The 5q35bp chromosomal abnormality characterizes certain CD30 positive anaplastic large cell lymphomas offering a new definition of malignant histiocytosis in childhood.

Anaplastic large cell CD30 positive lymphomas represent a heterogeneous group of lymphomas in which immunocytochemical and molecular investigations have demonstrated the existence of malignancies of T, B or undetermined origin. The recent identification, in a few cases, of a chromosomal 5q35 breakpoint may allow distinction of a specific disease. In these cases, the 5q35bp has been found to be a permanent abnormality present in 5 cell lines and associated with various translocations including most often t(2;5) but also t(7;5), t(5;6) and t(3;5). A primitive myelomonocytic origin of these 5q35bp cells is suggested on the basis of the following arguments: i) they spontaneously express CD68; ii) they reduce tetrazolium blue; iii) they express the c-fms proto-oncogene which encodes the macrophage growth receptor (CSF-1); iv) c-fms, which is not rearranged, has been mapped to 5q33 close to the 5q35 breakpoint; v) treatment by phorbol-diester of a 5q35bp cell line (DEL) induces immunodependent phagocytosis and modulation of the expression of c-fms, CSF-1 and TNF alpha. Since some 5q35bp cell lines also present rearrangements of TCR beta or Ig (jH), these data suggest an ancestral stem-cell origin, prior to T, B or myelomonocytic differentiation. Whatever its origin, the 5q35bp abnormality is mainly encountered in childhood malignancies. As it is constantly associated with the clinical and biological manifestations of a condition recognized by pediatricians as malignant histiocytosis, 5q35bp may today represent the best criterion for the identification of malignant histiocytosis in childhood.