T Cell Receptor Gene Rearrangement

T lymphocytes recognize and respond to foreign antigens by means of specific T cell receptors (TcR). The human genome does not contain enough unique DNA sequences to code for the vast repertoire of receptors needed to recognize all of the foreign antigens a person is exposed to during their lifetime. To overcome this limitation, TcR genes undergo a complex series of DNA rearrangements to produce more than one billion unique receptors.

Two types of TcR have been identified. The alpha-beta receptor is expressed on 95% of mature T cells, whereas the remaining 5% of T cells express a gamma-delta receptor. TcR beta and gamma genes are located on chromosome 7 and TcR alpha and delta genes are located on chromosome 14. Beta genes rearrange in the early thymocyte of stage of T cell development. Alpha chains rearrange a little later during the common thymocyte stage. Both rearrangements occur before the CD3-TcR complex is expressed on the cell surface.

A leukemia or lymphoma is derived from clonal expansion of a single abnormal cell. All malignant cells contain an identical gene rearrangement pattern that is easily identifiable by Southern blot analysis. In contrast, a polyclonal proliferation of lymphocytes is due to division of millions of different B and T cells, each containing a different set of gene rearrangements. No single gene rearrangement predominates and none are detectable by Southern blot. A clone must comprise at least 5% of the total number of lymphocytes to be detected. Detection of a TcR gene rearrangement indicates that a monoclonal population of T cells is present.

The percentage of difficult to classify T cell lineage disorders is higher than B cell disorders. Often, T cell disorders cannot be definitively diagnosed by morphology and flow cytometry. T lineage disorders lack a phenotypic marker of clonality like the kappa and lambda light chain restriction in B lineage disorders. Sometimes T cell clonality can be inferred when the lymphocytes express a predominance of CD4 or CD8 antigens. Molecular analysis of the beta TcR gene rearrangements provides the most definitive assessment of T cell clonality. TcR gene rearrangements are useful in the diagnosis of T cell malignancies such as T cell ALL and lymphoblastic lymphoma, mycosis fungoides, Sezary syndrome, T cell PLL/CLL, and adult T cell leukemia. A pathologist interprets the results.

Specimen requirement is:

• Two 7 mL lavender top (EDTA) tubes of blood
• One 7 mL lavender top tube of bone marrow
• 50 mL of body fluid
• 5x5x5 mm piece of tissue

Blood and bone marrow should be transported to the laboratory at room temperature. Tissue should be shipped in dry ice.