Southern Regional Meeting 2017, New Orleans, LA, February 11-13 ...

Chromosome Abnormalities And Cancer Cytogenetics

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Human Chromosome Translocations And Cancer

Translocations were first detected cytologically in the late nineteenth and early twentieth centuries as novel chromosomes that appeared prominently in tumor cells. Some of the earliest, most complete descriptions of tumor cell chromosomes were provided by the great German cytologist Theodor Boveri, considered by many to be first cancer geneticist. Based on his observations of tumors, Boveri postulated that tumor cells possessed "growth-stimulatory chromosomes" that played a role in malignancy (Balmain, 2001). At the time, cytological markers were not available for human chromosomes, so Boveri was not able to identify specific chromosomal changes in tumors. Today, we know that Boveri's insights were correct. Translocations are indeed common in cancer cells, and some translocations produce oncogenes that are responsible for malignant transformation.

With the development of genetic and cytological models in the early twentieth century, the existence of translocations became firmly established. Geneticists had discovered that genes were physically linked on chromosomes, and that the strength of genetic linkage could be used to provide a rough map of each chromosome. Occasionally, however, geneticists discovered mutations in which genes from two different chromosomes behaved as if they were physically linked. Furthermore, the number of these mutations could be increased by orders of magnitude when organisms were treated with X-rays. Examination of these organisms' chromosomes provided an explanation for the appearance of novel linkage groups; specifically, new chromosomes had appeared, and existing chromosomes had been altered. Moreover, in organisms like Drosophila and maize, for which cytological markers were available, scientists observed that the novel chromosomes contained parts of normal chromosomes. Thus, they were able to infer that breaks had occurred in normal chromosomes, and that the broken ends of nonhomologous chromosomes had fused together, producing translocations.

These early observations provided the foundation for our modern understanding of translocations. We now appreciate that translocations require double-strand breaks (DSBs) in DNA at two locations. The frequency of these DSBs is greatly increased by ionizing radiation, which is still used experimentally to generate translocations. Cells are not tolerant of DSBs; these breaks cause cells to arrest in mitosis or to undergo apoptosis. Therefore, the appearance of DSBs activates the cellular DNA repair machinery that catalyzes the joining of broken chromosome ends (Lieber et al., 2003). A variety of rearrangements can result from this joining. For instance, precise joining of broken ends can regenerate a normal chromosome. Deletions, duplications, and inversions can occur when joining involves two broken ends on the same chromosome. Furthermore, translocations may occur when the broken ends of two nonhomologous chromosomes are joined together. Nonhomologous end joining is often imprecise, so some nucleotides may be lost altogether during the joining process.

Acute Myeloid Leukemia Diagnosis

If a patients presents with the symptoms of acute myeloid leukemia a battery of tests are ordered. The tests used to diagnose AML include the following.

Blood tests

First a complete blood count is prescribed. Anemia is commonly detected with haemoglobin levels usually less than 5g/dl. In addition there is thrombocytopenia of varying degrees. This means there is a low platelet count.

White blood cell (WBC) count is usually high but may be normal or low. The number of neutrophils is usually low.

When a small sample of blood is smeared onto a glass slide and examined under the microscope there may be presence of blast cells. This is called peripheral blood smear. The blood smear may be normal if the blast cells are confined to the bone marrow.

Blood clotting is tested by checking elevated prothrombin time, reduced fibrinogen level and the presence of fibrin degradation products. Lactic dehydrogenase levels may be raised in blood. In addition due to increased number of cell death and formation there may be raised uric acid levels.

Liver and renal function is checked especially before administering chemotherapy. In case of infections blood cultures and other appropriate tests may be prescribed.

Bone marrow biopsy

In case of an abnormal blood test, bone marrow biopsy is prescribed. The biopsy involves taking a sample of bone marrow using a syringe. The needle is pierced into the hip bone commonly and a sample is drawn. The procedure is painful and performed under a local anesthetic. The procedure takes around 15 minutes to complete and is performed on an outpatient basis.

The sample of bone marrow is checked for cancerous cells and abnormal cells. The cells are also checked to see the type of leukemia.

Lumbar puncture or spinal tap

Lumbar puncture involves taking a small sample of cerebrospinal fluid from the spinal column and checking for presence of abnormal cancer cells. This may be positive in patients in whom the cancer has spread to the central nervous system.

A lumbar puncture is not often used to test for AML, unless the patient has symptoms that the leukemia has affected the central nervous system. A lumbar puncture is sometimes used to deliver chemotherapy drugs into the CSF.

Cell examination

Samples of blood, bone marrow, or CSF are looked at under a microscope to check the size, shape, and other traits of the white blood cells. This helps in classifying the type of AML.

The percentage of cells in the bone marrow or blood that are blasts is particularly important. Having at least 20% blasts in the marrow or blood is generally required for a diagnosis of AML. It can also be diagnosed if the blasts have a chromosome change that can be seen in specific types of AML even if the blast percentage does not reach 20%.

Sometimes the blasts look similar to normal immature cells in the bone marrow. However, in normal bone marrow the blast count is 5% or less. In order for a patient to be considered to be in remission after treatment, the blast percentage in the bone marrow must be less than 5%.

Cytochemistry

These tests include staining the leukemia cells with special dyes. For example, some of these stains cause the granules of most AML cells to appear as black spots under the microscope, but it does not cause ALL cells to change color. This helps in differentiating between the cancers.

CT scan and imaging studies

Those with AML may need to undergo imaging studies like MRI scans and CT scans to check for spread to major organs like heart, lungs, brain and liver.

Flow cytometry and immunohistochemistry

This helps in assessing the cells from the bone marrow and blood samples. These tests are helpful in determining the exact type of leukemia.

A sample of cells is treated with special antibodies that stick to the cells only if their specific protein is present on the cell surfaces. The cells are then passed in front of a laser beam. The laser light gives off light from the cells that have antibodies attached to them.

The computer can measure the number and nature of these cells. In immunohistochemistry tests blood and bone marrow samples are treated with specific antibodies so that cancer cells with proteins that bind to these antibodies change color and are visible under the microscope. These tests are used for immunophenotyping.

Cytogenetics

This involves identifying the genetic blueprints of the cancer cells. The leukemia cells atypical genetic makeup that can be identified using these tests. Identifications of the genetic abnormalities help in determining treatment.

One of the common errors or genetic mutations includes translocations.  This means part of one chromosome may be replaced by part of another.

Recognizing these changes can help identify certain types of AML and predict the possible outcome of the cancer.

Common translocation include t(8;21) meaning part of chromosome 8 is now located on chromosome 21, and vice versa.

Another type of error is inversion where a segment is reversed in alignment. For example, inv(16) means that part of the chromosome 16 is upside down. Deletion, addition and duplication are other forms of errors.

Fluorescent in situ hybridization (FISH)

This is similar to cytogenetic testing. This technique uses fluorescent dyes that only attach to specific parts of particular chromosomes. Once tagged to the chromosomes they are visible under a microscope in standard cytogenetic tests.

FISH can be used to look for specific changes in chromosomes. It can be used on blood or bone marrow samples.

Polymerase chain reaction (PCR)

This is yet another DNA analysis test that can find some chromosome changes too small to be seen under a microscope.

Further Reading

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