (ASH) in 1958; he served as president of both societies in 1956 and 1964, respectively.⁷⁸ BLOOD subsequently (1976) became the official ASH publication. Dr Dameshek retired in 1966 from his position as chair of Medicine at Tufts-New England Medical Center, and moved to Mount Sinai Medical School in New York, where he was professor of medicine until he died on 6 October 1969.⁷⁹

Although Dameshek's laboratory and clinical interests were broad, he is best remembered for describing the concept of 'MPDs' in 1951.⁸ It should be noted, however, that others before Dameshek had recognized trilineage myeloproliferation in MPDs. As early as the first decade of the twentieth century, Turk, Weber and Watson had noted both erythroblastic and leucoblastic proliferation in PV.^{64, 80} Similarly in the 1910s and 1920s, Di Guglielmo, Pianese, Minot and Buckman had underscored trilineage involvement in PV and other MPDs.^{67, 72, 81, 82} The description, in 1935, of post-PV myelofibrosis supported these earlier notions and the suggestion that the MPDs are interrelated.^{53, 54} In 1939, Janet Maria Vaughan (1899–1993) and Harrison reported two cases with 'leuco-erythroblastic anemia and myelosclerosis' and suggested that trilineage myeloproliferation in their patients arose from a common primitive reticulum cell and was induced by a single unidentified stimulus.⁵⁴ The summary of their case report from 1939 contained the following lines 'It is suggested that polycythaemia vera, megakaryocytic leukaemia, and myelosclerosis with leuco-erythroblastic anaemia form a group of closely related conditions'.⁵⁴

1960: the discovery of the Philadelphia chromosome

Peter Nowell (1928–), an American tumor biologist, was working at the University of Pennsylvania, PA, USA, when he unexpectedly visualized individual metaphase chromosomes in leukemia cell culture slides that were tap water-rinsed before Giemsa staining.⁸³ This was not intentional, from his part, although the use of hypotonic chromosome spreading was described earlier in 1953.⁸⁴ Nowell subsequently teamed up with David Hungerford (1927–1993), who at the time was working as a graduate student studying human chromosomes, at the Fox Chase Institute for Cancer Research, PA, USA. Together, they discovered an abnormally small chromosome, which looked like a Y chromosome, in two male patients with CML and published their seminal observation in 1960.¹⁶ The two further demonstrated the invariable presence of the specific chromosomal abnormality in seven other typical CML cases that included two females.⁸⁵ This historic discovery followed the establishment of the human diploid chromosome number as 46 in 1956⁸⁶ and the demonstration of constitutional chromosomal abnormalities in 1959.⁸⁷ Because of the coexistence of cells with normal karyotype, Nowell and Hungerford concluded that the abnormally small chromosome in CML was not constitutional and might be causally related to the disease.⁸⁵

At the First International Conference on Chromosomal Nomenclature in 1960 (Denver, CO, USA), the abnormal chromosome in CML was named after the city of its discovery, as the Philadelphia chromosome (Ph¹). The superscript '1' was used in anticipation of additional similar discoveries originating from work in Philadelphia. The Philadelphia (Ph) chromosome is the first disease-specific chromosomal abnormality in cancer and supported the 1914 somatic mutation theory of Theodore Boveri (1862–1915), a German biologist, who suggested that cancer was caused by acquired chromosomal aberrations.⁸⁸

1967: Fialkow and the stem cell origin of clonal myeloproliferation

In establishing the MPDs as stem cell-derived clonal diseases, Philip Fialkow (1934–1996; an American physician scientist) and colleagues exploited previous observations by Ernest Beutler (b. 1928; an American hematologist) and Mary Frances Lyon (b. 1925; an English geneticist) regarding X chromosome mosaicism in female humans (1962)⁸⁹ and mice (1961).⁹⁰ These scientists, in turn, took the lead from previous work by Susumu Ohno (1928–2000), an American scientist of Japanese descent, who described the heterochromatic nature of one of the two X chromosomes in females.^{91, 92} Beutler and Lyon demonstrated the functional inactivity of the heterochromatic X chromosome and that this inactivity affected both the paternal and maternal X chromosomes in a random fashion. Based on this concept, in 1965, polymorphisms in the X-linked glucose-6-phosphate dehydrogenase (G-6-PD) locus was used to demonstrate the unicellular origin of leiomyoma.⁹³ Subsequently, Fialkow and colleagues applied a similar technique to confirm the clonal nature of CML (1967),⁹⁴ PV (1976),⁹⁵ PMF (1978),⁹⁶ and ET (1981).⁹⁷ They also showed the presence of a single G-6-PD isoenzyme type in different myeloid lineages^{94, 95, 96, 97} as well as certain lymphoid cell types,^{98, 99} thus establishing the common stem cell origin of the clonal process.

