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CLONALITY IN ACQUIRED HEMATOLOGIC DISORDERS
Annu. Rev. Med. 1991.42:491-506. Downloaded from www.annualreviews.org by Laurentian University on 03/28/13. For personal use only.
D. Gary Gilliland, M.D. , Ph.D., Kerry L. Blanchard, M.D., Ph.D., and H. Franklin Bunn, M.D.
Hematology Division, Department of Medicine, Brigham and Women's Hospital, and Harvard Medical School, Boston, Massachusetts 02115 KEY WORDS:
myeloproliferative, myelodysplastic, polycythemia vera, leukemia, lymphoma
ABSTRACT
Clonal populations of cells can be identified by a number of different independent approaches, including analyses of karyotype, gene rearrange ments, deletions or point mutations, X-linked polymorphisms, and inte gration of virus into the genome. Assessment of clonality has yielded valuable and surprising clues to the pathogenesis of acquired hematologic disorders. In the myeloproliferative states, clonal expansion of a mutated pluripotent stem cell can induce production of blood cells having a normal phenotype. The transition to acute leukemia is often associated with additional mutations. A similar progression has also been noted in lym phoproliferative disorders, again supporting a multistep pathogenesis of malignancy. Thus, a mutation inducing clonal growth may be a necessary but not sufficient step in induction of malignancy. INTRODUCTION
A central question in the study of neoplasia is whether abnormal growth is initiated by a mutation in a single cell or whether normal cells are stimulated to proliferate abnormally by exogenous factors (1, 2). The question of clonal growth vs a "field effect" is of more than academic interest. Identification of clonal cells strongly suggests the presence of either a single or a limited number of somatic mutations that give rise to 491 0066-4219/91/0401-0491$02.00
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GILLILAND, BLANCHARD
& BUNN
the neoplastic disease. Characterization of the specific mutations causing neoplasia may in turn lead to the design of more rational therapy. Clonal cell populations can be identified by a variety of methods, listed in Table 1. Each has its limitations. The most widely used approach is
Annu. Rev. Med. 1991.42:491-506. Downloaded from www.annualreviews.org by Laurentian University on 03/28/13. For personal use only.
the morphologic analysis of chromosomes in metaphase cells. The great
majority of leukemias and lymphomas are associated with chromosomal abnormalities, many of which are specific for a particular disease (3-5). Although karyotype analysis has proven valuable in identifying clonally derived cells, in most instances the relationship between the chromosomal anomalies and pathogenesis of the disease is uncertain. Moreover, not all neoplastic cells contain gross cytogenetic defects. A given chromosomal abnormality is informative only if it is present in a substantial proportion of metaphases, or if it increases significantly with time. At sites involving known genes, sensitivity and specificity of the technique can be enhanced by Southern blotting. For example, the breakpoints of a rearrangement can be precisely delineated. A chromosomal deletion in tumor cells too small to be detected by karyotype analysis can sometimes be identified by the use of restriction fragment length polymorphisms (RFLP): a loss of a polymorphic allele on Southern blot indicates a deletion and identifies a clonal population of cells. Further sensitivity can be gained through the use of the polymerase chain reaction (PCR). For example, PCR analysis of the 9;22 t ranslocation in chronic myelocytic leukemia (6) and the 14;18 translocation in foIlicular lymphoma (7, 8) has provided information on molecular pathogenesis as weIl as a means of detecting minimal residual disease after therapy. PCR has also been used to identify specific point mutations such as in the ras family of oncogenes (9, 10); these mutations provide markers of clonality in leukemias and solid tumors. In certain diseases, it has been possible to take advantage of the normal function of the involved cells to establish clonal derivation. For example, a population of normal B cells exhibits a pol yclonal pattern of immu noglobulin gene rearrangements that enables a diversity of immu noglobulins. Clonal derivation can be demonstrated by the presence of a single or predominant immunoglobulin gene rearrangement. Similarly, in a clonally derived T-cell malignancy, a unique rearranged T-cell receptor gene can be demonstrated. These clonal antigen receptor gene rearrange
ments occur almost exclusively in lymphoid malignancies. In a small subset of human malignancies, particularly in lymphomas, the integration of virus into the genome has provided an independent means of establishing a clonal cell population. Epstein-Barr virus may play a critical role in the pathogenesis of transplant-associated lymphoid mali g nanci es (11, 12), whether it is integrated into the genome or not. Although chromosomal rearrangements and deletions, point mutations,
Annu. Rev. Med. 1991.42:491-506. Downloaded from www.annualreviews.org by Laurentian University on 03/28/13. For personal use only.
Table 1
Clonal markers in disorders of hematopoiesisa
Karyotype
Gene
X -chromosome
Oncogene
Clonal antigen
rearrangement
polymorphism
mutations
receptor
ber-abl (95%)
Pluripotent
p53in
1
stem cell
blast crisis
Pluripotent
ras
Myeloproliferative CML
t(9;22)
p2 1 0 20q - (10%)
MM-MF
stem cell Pluripotent
Iq+, 8+, 9+, 9p+ etc.
PCV
stem cell Pluripotent
ET
stem cell Myelodysplasia
5q-, 7 -, 7 q - , 8+ etc.
(") t"" o Z > t"" =l
ras
Pluripotent stem cell
AML
t(8;21 ), t(l5; 1 7)
PPSC or
inv 1 6, etc
myeloid
t(8; 1 4) , t(4; I I); t(9; 22) --;
ALL
ber-abl (10%)
ras
-