A mutated promoter of a human Ig V lambda gene segment is associated with reduced germ-line transcription and a low frequency of rearrangement.

Abstract

Several explanations for skewed variable (V) gene usage in the expressed Ig repertoire have been put forth, ranging from cellular selection to differences in the actual rearrangement frequencies of individual V gene segments. In this report we study V gene usage in human Ig lambda rearrangements in an in vitro system, thus largely avoiding selective forces. We find a significant bias in V lambda gene usage in these rearrangements. Through deletional mapping on Southern blots, the relative chromosomal organization of members of the V lambda I, V lambda V, V lambda VI, and V lambda X families was established, which revealed that the bias in V lambda gene usage cannot be readily explained by chromosomal location. We further found that the qualities of the recombination signal sequences, nucleotide sequence/regions of homology of the coding ends or the CpG methylation patterns, do not explain this bias either. We do, however, find a direct correlation between a low frequency of rearrangement and a reduced level of germ-line transcription of a particular V gene segment. We were able to identify three mutations in the octamer motif of the promoter region of this V gene segment. This is the first report showing that mutations in important transcriptional control motifs of individual V gene segments are associated with reduced levels of transcription in the germ line, which in turn may influence the frequency of rearrangement and the shaping of the expressed Ig repertoire.