The β2-Microglobulin Locus of Rainbow Trout (Oncorhynchus mykiss) Contains Three Polymorphic Genes

Abstract

β2-microglobulin (β2m) associates with MHC and related class I H chains to form cell surface glycoproteins that mediate a variety of functions in defense. In humans, monomorphism of a single β2m gene contrasts with the diversity and polymorphism of the class I H chain genes, and a similar picture was seen in almost all other species examined. In this regard, rainbow trout (Oncorhynchus mykiss) appeared unusual: trout β2m genes gave a complicated and polymorphic pattern in Southern blots, and a minimum of 10 different mRNA encoding two distinct types of β2m were expressed by a single fish. Characterization of genomic clones from the same fish now shows that the rainbow trout β2m locus consists of two expressed genes and one partial gene that are closely linked. Four copies of the locus were identified and allelic variants of each gene defined, largely through comparison of the noncoding regions. A dramatic variation in the lengths of introns is caused by variable repetitive elements and accounts for the complex pattern seen in Southern blots. By comparison to noncoding sequences, the coding regions are conserved but the three loci differ within a cluster of codons that encode residues of β2m that do not interact with class I H chains. Additional diversity in the trout β2m genes appears to be due to somatic mutation that might be facilitated by the abundance of repetitive DNA elements within the 12 β2m genes of an individual rainbow trout.

Genomic characterization and sequence diversity of the β2-microglobulin gene in the miiuy croaker, Miichthys miiuy

“…Alignments revealed a high degree of sequence similarity between the three sequences obtained (99.6%). Higher-vertebrate β 2 m has no isoforms and is not polymorphic, but polymorphisms between different alleles have been found in some fish species (Shum et al, 1996;Magor et al, 2004;Xu et al, 2010b). In this study, three alleles were found in the miiuy croaker, which provides further evidence that confirms the above phenomenon.…”

“…In general, the β 2 m gene contains four exons and three introns in higher vertebrates, and consists of three exons and two introns in teleosts (Parnes and Seidman, 1982;Gussow et al, 1987;Lundqvist et al, 1999;Xu et al, 2010a); however, two distinct types of β 2 m have been identified in the red sea bream (Pagrus major) and the yellowtail (Seriola quinqueradiata) using expressed sequence tags (ESTs) (Kondo et al, 2010). β 2 m has no isoforms and is not polymorphic in higher vertebrates, but polymorphisms between different alleles have been found in some fish species (Shum et al, 1996;Magor et al, 2004;Xu et al, 2010b). The miiuy croaker (Miichthys miiuy) is an important economic marine fish in China, and the MHC class Iα and class II genes in this species has already been described (Xu et al, 2010c(Xu et al, , 2011.…”

The Structure of the MHC Class I Molecule of Bony Fishes Provides Insights into the Conserved Nature of the Antigen-Presenting System

“…Since then, class I genes and their genomic organizations have been analyzed in sharks (18), and in zebrafish, rainbow trout, Atlantic salmon, fugu and grass carp (19). These studies revealed that the class I amino acid sequences of fish and mammals share ,50% similarity and that many bony fish have MHC features distinct from those typical in mammals, like nonlinkage of class I and class II genes, extreme levels of MHC class I polymorphism, and multiple b2m loci (20). Moreover, similar to findings in humans, CD2, CD3, CD4, CD8a/b, and TCRa/b homologs have also been identified in bony fishes (21), suggesting that p/MHC I-based CD8 + T cell-mediated immunity might be present in fish species.…”

Crystal Structure of a Bony Fish β2-Microglobulin

“…The genomic structure of these genes contains four exons and three introns. The ␤2m genes are genetically unlinked and are located outside of the MHC region (9).…”