Two independent marker systems, G-6-PD isoenzymes and cell membrane-associated IgM, were used to trace the cellular origin of Burkitt lymphoma. Application of the G-6-PD system is dependent upon the fact that, in accordance with inactivity of one X chromosome in each somatic cell, females heterozygous for the usual B gene (GdB) at the X-linked G-6-PD locus and the variant allele GdA (or GdA-) have two types of cells. GdB is active in one cell population, which consequently produces B type enzyme; in the other population GdA is active, producing the variant A enzyme. Therefore, tumors with a clonal origin in a GdB/GdA heterozygote should exhibit only one enzyme type (B or A) whereas those with multicellular origin may show both A and B enzymes. Utilization of the immunoglobulin system is based upon the supposition that in lymphoid neoplasms with clonal origin either all or none of the tumor cells should have surface-associated IgM and κ-reactivities.
33 of 34 relatively homogeneous (with respect to content of neoplastic cells) individual Burkitt tumors from 19 G-6-PD heterozygotes had single enzyme phenotypes. Similarly, of 95 tumors tested, 92 consisted essentially of IgM(+) or (-) cells. Two neoplasms could not be definitely classified and one tumor had two cell populations. These data suggest a clonal origin for most Burkitt tumors, but the one neoplasm with a double G-6-PD phenotype (A/B) and the one tumor that had two populations of cells with respect to surface IgM, could have originated from multiple cells.
G-6-PD was determined in each of two tumors from seven heterozygotes and in all cases both tumors had the same single enzyme phenotype. Surface-associated IgM was tested in four tumors from one patient, three from another, and in two neoplasms from 11 patients. With one exception, all tumors from the same patient were concordant with respect to IgM. These findings suggest that the entire disease has a clonal origin, i.e., it emerges at one focus and then spreads to other parts of the body.
Cells from 36 recurrent neoplasms were typed for G-6-PD (in heterozygotes) and/or IgM. In one previously reported patient, initial and recurrent tumors were discordant for G-6-PD. Two other patients had IgM phenotypes in recurrences that were discordant with those found in their initial tumors. Phenotypes from three of nine relapses which occurred after 5 mo were discordant for G-6-PD or IgM but no discordance was detected among 27 earlier recurrences. Thus, some "late" recurrences may be due to emergence of "new" maligant cell lines whereas most early relapses are due to reemergence of the original malignant clones.
The probable unicellular origin of Burkitt lymphoma and the findings in tumor recurrences are discussed in terms of the disease's putative viral etiology.