TTF, a gene encoding a novel small G protein, fuses to the lymphoma-associated LAZ3 gene by t(3;4) chromosomal translocation.

We have previously shown that the LAZ3/BCL6 gene encoding a potential transcription factor, is disrupted in B-diffuse large cell non-Hodgkin's lymphomas (NHL) with 3q27 chromosomal abnormalities involving the immunoglobulin (IG) genes. However, LAZ3 rearrangement also occurs in NHL bearing 3q27 translocations without involvement of the IG genes: for example the VAl cell line exhibits t(3;4)(q27;p11). In the present work we have used a RT-PCR method to detect and to sequence the LAZ3 mRNA products from the VAL cell line. We report that the consequence of the t(3;4) is the expression of a chimeric transcript of LAZ3 with a new gene encoding a small G-like protein, termed TTF (Translocation Three Four). Nucleotide sequence analysis of a 1.4 kb cDNA predicts that the TTF gene encodes a protein of 191 amino-acids similar to members of the RAS superfamily including HRAS (27% identical), RAB1A (30% identical) and RHO proteins: the human RAC1, RHOB and CDC42Hs proteins (respectively 43, 44 and 45% identical) and the yeast RHO2 protein (44% identical). Unlike most other small G proteins which are expressed ubiquitously, TTF was transcribed only in hemopoietic cells as a 2.2 kb transcript. TTF may define a new subgroup of RHO-like proteins.