Implications of heavy chain disease protein sequences for multiple gene theories of immunoglobulin synthesis.

The sequence of the amino-terminal 34 amino acids of gamma-heavy chain disease (gamma-HCD) protein Hi is homologous with the amino-terminal region of immunoglobulin heavy chains. gamma-heavy chain disease is smaller than a normal gamma-chain, but has the carboxy-terminal composition expected for gamma-chains and must, therefore, contain an internal deletion. Comparison of the Hi sequence with that of gamma-heavy chain disease Zu, which also has an internal deletion, indicates that the site of internal deletion is not a constant characteristic of gamma-heavy chain disease proteins. Heavy chains can be assigned to subgroups on the basis of variable region sequences. The variable regions of Hi and one other protein differ significantly from those determined for other heavy chains, and these two proteins are assigned to a new heavy chain variable region subgroup, V(HIV). It has been suggested that single immunoglobulin heavy chains are the products of two separate structural genes and that variable region genetic information is translocated and integrated into common region information. These multiple gene theories make no prediction as to whether DNA or RNA is translocated. gamma-heavy chain disease proteins provide unique information that indicates that if translocation is required for the production of immunoglobulin heavy chains, it is DNA, not RNA, that is translocated.