Somatic mutations in the Ig variable region genes and expression of novel Cmu-germline transcripts in a B-lymphoma cell line ("Farage") not producing Ig polypeptide chains.

Non-Hodgkin's B-lymphomas (B-NHL) are a very heterogeneous group of B-cell neoplasias originating from the germinal centers of lymphatic follicles. Thus, they represent a suitable experimental model to study the molecular basis of certain key events which take place in the lymphatic follicles, including somatic hypermutation and heavy chain isotypic switch. An unusual B-NHL cell line ("Farage") not producing Ig polypeptide chains was previously shown to rearrange its IgH and Igkappa genes and transcribe seemingly normal size mu and kappa mRNAs. In an attempt to characterize the phenotype of Farage cells better and to elucidate the molecular basis of the failure of Farage cells to synthesize Ig chains, we sequenced its VH and Vkappa rearranged gene segments by PCR and RT-PCR. It was found that both V genes are somatically, heavily mutated compared to their germline counterparts. In addition, this rearranged VDJ gene of the heavy chain is not transcribed. Instead, the Farage cells express a low level of a new family of germline transcripts starting with a VH like sequence, continuing with a small segment of the 3'VH germline flanking region, and ending within the Cmu region. These transcripts lack D and J segments and do not contain the open reading frame of the full-length Cmu protein. Thus, Farage cells fail to produce mu heavy chains due to silencing of the expression of the conventional VDJCmu transcript and expression of unusual Cmu-germline transcripts. In contrast to the IgH genes, the rearranged VJ gene of Farage is transcribed and gives rise to a full-size kappa-mRNA. This transcript, however, is not translated to a full-length kappa-chain, as it contains a stop codon in its coding region. All the above show that Farage cells are unable to produce Ig polypeptide chains, due to somatic mutations altering the kappa-chain gene, and mutations and/or regulatory events that shutoff the transcription of the IgH gene. The heavily mutated Vkappa and Vkappa genes found, support the conclusion that the Farage cell line originated either from germinal center cells or from the mantle zone of the lymphoid follicle.