A lymphoma is a tumor of the immune system, usually involving cells called lymphocytes, which are cells that circulate in the lymphatic system. Lymphocytes originate in the bone marrow and carry on their surface special receptors that recognize antigens. Antigens are 'foreign' particles (bacteria, viruses, molecules on the surface of a tumor cell, etc.), which stimulate the immune system to attack. It is the job of lymphocytes to recognize, and then kill off these antigens.

There are two types of lymphocytes: 'B' lymphocytes and 'T' lymphocytes (also known as B cells and T cells). B cells make antibodies, which are proteins that attach to and help destroy the corresponding antigens (or foreign particles). The T cells (so named because they mature in a gland known as the thymus) help the immune system to work in a coordinated fashion.

How Do Lymphocytes Work?

Though every cell in the body carries the same genetic information (known as DNA), different cells 'have different jobs', or express different parts of that DNA. In lymphocytes, the genes for the antigen receptor get assembled differently in every new lymphocyte. Each lymphocyte then carries a unique receptor expressed as a protein on its surface. This random process assures that a person's body can fight off many different kinds of infection, even if the person has never been exposed to that particular infection or disease before. Upon encounter with the 'right' antigen (one that would fit into the receptor like a key in a lock), the cell will become stimulated, make copies of itself (all the copied cells will then carry the same receptor) and mature in preparation for an attack on the foreign invader. In B cells this receptor is secreted and becomes an antibody, whereas in T cells the receptor remains attached to the cell's surface as it hunts for the antigen. Once the cells have eliminated the antigen, most of them will die except a very small fraction that becomes one's immune memory.

The Beginning of a Cancer Cell

Close to billions of new cells are made in our immune systems every day. New cells are made when old cells make copies of themselves and divide.

Apoptosis: natural cell death This constant process of cell division is compensated by an equal measure of natural cell destruction: once the cells get old or have finished their job (for example clearing the antigen in the case of the lymphocytes), they will automatically die in a suicide-programmed process called apoptosis. Apoptosis is like the body's natural 'death sentence' for each cell.

The cell copying process In humans, DNA is found in our 46 chromosomes which contain thousands of genes. Before the process of cell division, the DNA material needs to be copied. During the copy process, DNA is stretched out like a very long necklace made of a succession of beads, each bead representing a different gene. After the copy process, the DNA is condensed (packed) tightly back into chromosomes, very much like a twisted telephone cord. The copying and reassembly of this entire DNA is a delicate process and sometimes the dividing cell makes mistakes. Usually these mistakes are corrected by built-in repairing mechanisms, though sometimes these mechanisms fail and the cell will automatically die through apoptosis.

Mistakes in cell copying process Certain mistakes, especially DNA breaks, may give a cell growth-advantage over other, normal cells, by either activating the cell division process, or by affecting the process of apoptosis. This cell then tends to survive for an abnormally long time and often ends up multiplying more than other cells. With time, the abnormal cell and its copies that survive will continue to accumulate more genetic abnormalities. Eventually, these increasingly abnormal cells can lead to the development of a cancer cell.

The cancer cell This process of abnormal cells thriving and dividing is thought to be the process that leads to all types of cancer. It has been well studied in lymphoma and leukemia, as the cells in these two cancers often circulate in the blood and are more readily available for study. The catalog of known genetic abnormalities seen in lymphomas keeps evolving as research progresses. Some of the abnormalities have clinical importance and may relate to prognosis (how well you can expect to respond to treatment).

There are two main types of genetic abnormalities, either of which may contribute to cancer development:

• Quantitative: In a quantitative genetic abnormality, there is a loss or gain of DNA. This change can lead to either a missing gene or chromosome, or an extra copy of a gene or chromosome.
• Qualitative: In a qualitative genetic abnormality, there are no changes in the amount of DNA, but genes are re-arranged due to errors in the cell copying process. These types of genetic mutations often lead to an altered protein product (a protein with abnormal function), which means that these cells' function or behavior becomes aberrant.

In a biopsy of a lymphoma (a study of actual cancer cells), there may be unique looking cells called double-eyed cells, first described by Sir Hodgkin 150 years ago; hence, if these cells are present, the person is said to have Hodgkin's disease (HD). All lymphomas that do not contain these special cells are called Non-Hodgkin lymphomas (NHL). NHL is a broad category of lymphoma and contains more than 25 subtypes, which are classified based on the speed of progression into 'low grade', 'intermediate grade' and 'high grade'.

