SOCS1 mutations in DLBCL and FL. (Top) A scheme of the SOCS1 protein with functionally important regions.20 For DLBCL and FL primary cases and DLBCL cell lines, only replacement mutations are shown. For missense mutations, the amino acid exchanges are shown in the single letter code. In DLBCL2, a nonsense mutation, in DLBCL3 and 6, and SU-DHL-8, small out-of-frame deletions (resulting from reading frame shifts 84, 48, and 19 amino acids, respectively, unrelated to SOCS1 followed by stop codons were encoded) and in OCI-LY-19 a large deletion/insertion (encoding 14 amino acids unrelated to SOCS1 before a stop codon) caused obviously function-impairing truncations of SOCS1 protein, and the same applies to the 9- and 5-amino-acid deletions in functionally important regions in Farage and FL1. The mutations in the Farage cell line were detected as single peaks in sequence electropherograms, indicative of a lack of an unmutated allele. All other mutations in primary cases and cell lines appeared as double peaks. Thus, a second unmutated allele was always present in the 4 cell lines with double peaks. In biopsies the unmutated alleles could be the result of the presence of a second unmutated allele in the tumor cells but also originate from nontumor cells present in the lymphoma biopsies. For the 2 DLBCL samples with several mutations, dilution of the genomic DNA and subsequent 2-round PCRs to amplify single molecules revealed that in both cases all mutations were on one SOCS1 allele.