The stem cell origin of the classic MPDs has been confirmed by more recent studies demonstrating clonal involvement of B,^{100, 101, 102} T,^{100, 101, 102} CD4-positive T¹⁰³ and natural killer¹⁰⁴ lymphocytes. Furthermore, early reports¹⁰⁵ on the occurrence of Ph-negative clonal B lymphocytes in CML have suggested that preclinical clonal myelopoiesis might antedate disease-causing mutations in the classic MPDs. More recent observations that support such a contention include the emergence of new cytogenetic clones during successful treatment of CML with imatinib,¹⁰⁶ the imperfect association between overall clonal load and JAK2V617F mutant allele burden in PV¹⁰⁷ and the observation that JAK2V617F or MPLW515L/K mutation positive and negative endogenous colonies coexist in both PV and other MPDs.¹⁰⁸ The recent discovery of MPD-specific clonal markers (for example, JAK2V617F) has allowed further clarification regarding the clonal basis of ET even in those patients who feature 'polyclonal' hematopoiesis by X-linked clonality studies.^{109, 110}

1967: establishment of the Polycythemia Vera Study Group

In 1967, Louis Wasserman (1912–1999), an American hematologist from the Mount Sinai Hospital in New York, assembled a multinational group of clinical investigators and founded the Polycythemia Vera study Group (PVSG), under the auspices and funding support of the US National Cancer Institute.¹¹¹ Their mandate was to study the natural history of PV, establish diagnostic criteria and conduct large-scale clinical trials. One of the major incentives for the creation of the PVSG was the concern regarding the leukemogenicity of radioactive phosphorus (P-32), then the major myelosuppressive agent used in PV. The first case reports of P-32-associated acute myeloid leukemia (AML) appeared in 1943¹¹² and 1948.¹¹³

Treatment of polycythemia vera prior to the PVSG studies

Historically, treatment modalities in PV included skeletal radiation therapy (1917),¹¹⁴ acetylphenylhydrazine (1918),¹¹⁵ potassium arsenite (1933),¹¹⁶ P-32 (1940),¹¹⁷ lead acetate (1942),¹¹⁸ nitrogen mustard (1950),¹¹⁹ triethylene melamine (1952),¹²⁰ pyrimethamine (1954),¹²¹ busulfan (1958),¹²² 6-mercaptopurine (1962),¹²³ pipobroman (1962),¹²⁴ uracil mustard (1964),¹²⁵ chlorambucil (1965)¹²⁶ and dapsone (1966).¹²⁷ Hydroxyurea and melphalan were added to the list in 1970.^{128, 129} However, immediately before the initiation of the PVSG clinical trials, the two most frequently used therapeutic modalities were phlebotomy and intravenous P-32. Phlebotomy has been the cornerstone of treatment in PV for over a century.¹³⁰ However, early retrospective studies in PV had suggested a superior median survival with myelosuppressive therapy as opposed to either no treatment (median survival 18 months) or treatment with phlebotomy alone (median survival close to 4 years).¹³¹

The PVSG clinical trials

The first PVSG study (PVSG-01) randomized 431 patients to treatment with either phlebotomy alone or phlebotomy supplanted by either oral chlorambucil or intravenous P-32. The results favored treatment with phlebotomy alone with a median survival of 12.6 years compared to 10.9 and 9.1 years for treatment with P-32 and chlorambucil, respectively (P=0.008). The difference in survival was attributed to an increased incidence of AML in patients treated with chlorambucil or P-32 compared to those treated with phlebotomy alone (13.2 vs 9.6 vs1.5% over a period of 13–19 years).¹³²

In a subsequent nonrandomized study by the PVSG (PVSG-08), treatment with hydroxyurea was associated with a lower incidence of early thrombosis compared to a historical cohort treated with phlebotomy alone (6.6 vs 14% at 2 years). Similarly, the incidence of AML in patients treated with hydroxyurea, compared to a historical control treated with either chlorambucil or P-32, was significantly lower (5.9 vs 10.6 vs 8.3%, respectively, in the first 11 years of treatment).¹³³

