Somatic hypermutation analysis in follicular lymphoma provides evidence suggesting bidirectional cell migration between lymph node and bone marrow during disease progression and relapse.

In follicular lymphoma, somatic hypermutation of the immunoglobulin heavy chain genes facilitates the identification of different lymphoma cell clones, and the construction of genealogical trees. To investigate the dissemination of lymphoma cells, and the role of bone marrow in disease progression, we simultaneously analyzed the somatic hypermutation patterns of lymph node and bone marrow specimens taken from three patients at onset and relapse of their disease. Immunoglobulin heavy chain genes were amplified by polymerase chain reaction, cloned and sequenced. Mutational pedigrees were constructed in a hierarchical order. When direct transition of one mutation pattern into that of a successor clones was not feasible, hypothetical predecessor clones were created, and a probability measurement calculation was introduced. Eighty-five sequenced clones were generated. The average mutation rates were 13.45% for the lymph node specimens, and 9.78% for the bone marrow ones. Forty-two hypothetical predecessor clones were introduced into inter-compartment pedigrees. The genealogical trees showed that early lymphoma clones with a low mutational load quickly migrate from lymph nodes into the bone marrow. Bi-directional lymphoma cell migration was detectable between the two compartments. In one case of follicular lymphoma, a clone identical to the initial lymph node clone was detected 2 years later in the bone marrow. The newly introduced algorithm allows the evaluation of both time and direction of follicular lymphoma cell migration. We found evidence that follicular lymphoma originates in the lymph node, and infiltrates the bone marrow early in the course of the disease. Moreover, inter-compartment migration between lymph nodes and bone marrow occurs in both directions.