Some subtypes of lymphoma are associated with certain specific genetic abnormalities. These genetic mutations are called primary mutations and are thought to be involved in the creation of the lymphoma, whereas other abnormalities are seen with time and are associated more with tumor progression. The following are a few brief examples from the most common subtypes of lymphoma:

Follicular lymphoma (FL): This sub-type is a very common low-grade lymphoma with a typically slow and sometimes waxing and waning progression. In 85-90% of cases, this lymphoma carries a unique genetic mutation that affects B cells (which are the cancerous cells). This mutation leads to an increased production of a protein called BCL2 that inhibits normal cell death (apoptosis) and leads to an abnormal accumulation of B cells.

Diffuse large cell lymphoma (DLCL): This more aggressive lymphoma is the most common subtype of intermediate-grade lymphoma. It can arise by itself or result from the progression (also called transformation) of a low-grade lymphoma. DLCL are rapidly fatal if not treated but they respond well to chemotherapy (75-80% of people with this disease respond to treatment). Patients often relapse, but high dose 'salvage' chemotherapy and a bone marrow transplant can cure 30% of relapsed cases.

The most frequent mutation seen in DLCL involves a gene whose protein regulates cell division. The frequent occurrence of this mutation in DLCL lymphomas suggests it has a role in causing lymphomas. This mutation may also carry important clues about a patient's prognosis.

Burkitt's lymphoma: This is a high-grade (very aggressively progressing) lymphoma characterized by a unique genetic mutation. The succession of events in this lymphoma is felt to occur in the following steps and represents a nice example of lymphomagenesis (development of lymphoma):

• Most humans meet the Epstein Barr virus (EBV) early in life (which causes infectious mononucleosis also known as Mono). The EBV virus has the capacity to 'turn on' B cell division, but in normal individuals the immune system is able to keep the virus 'dormant' in B cells for life.
• When there is a chronic activation of the immune system by infection, such as with malaria (in Africa and parts of Asia) or HIV-1 virus (in AIDS patients), some of the dormant EBV virus containing B cells get reactivated and can start dividing again.
• This over-activation of the immune system leads to mistakes in B cells, especially a mutation that activates a rapid cycle of cell division.

Hodgkin's Disease (HD): In comparison with NHL, in Hodgkin's Disease, the data are very limited. This is due to the fact that the number of tumor cells in a lymph node biopsy for HD is very small (less than 2% of the cells), most of the cells being reactive normal lymphoid cells. PCR (Polymerase Chain Reaction) is a technique of DNA amplification in vitro that can be applied to single cells. Studies of the antigen receptor in the cells of HD (Reed Sternberg cells), suggest these cells are of B cell origin.

How Does Genetic Research Help the Patient?

How does it help the diagnosis?

• In the case of a difficult diagnosis of lymphoma, we can analyze the antigen receptor. In a lymphoma, all the tumor cells will carry the same antigen receptor, showing that they are all derived from the same original cell (the cells are called monoclonal). If the cells have all different antigen receptors, this will confirm this is a non-malignant (or polyclonal) process.
• As mentioned above, some genetic mutations are associated with specific subtypes of lymphoma and therefore can be used as markers for the disease (for example, detection of translocated chromosomes on a karyotype).

• Certain mutations have been described as being of good prognostic value (these lymphomas generally have a good outcome) and patients can be reassured.
• Other chromosomal abnormalities may have a negative prognostic value (these lymphomas generally have a poor outcome) and can help patients and their doctors to make treatment decisions.

• Physicians monitor the response to treatment by doing physical examination, blood work and CT scans. However even if the routine tests appear negative, there is occasionally some undetectable microscopic disease after treatment, called residual disease. These residual tumor cells can be detected by doing genetic tests, for example taking the blood or the bone marrow of a patient to see if there are still some cells carrying the same DNA abnormalities corresponding to the original lymphoma cells. PCR studies allow us, for example, to detect 1 residual tumor cell out of 100,000 to 1,000,000 normal cells.

• We can imagine in the near future that the molecular characterization of the tumor cells at presentation will help in deciding how much and what type of chemotherapy to use for a given patient.
• In addition, a better understanding of the genetic defects in the cells might help to design new targeted therapies aiming at correcting the genetic abnormality itself. For example, in a cell with abnormal apoptosis, a molecule that would neutralize BCL2 (anti-sense therapy) might help force the abnormal cell to die or even overcome the resistance of the cells to the conventional chemotherapy.

Conclusion

Lymphoma has taught us much about cancer and how it progresses. This is a very exciting time, as we are developing a better understanding of the genetic abnormalities and mechanisms involved in causing lymphomas. In the near future, this research will lead to the development of new therapeutic tools for a disease that is, unfortunately, becoming more common.