Additional studies by the PVSG (PVSG-05) demonstrated that the addition of antiplatelet agents to phlebotomy provided no benefit in terms of thrombosis prevention but increased the risk of gastrointestinal bleeding. The latter observation was based on a randomized study of P-32 plus phlebotomy against phlebotomy plus high-dose aspirin (900 mg day^-1) in combination with dipyridamole (225 mg day^-1).¹³⁴

What is and what is not considered still valid regarding the PVSG experience

The results of the PVSG studies have impacted clinical practice in MPDs for the last several decades. However, several changes have recently taken place. First, the PVSG-promulgated concern regarding the safety of aspirin use in PV has now been refuted through a well-conducted randomized study that showed safety as well as antithrombotic activity of low-dose aspirin.¹³⁵ Second, the recent discovery of JAK2V617F as a molecular marker in PV and related MPDs has undermined the utility of the PVSG diagnostic criteria in these diseases, which are now diagnosed according to the recently revised WHO criteria.¹³⁶ What has not changed since the PVSG experience is the therapeutic role of hydroxyurea; recent controlled studies have confirmed the value of hydroxyurea over both no treatment¹³⁷ and anagrelide,¹³⁸ in high-risk ET. Furthermore, both prospective and well-designed retrospective studies, in ET and PV, do not support the concern of drug leukemogenicity associated with single-agent hydroxyurea therapy.^{139, 140}

Current status of the PVSG

The PVSG received NIH support until 1987 and had conducted 14 separate studies during its existence; its last official publication was in 1997¹⁴¹ and its activities are summarized in a superb book entitled Polycythemia Vera and the Myeloproliferative Disorders, edited by Louis Wasserman, Paul Berk and Nathaniel Berlin (Saunders in 1995). However, several of the original PVSG participants have continued their scientific and clinical efforts in MPDs, including those who are no longer with us: Murray N Silverstein (1928–1998), Louis Wasserman (1912–1999), Harriet S Gilbert (1930–2003) and Scott Murphy (1936–2006).

1973: cytogenetic characterization of the Ph chromosome

The introduction of Giemsa¹⁴² and quinacrine¹⁴³ chromosome banding techniques allowed Caspersson and colleagues to morphologically identify the Ph chromosome, in 1970, as a number 22 chromosome with a deletion.¹⁴⁴ In 1972, Janet Rowley (b. 1925), an American geneticist, confirmed the particular observation and in addition revealed the constitution of the Ph chromosome as a reciprocal translocation between chromosomes 9 and 22; t(9;22)(q34;q11).¹⁷ This was but one of several chromosomal translocations that Rowley has described including t(15;17) in acute promyelocytic leukemia¹⁴⁵ and t(8;21) in acute myeloid leukemia.¹⁴⁶ Her discoveries strongly supported the crucial role of genetic changes in leukemogenesis.

1982 to 1990: molecular and oncogenic characterization of BCR–ABL

The Abelson murine leukemia virus, containing the P160 gag/v-abl oncoprotein, was described¹⁴⁷ not long (1969) before the description of the Ph translocation, t(9;22)(q34;q11).¹⁷ In 1982, a group of American scientists working at the Laboratory of Viral Carcinogenesis at the US National Cancer Institute mapped the human homolog (ABL; 225 kb total gene size) of v-abl to chromosome 9 (Heisterkamp and colleagues)¹⁴⁸ and subsequently collaborated with the Department of Cell Biology at the Erasmus University in Rotterdam, Holland, to demonstrate its involvement during the Ph translocation (de Klein and colleagues).¹⁸ In 1984, the same collaborative effort resulted in pinpointing the chromosome 22 breakpoint to a 5.8 kb area (Groffen and colleagues),⁹ which they named the 'breakpoint cluster region (bcr)', later (1985) shown to be part of the BCR gene (135 kb total gene size).¹⁴⁹ Also in 1984, a new 8.5 kb ABL transcript was described in CML patients,^{150, 151} and subsequently (1985) identified as a BCR–ABL fusion transcript^{152, 153} that translated to the corresponding P210 fusion protein (1986).¹⁵⁴ A similar series of reports in 1986 and 1987 revealed the presence of another BCR–ABL fusion oncogene, with a different BCR breakpoint cluster in acute lymphoblastic leukemia with its corresponding transcript and P190 protein product.^{155, 156, 157, 158}

