c-sis is translocated from chromosome 22 to chromosome 9 in chronic myelocytic leukemia.

By analysis of a series of somatic cell hybrids derived by fusion of either mouse or Chinese hamster cells with leukocytes from different chronic myelocytic leukemia (CML) patients or from normal donors, we have localized the human oncogene, c-sis, on the q11 to qter segment of chromosome 22 and demonstrated its translocation from chromosome 22 to chromosome 9 (q34) in CML.

carcinoma DNA (15) using, as a probe, a 1.2 kb PstI v-s/s restriction fragment in pBR322 (8), generously provided by K. Robbins and S. A. Aaronson. Isolation of a 1.7 kb BamHl v-s/s-homologous restriction fragment from the cosmid clone was performed according to previously described methods (I 5).
Molecular Hybridization. Restriction enzymes were purchased from New England Biolabs, Beverly, MA and Bethesda Research Laboratories, Rockville, MD and were used according to the suppliers' specifications. DNA were digested with restriction enzymes, subjected to electrophoresis through 0.75% agarose gels, and transferred to nitrocellulose essentially as described by Southern (16). Nick translation of probes and filter hybridization were as described (15). Specific activity of the probes was 2-5 × 10 s cpm/#g. After hybridization, filters were washed under high stringency conditions (10% standard saline citrate, 65 °C) and exposed to XAR-2 film (Eastman Kodak Co., Rochester, NY) for up to 5 d at -70°C with Dupont Lightning Plus intensifying screens (Dupont Instruments, Wilmington, DE).

Results
To prepare a probe suitable for identification of somatic cell hybrids containing human c-s/s sequences, a previously described (15) cosmid library of human lung carcinoma DNA was screened for clones containing sequences homologous to the 1.2 kb PstI v-s/s probe. As shown in Fig. 1, a single cosmid clone was obtained containing a 30 kb cellular insert with v-s/s-homologous cellular sequences. By restriction endonuclease analysis this clone was shown to correspond to a previously described v-s/s-homologous human sequence, designated c-sis (12). For generation of a c-s/s-specific probe, a single 1.7 kb BamHl fragment, possessing strong homology to v-s/s (c-s/s B~.7), was isolated from the cosmid cellular DNA insert.
Human and mouse control cellular DNA were digested with Sst-I and analyzed for homology to the above described c-sis Bi.7 probe. As shown in Fig. 1 To independently show the localization of c-s/s on chromosome 22 (ql 1 to qter) and demonstrate its translocation to chromosome 9 in CML, a series of Chinese hamster × human somatic cell hybrids were analyzed for human c-sis sequences. As shown in Fig. 1, lanes E and F, the only c-s/s B1.7 cross-hybridizing Sst-I restriction fragment in Chinese hamster cellular DNA is around 2.7 kb in size and thus clearly resolved from the 3.6 kb human Sst-I fragment. Hybrid 1CB-17aNU, which contains chromosome 22q-, lacks detectable human c-s/s sequences (lane F), while a second hybrid, 14CB-21A, containing chromosome 9q+ but not chromosome 9 or 22 (13), is positive for human c-s/s sequences (lane  Table I (Table I). As internal controls, each of the above hybrids were also analyzed for c-abl, a marker for the portion of chromosome 9 translocated to chromosome 22 in CML (6), and for AK1, which maps near the breakpoint but within the nontranslocated portion of chromosome 9 (Table I).

Discussion
Several of the human cellular homologues of viral oncogenes studied to date including c-abl (5,6), c-sis (11,12), c-fes (5,17), and c-mos (18,19) have been localized on human chromosomes frequently involved in translocations associated with specific human cancers. One of these, c-myc, is translocated from chromo-  (13,14). Identification of hybrid clones containing c-abl sequences (6) and analysis of human adenylate kinase (AK 1) enzymatic activity (6) have been previously reported. Cells were analyzed for human c-sis-specific sequences as described in Fig. 1. some 8 to chromosomes 14, 2, or 22, each of which contain immunoglobulin sequences, in Burkitt's lymphoma (1,(19)(20)(21). Similarly, c-abl maps on the region of chromosome 9 which translocates to chromosome 22 in CML (5,6). Other translocations, such as the t(15; 17) reciprocal transiocation associated with acute promyelocytic leukemia (3), involve regions to which human cellular oncogenes (in this case c-fes) have been mapped (22), but appear to be independent of immunoglobulin sequences. The present demonstration that c-sis is translocated from chromosome 22 to chromosome 9 in CML raises the possibility that c-sis rather than c-abl may be involved in CML. Resolution of these alternatives will require a determination of the proximity of these genes to the breakpoints in chromosomes 22 and 9, respectively, and analysis of the expression of their transcriptional and translational products in CML cells.
In addition to possible implications regarding the cause of CML and the significance of the associated t(9;22) (q34;ql 1) translocation, the localization of c-sis within the translocated region of chromosome 22 (Fig. 2) provides a unique molecular marker for studies of the more complex translocations associated with minority populations of CML patients. These can involve translocation of the ql 1 to qter region of chromosome 22 to chromosomes other than chromosome 9, or can in some instances involve more complex rearrangements including three or occasionally even four or five chromosomes (23). Analysis of these translocations using c-abl and c-sis probes should allow a determination of the critical translocation event resulting in the generation of CML.