c-myc gene abnormalities in mucosa-associated lymphoid tissue (MALT) lymphomas.
c-myc gene abnormalities associated with lymphomagenesis, including rearrangements and mutations in the regulatory region between exon I and intron I, have been studied in 54 MALT lymphomas (43 low and 11 high grade) and 36 nodal lymphomas (27 low and 9 high grade). By Southern blot analysis, none of the 54 MALT lymphomas but 2 of 36 nodal lymphomas had c-myc gene rearrangements. Defined tumour cell populations from all MALT lymphoma cases were isolated by microdissection from frozen tissue sections and analysed by polymerase chain reaction-single-strand conformational polymorphism (PCR-SSCP) and direct sequencing for somatic mutations in the exon I/intron I region of the gene. Point mutations in this region were identified in nine cases of MALT lymphomas (7/43 = 16.2 per cent of low grade; 2/11 = 18.1 per cent of high grade). These mutations were located at either the exon I/intron I border of myc intron factor (MIF) binding sites, which are critical in the negative regulation of c-myc expression. Of the nodal lymphomas, only the two cases (5-6 per cent) with c-myc gene rearrangement showed scattered or clustered mutations. These results suggest that c-myc mutations in MALT lymphomas are unlikely to be associated with chromosome translocation, which is the main cause of somatic mutations observed in other types of lymphomas. The mutations involving the c-myc regulatory regions may play a pathogenetic role in at least a proportion of MALT lymphomas.