In 1987 and1988, the transforming activity of P210 BCR–ABL was demonstrated both in mouse bone marrow cells and cell lines (interleukin (IL)-3-dependent Ba/F3).^{159, 160} Earlier in 1984, the CML-associated ABL was shown to possess an enhanced protein tyrosine kinase activity.¹⁶¹ In 1989¹⁶² and 1990,¹⁶³ the BCR–ABL fusion gene was shown to induce lymphoma (bcr-v-abl construct) and acute leukemia (P190 BCR–ABL), respectively, in transgenic mice. In 1990, retroviral infection of hematopoietic stem cells with P210 BCR–ABL was shown to induce CML-like disease in mice.^{164, 165, 166} Thus, BCR–ABL was established as the disease-causing mutation in CML.

1996: the discovery of imatinib, a paradigm shift in cancer treatment

The three most popular CML drugs before the imatinib era were busulfan, hydroxyurea and interferon (IFN)- used first as treatment for CML in 1953,¹⁶⁷ 1965¹⁶⁸ and 1983,¹⁶⁹ respectively. These drugs were antedated by a series of therapeutic attempts with arsenic trioxide (Fowler's solution) in 1865,^{170, 171} Röntgen ray therapy in 1903,¹⁷² arsenic again in 1931,¹⁷³ antileukocyte sera in 1932,¹⁷⁴ benzene in 1935,¹⁷⁵ intravenous radiophosphorus in 1940,¹¹⁷ urethane in 1950¹⁷⁶ and titrated total body irradiation in 1952.¹⁷⁷ Other early treatment modalities in CML included thio-Triethylenethiophosphoramide,¹⁷⁸ leukapheresis,¹⁷⁹ splenectomy,^{180, 181} immunotherapy with Bacillus Calmette-Guerin and vaccination with allogenic myeloblasts,¹⁸² and intensive chemotherapy.^{183, 184} However, none of these treatment approaches, including early comparative trials of busulfan vs radiotherapy,¹⁸⁵ cyclophosphamide vs uracil mustard¹⁸⁶ and busulfan vs cyclophosphamide,¹⁸⁷ were successful in either identifying 'the best treatment' or substantially improving overall survival in CML, which was estimated at 20 months in untreated cases.¹⁸⁸

In 1980, recombinant human leukocyte IFN was successfully prepared¹⁸⁹ and Talpaz and colleagues were the first to use it in patients with chronic phase CML and demonstrate cytogenetic remissions.^{169, 190} This was followed by two landmark treatment studies, reported in 1994 and 1993, which respectively established the superiority of IFN- over chemotherapy (hydroxyurea or busulfan)¹⁹¹ and hydroxyurea over busulfan;¹⁹² median survivals were 72 months, 58 months and 45 months, respectively. Nevertheless, these results were inferior to those achieved with the use of allogeneic stem cell transplantation for chronic phase CML where it is reasonable to expect a projected 15-year survival of 53%.¹⁹³

In 1996, Brian Druker discovered imatinib mesylate—a 2-phenylaminopyrimidine class tyrosine kinase inhibitor of ABL (it also inhibits ARG, PDGFRA, PDGFRB and KIT).¹⁹ The drug targets the ATP binding site within the BCR–ABL tyrosine kinase and disrupts the oncogenic signal by stabilizing the oncoprotein in an enzymatically inactive conformation.^{19, 194, 195} In 2003, the results of a large randomized study that compared imatinib therapy with the combination treatment with IFN- and low-dose cytarabine in chronic phase CML were reported; at a median follow up of 19 months, the former was superior in terms of both CCR (76.2 vs 14.5%) and progression-free survival (96.7 vs 91.5%).¹⁹⁶ The 5-year follow-up of the imatinib arm of the study was reported in 2006 and disclosed an 87% 5-year survival, 87% complete cytogenetic remission rate, and close to 80% major molecular remission.¹⁹⁷ Imatinib is not as effective in advanced phase CML¹⁹⁸ and imatinib-resistant BCR–ABL oncoproteins are emerging as an important clinical challenge. The latter are most often due to acquired ABL point mutations that usually, but not always (for example, T315I), remain sensitive to second-generation kinase inhibitors.^{199, 200, 201, 202, 203, 204}

