Molecular features responsible for the absence of immunoglobulin heavy chain protein synthesis in an IgH− subgroup of multiple myeloma

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Abstract
This study involved 12 patients with multiple myeloma (MM), in whom malignant plasma cells did not contain immunoglobulin heavy chain (IgH) protein chains. Southern blot analysis revealed monoallelic Jh gene rearrangements in 10 patients, biallelic rearrangement in 1 patient, and biallelic deletion of the Jh and Cμ regions in 1 patient. Heteroduplex polymerase chain reaction analysis enabled the identification and sequencing of 9 clonal Jhgene rearrangements. Only 4 of the joinings were complete Vh-(D)-Jhrearrangements, including 3 in-frame rearrangements with evidence of somatic hypermutation. Five rearrangements concerned incomplete Dh-Jh joinings, mainly associated with deletion of the other allele. Curiously, in at least 1 of these 5 cases the second allele seemed to be in germline configuration, whereas the in-frame Vκ-Jκgene rearrangements contained somatic mutations. The configuration of the IGH genes was further investigated by use of Ch probes. In 5 patients the rearrangements in the Jh and Ch regions were not concordant, probably caused by illegitimate IGH class switch recombination (chromosomal translocations to 14q32.3). These data indicate that in many IgH− MM patients illegitimateIGH class switch rearrangement or illegitimate deletion of the functional Vh-(Dh)-Jhallele are responsible for IgH negativity. For example, the exclusive presence ofDh-Jhrearrangements in combination with mutated IGK genes can only be explained in terms of normal B-cell development, if the second (functional) IGH allele is deleted, which was probably the case in most patients. Therefore, defects at the DNA level are responsible for the lack of IgH protein production in most IgH− MM patients.

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