Molecular Basis of Hematologic Malignancy

Chromosomal Translocations

common in hematologic malignancy; uncommon in solid tumors

often (not always) reciprocal, but one of the new fusion genes predominates

Oncogenes

are growth regulatory genes which if abnormally expressed or altered contribute to the pathogenesis of cancer cells.

Some examples include: Growth factors and receptors (Erb-B2, IL-3, TCR), cell-cycle regulators (p53, p21, p27, p16, Rb, cyclin D, ATM), transcription factors (myc, jun, tal (ALL), creb, p53), DNA repair (Mlh1, Msh2, Xpa-G, Blooms); Membrane bound (Ras, SRC); and nuclear membrane Þ Apoptosis (Bcl2).

Oncogenes in Hematologic Malignancies:

C-myc overexpression and amplification seen in Burkitt’s lymphoma.
p53 mutation can be seen in AML, as well as in intermediate and high grade lymphomas.

Reciprocal Translocations – cause many hematological malignancies!

Chronic Myelogenous Leukemia/Acute Lymphocytic Leukemia

t(9;22): Philadelphia chromosome – ABL gene on chromosome 9 is translocated to BCR gene on chromosome 22 (c-ABL encodes P145 protein which has weak tyrosine kinase activity, however BCR/ABL hybrid has enhanced tyrosine kinase activity) Þ Ý tyrosine kinase Þ nuclear transcriptional (via Stat 1,5, myc, ras, PI-3k).

BCR/ABL – longer chimeric protein (p210) Þ Chronic Myelogenous Leukemia; if shortened, chimeric protein (p190) Þ Acute Lymphocytic Leukemia (higher kinase activity), acute presentation.

CML – chronic phase can last 1-5yrs., accelerated phase – 6 months, and a blast crisis 3 weeks. Myeloid proliferation is present.

ALL – acute presentation, lymphoblasts present, CNS involvement, and poor prognosis.

Acute Promyelocytic Leukemia (APL)

Proliferation of promyelocytes in bone marrow and peripheral blood

Disseminated Intravascular Coagulation Þ most common cause of death

Unique karotypic marker t[15;17] [q22;q11-12]

Links PML gene on 15 to RAR (retinoic acid) on 17 – a regulator of transcriptional activation

Protein expressed is much more resistant to retinoic acid Þ blocks maturation of myelocytes due to ß sensitivity to retinoic acid, however, can be overcome by increased amounts of retinoic acid. In tissue culture, retinoic acid can induce APL cells to differentiate into granulocytes Þ trans-retinoic acid can cause remission of APL.

Core Binding Factor (CBF) Leukemias

CBF is a transcription factor complex expressed in hematopoietic cells

Translocations of the CBF gene result in increased translocations within the genes

Heterodimer transcription unit made of alpha and beta portions. Alpha binds to the DNA and beta is responsible for differentiation signaling.

CBF = AML1, alpha domain gene

t(8;21) Þ Acute Myelogenous Leukemia, due to fusion of AML1/ETO genes

t(12;21) Þ Acute Lymphoblastic Leukemia, due to fusion of TEL/AML1

t(2;21) Þ Myelodysplastic Syndrome, Chronic Myelogenous Leukemia; due to fusion of AML1/EV1

inv (16) Þ AML , due to inversion of CBFbeta/MYH11; has a good prognosis with a high eosinophilic presentation.

Associated with good prognosis – chemosensitivity, do not bone marrow transplant while in 1^st remission

The fusion gene can be detected by PCR in remission [poorer prognosis].

Burkitt’s Lymphoma

C-myc (8) translocated to

Heavy chain (14); t(8;14) Þ Ý c-myc

Light chain (22); t(8;22) Þ Ý c-myc

Myc expression in B cells blocks differentiation, clonal extinction, and prevents apoptosis.

Transformation is associated with EBV virus

B-Cell Lymphoma

t(14;18) – BCL2 is translocated to heavy chain (chromosome 14) Þ Ý expression of BCL2.

Ca²⁺ influx (important) in the mitochondria is controlled by BCL2. Bax (another protein) can come in and be proapoptotic (Bax Þ cytochrome c Þ caspase 3 Þ caspase 9 Þ apoptosis); however, BCL2 is anti-apoptotic by sequestering Bax and stopping the cell from apoptosis. Therefore, Ý BCL2 Þ no apoptosis of B-cells Þ lymphoma.

Expansion of the lymphocyte pool by BCL2 allows for further genetic aberration to occur Þ lymphoma.

Decreases steroid and chemotherapy induced apoptosis, as well.

The c-myc oncogene can cooperate with Bcl-2 in promoting tumorigenesis. t(14;18) Þ B cell expansion Þ follicular B-cell lymphoma Þ c-myc amplification, p53 mutation Þ large cell lymphoma

Evidence that fusion genes cause hematologic malignancies – as seen in transgenic mice

BCR-ABL Þ myeloproliferative syndrome

Hc-BCL2 Þ lymphoproliferative syndrome

RAR-PML Þ PML

PCR has improved our prognosis of cancers since the translocations can be identified and characteristics of the translocation can be accurately assessed.