Genetic basis of hematologic neoplasia

Cell proliferation

Cell division requires that cells first replicate their DNA and then divide into two daughter cells. These two phases of the cell cycle , termed the synthesis phase ( S phase ) and the mitosis phase ( M phase ), are separated by two preparatory gap phases (G1 and G2) ( Fig. 12.1 ). Cells that are not actively dividing are in a resting state termed G0. Nonproliferating cells activated to proliferate first transit from the quiescent G0 phase to the G1 phase, during which time synthesis of enzymes required for DNA replication takes place. DNA replication that is completed during the S phase is followed by synthesis of enzymes required for mitotic cell division during the G2 phase. During the M phase, the tetraploid DNA is partitioned into two diploid daughter cells. The daughter cells may either continue to proliferate by directly passing into the G1 phase or cease cell division by passing into the G0 phase.

Regulation of the cell cycle is governed by the concerted actions of two families of proteins, the cyclins and the cyclin-dependent protein kinases (CDKs). Cyclins , each specific for the G1 or the G2 checkpoint, accumulate during either G1 or G2 and trigger activation of CDKs. Activated CDKs phosphorylate specific substrates that regulate transcription of genes involved in DNA synthesis or mitosis. One of the substrates phosphorylated by the G1-specific cyclin D–CDK4/6 complex is the retinoblastoma (RB) protein . RB is normally bound to the E2F transcription factor in a complex that represses transcription of E2F target genes. After phosphorylation, RB dissociates from E2F, allowing E2F to enter the nucleus and activate transcription of target genes involved in DNA synthesis, including DNA polymerase and thymidine kinase.

Important checkpoints in the cell cycle are strictly governed by the CDKs, in particular the restriction point in late G1. Before this point is reached, normal cells respond to extracellular proliferative and antiproliferative cytokines. After this point is passed, the cells no longer respond to extracellular signals, instead proceeding inexorably through cell division. Cancer cells often carry mutations in cell cycle regulatory genes that make them refractory to restriction point control, leading to unregulated cell proliferation. RB mutations that interfere with binding to E2F allow unregulated E2F transcriptional activity. Mutations in several other regulatory proteins that inhibit the cyclin D–CDK4/6 complex have been discovered in cancer cells. The most common mutations of this sort are those involving the TP53 gene that encodes the p53 protein . p53 regulates transcription of several genes involved in G1 or G2 cell cycle control, including the proapoptotic BAX gene and the WAF1/CIP1 complex that inhibits the cyclin D–CDK4/6 complex. Mutations in p53 lead to both unregulated cell proliferation and resistance to apoptosis.

Entry of hematopoietic cells into the cell cycle (G0 and G1) is stimulated by diffusible extracellular growth factors ( cytokines ) that bind to growth factor receptors. Most of these hematopoietic growth factor receptors are expressed on the cell surface and are composed of a ligand (cytokine)-binding extracellular domain, a transmembrane domain, and an intracellular signaling domain. Binding of ligand to the extracellular domain leads to activation of CDKs that are loosely bound to the intracellular domain of the receptor. The activated kinases detach from the receptor and activate specific transcription factors by phosphorylation of either their tyrosine residues or their serine and threonine residues. The activated transcription factors then migrate to the nucleus and activate transcription by binding to regulatory regions of specific genes. The JAK-STAT pathway is particularly important in hematopoietic cells. Cytokine receptor-mediated activation of Janus (JAK) kinase leads to phosphorylation of the transcription factor signal transducer and activator of transcription (STAT) , which induces cellular growth and differentiation through its effect on gene expression.

Neoplastic cells are characterized by gene mutations that lead to uncontrolled proliferation—often caused by dysregulation of cell cycle checkpoints in G1 and G2, growth factor independence, and resistance to apoptosis. Dysregulation of cell cycle checkpoints may be caused by inactivating mutations in cyclin-CDK inhibitors (including RB, p53, INK4, and CIP/KIP proteins) or, in the case of mantle cell lymphoma, activating translocation of the cyclin D1 gene . Growth factor dependence may be caused by a variety of mechanisms, including the following:

• 1.

  Autocrine production of growth factors by tumor cells

• 2.

  Increased sensitivity to growth factors caused by the increased number of receptors on tumor cells

• 3.

  Constitutive ligand-dependent activation by mutant growth factor receptors

• 4.

  Functional inactivation of tumor suppressor proteins, leading to cell activation by minute quantities of growth factors not normally sufficient to drive cell proliferation