Somatic hypermutation and junctional diversification at Ig heavy chain loci in the nurse shark.
We estimate there are approximately 15 IgM H chain loci in the nurse shark genome and have characterized one locus. It consists of one V, two D, and one J germline gene segments, and the constant (C) region can be distinguished from all of the others by a unique combination of restriction endonuclease sites in Cmu2. On the basis of these Cmu2 markers, 22 cDNA clones were selected from an epigonal organ cDNA library from the same individual; their C region sequences proved to be the same up to the polyadenylation site. With the identification of the corresponding germline gene segments, CDR3 from shark H chain rearrangements could be analyzed precisely, for the first time. Considerable diversity was generated by trimming and N addition at the three junctions and by varied recombination patterns of the two D gene segments. The cDNA sequences originated from independent rearrangements events, and most carried both single and contiguous substitutions. The 53 point mutations occurred with a bias for transition changes (53%), whereas the 78 tandem substitutions, mostly 2-4 bp long, do not (36%). The nature of the substitution patterns is the same as for mutants from six loci of two nurse shark L chain isotypes, showing that somatic hypermutation events are very similar at both H and L chain genes in this early vertebrate. The cis-regulatory elements targeting somatic hypermutation must have already existed in the ancestral Ig gene, before H and L chain divergence.
Malecek
Karolina
K
Department of Physiology and Pharmacology, State University of New York Health Science Center, Brooklyn, NY 11203, USA.
Brandman
Julie
J
Brodsky
Jennie E
JE
Ohta
Yuko
Y
Flajnik
Martin F
MF
Hsu
Ellen
E
D000595:Amino Acid Sequence; D000818:Animals; D022801:Complementarity Determining Regions; D019143:Evolution, Molecular; D005819:Genetic Markers; D007143:Immunoglobulin Heavy Chains / Q000235:genetics*; D007147:Immunoglobulin Light Chains / Q000235:genetics; D008969:Molecular Sequence Data; D009154:Mutation; D011995:Recombination, Genetic; D012754:Sharks / Q000276:immunology*; D027041:Somatic Hypermutation, Immunoglobulin
D022801:Complementarity Determining Regions; D005819:Genetic Markers; D007143:Immunoglobulin Heavy Chains; D007147:Immunoglobulin Light Chains