A cassette corresponds to the organization of small units at the genomic DNA level. Cassettes are found, for example,
for chondrichthyes (shark, skate) Ig-Heavy (IGH) genes and for teleostei Ig-Light-Iota (IGI) genes.
Chondrichtyes IGH cassettes
- Chondrichtyes IGH cassettes comprise V-GENE, D-GENE, J-GENE and C-GENE.
- The V-D-J rearrangements seem to occur only within a given cassette and not between cassettes.
- The number of cassettes may reach several hundreds and may be localized on different chromosomes.
- In the chondrichthyes IGH cassettes partial V-D (~VD)
or V-D, D-J (~VD,DJ)
rearrangements or complete V-D-J (~VDJ)
rearrangements are observed in the genomic DNA of any cell of the organism. This is in constrast with other vertebrates
in which V-D-J rearrangements only occur in cells belonging to the B and T lineage, for the IG and TR genes respectively.
- It is probable that partially joined ~VD or ~VD,DJ DNA can complete the V-D-J rearrangement in B cells and be transcribed into fully
rearranged cDNA (#c).
Teleostei IGI cassettes
- Teleostei IGI cassettes comprise V-GENE, J-GENE and C-GENE.
- The V-J rearrangements seem to occur only within a given cassette and not between cassettes, but this needs to be
confirmed by more data.
- Two cassette types are defined, depending from the orientation of transcription of the V-GENEs relative to the
J-GENE and C-GENE,
- Cassette type 1: The V-GENE(s) is (are) in inverted orientation of
transcription relative to the J-GENE and C-GENE,
- Cassette type 2: The V-GENE(s) is (are) in the same orientation of
transcription relative to the J-GENE and C-GENE.
The 'cassette' concept is part of the 'CLASSIFICATION' concept of IMGT-ONTOLOGY