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Comparison of Histone Variant Synthesis in Human Lymphocytic Leukemia Cells and in Normal Lymphocytes¹

Istituto di Ricerche Farmacologiche "Mario Negri", Via Eritrea, 62, 20157 Milan [C. M., A. B., P. U., M. D'I.], and Divisione di Ematologia Pedriatica, Monza, 20052 Milan [V. R., A. B., G. G., G. M.], Italy

The synthesis of core histone variants and of histone H1 variants was determined in fresh leukemic cells of eight patients with leukemia [seven acute lymphoblastic (ALL) and one chronic lymphocytic (CLL)], in normal lymphocytes from healthy donors or from ALL patients in complete remission. Histone variant synthesis was evaluated by incubating cells with [¹⁴C]Lys and [³H]Arg in medium without Lys and Arg and then by two-dimensional polyacrylamide gel electrophoretic separations (acetic acid-urea-Triton x-100 acetic acid-urea-hexadecyltrimethylammonium bromide for core histone variants; sodium dodecyl sulfate/acetic acid-urea-hexadecyltrimethyl ammonium bromide for H1 variants). As previously reported, quiescent lymphocytes and lymphocytes stimulated with phytohaemagglutinin (PHA) showed clearcut changes in the proportions of synthesis of core histone variants and H1 variants. Leukemic lymphocytes freshly obtained from blood showed a pattern of core histone synthesis and H1 synthesis intermediate between that of quiescent and PHA-stimulated lymphocytes; this is probably due to the presence of a mixture of resting and growing cells. When leukemic cells were stimulated to grow by mitogens, the pattern of core histone and H1 variant synthesis was similar to that in mitogen-stimulated normal lymphocytes. Histone variants whose synthesis is associated with the S-phase were not synthesized in leukemic cells treated with the DNA synthesis inhibitors hydroxyurea and 1-ß-D-arabinofuranosylcytosine (Ara-C). The pattern of acetylation of histone H4 was also apparently similar in leukemic cells and normal lymphocytes.

The radioactivity associated with the ubiquitinated forms of H2A increased in nongrowing lymphocytes and in leukemic cells treated with DNA synthesis inhibitors whereas they decreased after mitogenic stimulation. Variability was wide in the synthesis of ubiquitinated H2A in different cases of leukemia.

The only clear-cut difference between leukemic cells and normal lymphocytes was that leukemic cells from ALL patients, but not lymphocytes from normal donors or from ALL patients in complete remission, synthesized appreciable amounts of H1°, increasing after hydroxyurea/Ara-C treatment and decreasing after PHA-stimulation. In leukemic cells from a CLL patient H1° synthesis was undetectable.

¹ The generous contribution of the Italian Association for Cancer Research, Milan, Italy, is gratefully acknowledged.

² To whom requests for reprints should be addressed.

Received 9/21/87. Revised 2/ 3/88. Accepted 3/11/88.