Transcription of mouse kappa chain genes: implications for allelic exclusion.

Abstract

The nuclear RNA from a large variety of K-producing plasmacytomas was size fractionated and analyzed with a series of cloned probes representing sequences encoding variable (V), joining (J), and constant (C) regions and selected intervening sequences. All of the plasmacytomas produce a nuclear RNA component that contains V, and C, sequences as well as the intervening sequence between Jo and C,, and that has a distinctive size depending on which of the four Jo segments is expressed (i.e., is present in the secreted K chain). These RNAs are the precursors of K mRNAs, which are transcribed from productively rearranged C, genes. Half of the plasmacytomas examined produce, in addition to a K mRNA precursor, a discrete component of about 8.4 kilobases that contains C, and upstream flanking sequences but lacks the expressed V region sequence.The ability to produce this component is always associated with the persistence in the tumor genome of an unrearranged (germline) J,,-C region. In tumors rearranged at both K loci the nonproductive allele is either transcriptionally silent or, in a minority of cases, transcribed and processed into a "fragment" mRNA lacking V region sequences. These results reveal that allelic exclusion can be effected at several levels of gene expression. They also provide some insight into the relative contributions of the V and C gene elements to this expression. The formation of an active immunoglobulin gene requires a reorganization of the germline genome to bring the elements coding for the variable (V) and constant (C) regions within a single transcriptional unit (1). In the case of mouse K chains this reorganization appears to involve a deletion of the DNA that separates the expressed VK gene and any one of four different JK (joining) gene segments that are located from 2.7 (J4) to 3.9 kilobases (kb) (J1) upstream from the CK gene § (2, 3). Transcription of such a rearranged K locus results in the formation of a pre-mRNA that contains the intervening sequence between the CK gene and the expressed JI segment (J-C intron) (4-7). This intron is subsequently excised during the processing of the K mRNA. Because the length of the J-C intron is different for each of the four IK segments, plasmacytomas expressing different JK segments contain pre-mRNAs of characteristic and distinctive sizes (7). The observation that productive rearrangements are found on only one of the two chromosomes bearing the K genes is thought to provide a molecular basis for the phenomenon of allelic exclusion (1, 8). However, there is presently some uncertainty about the nature and extent of rearrangements on the chromosome bearing the "nonexpressed" CK allele (8-i1). Moreover, the relationship between the structural organization of the K locus and its transcriptional activity is also obscure.In several plasmacytomas a large (n-9 kb) CK-containing transcript is produced, which, in contrast to the pre-mRNAs described above, has a uniform size irrespective of which JK segment is expressed (7). We initially thought th...

Developmental hierarchy of immunoglobulin gene rearrangements in human leukemic pre-B-cells.

“…Such cases may represent pre-B-cells that have thus far failed in their attempt to form effective immunoglobulin gene recombinations. Indeed, an error-prone somatic recombinational system (16)(17)(18)(19) would be expected to produce just such a population of cells.…”

“…For example, mature B cells and plasma cells demonstrate the phenomena of allelic and isotypic exclusions, producing an immunoglobulin from only the maternal or paternal allele, never both, and, in the case of the light chain, using only one of the two available isotypes (i.e., the K or the A light chain) (14). The excluded allele may remain in the germ line form or may be rearranged in an ineffective, aberrant fashion or may be deleted (7,(15)(16)(17)(18)(19)(20). Sim-ilarly, the genes corresponding to the excluded isotype may remain in the germ line form, as do the A genes in K chain-producing cells, or they may be deleted or rearranged, as K genes are, in A chain-producing B cells (7).…”

Pro-B cells sense productive immunoglobulin heavy chain rearrangement irrespective of polypeptide production

“…In a stochastic model, allelic exclusion is considered to be a statistical consequence of a low frequency of rearrangements encoding functional H chains (8,9). In its barebones form, the stochastic hypothesis seems to be disproven for the IgH locus, because pro-B cells expressing signaling-defective forms of the pre-BCR have a large proportion of μH chain double producers (10).…”

Antibody‐producing Cells: a Survey of Four Decades of Research Development The Tenth Annual Ernest Witebsky Memorial Lecture, 22 April 1980

“…New techniques, as exemplified by the ones that the groups of Leder [32,36] and Tonegawa [25] have elaborated, will permit a profound analysis of the germ-line genes and the potential of the joining genes (J-genes). This might enable us to answer the question raised by Medawar [26]: By what mechanism shall it be that a rabbit yet unborn should have the capacity in its genes to make antibody to a chemical determinant not yet synthesized?…”