2005: the JAK2 mutations era

In 2005, four independent laboratories led by D Gary Gilliland,¹⁰ William Vainchenker,¹¹ Radek Skoda¹² and Anthony Green,¹³ made the historic observation that the majority of patients with classic BCR–ABL-negative MPDs carried a JAK2 gain-of-function mutation (JAK2V617F; a G to T somatic mutation at nucleotide 1849, in exon 14, resulting in the substitution of valine to phenylalanine at codon 617). JAK2V617F mutational frequencies are estimated at approximately 95% in PV,^{205, 206, 207, 208} 50% in ET,^{110, 209, 210, 211} PMF²¹² or refractory anemia with ringed sideroblasts and thrombocytosis (RARS-T),^{213, 214} 20% in nonclassic MPDs^{215, 216} and 3% in de novo AML or MDS.^{217, 218, 219} The mutation is not seen in nonmyeloid neoplasms^{220, 221} or reactive myeloproliferation.²²²

In 2006, Gary Gilliland's group discovered a somatic GOF MPLW515L mutation (a G to T transition at nucleotide 1544 resulting in a tryptophan to leucine substitution at codon 515 of the transmembrane region) in JAK2V617F-negative PMF.¹⁵ Subsequently, an additional MPL mutation involving the same 515 codon (MPLW515K) was incidentally discovered during screening for MPLW515L and the prevalence of both mutations was determined at approximately 5% in PMF and 1% in ET.²²³ Interestingly, some patients with MPL mutations also displayed a minor JAK2V617F clone, an observation that is not easily explained and underscores the complexity of pathogenetic mechanisms in MPD.²²⁴

In 2007, other JAK2 mutations in JAK2V617F-negative patients with PV were described by Anthony Green and colleagues; four exon 12 JAK2 mutant alleles were described including F537-K539delinsL, H538QK539L, K539L and N542-E543del.¹⁴ At present, mutational frequencies, in PV, are estimated at 95% for JAK2V617F and 3% for JAK2 exon 12 mutations.²²⁵ Furthermore, current studies suggest substantial overlap in both bone marrow histological and clinical presentation between the two mutations.²²⁵

From a pathogenetic standpoint, all of the aforementioned MPD-associated mutations have been shown to be stem cell-derived, result in constitutive JAK–STAT hyperactivation, induce a PV- (JAK2) or PMF-like (MPL) phenotype in mice, and be vulnerable to small molecule JAK2 inhibition.²²⁶ Further clarifications on the precise pathogenetic role of these mutations as well as their importance as targets for small molecule therapy are being eagerly awaited. For now, the aforementioned discoveries have changed our diagnostic approach to PV and related MPDs.^{136, 227, 228}

The future

'What is past is prologue'. The best is yet to be. The discovery and successful therapeutic application of imatinib in both CML and platelet-derived growth factor receptor-rearranged eosinophilic MPDs has provided an incontrovertible proof of principle in regards to targeted therapy in cancer.^{197, 229, 230} Other examples in this regard, although not as impressive, include the use of imatinib in gastrointestinal stromal tumors,²³¹ trastuzumab in HER2-positive breast cancer,^{232, 233} rituximab in large cell lymphoma²³⁴ and all-trans-retinoic acid in acute promyelocytic leukemia,^{235, 236} to name a few. The possibility of a similar success story in BCR–ABL-negative classic MPDs has been raised by a series of recent discoveries that have linked these disorders with activating mutations of JAK–STAT pathway molecules including JAK2^{10, 11, 12, 13, 14} and MPL.¹⁵ The precise pathogenetic contribution of these novel mutations, and the underlying molecular lesions in their absence, are currently under intense investigation. In the interim, small molecule drugs that effectively and selectively target JAK2 have been developed²³⁷ and the corresponding clinical trials have already been launched (for more information, visit the MPD clinical trials web page at http://www.mpdinfo.org/).²³⁸ These are exciting times for the MPD community and the future looks bright for MPD patients whose courage, tenacity, involvement in patient education and advocacy and financial support to accelerate scientific progress^{238, 239} is increasingly being recognized.²⁴⁰

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