Gene Conversion of Sialic Acid Binding Domains in CD33-Related Siglecs by Adjacent Pseudogenes: A Novel Mechanism To Change Sialic Acid Binding Specificity.

Siglecs (sialic acid-binding immunoglobulin superfamily lectins) are a family of cell surface receptors involved in regulating the immune response. The CD33-Related Siglecs (CD33rSiglecs, namely Siglec-3, -5 through -11 and -XII in humans) are a subgroup of these molecules found primarily on cells of the innate immune system. All are type-1 transmembrane proteins with an N-terminal sialic acid-recognizing V-set domain followed by a variable number of C-2 set domains, a transmembrane region and a cytosolic C-terminal domain that includes two tyrosine-based signaling motifs. Available data suggest an inhibitory signaling role in the innate immune response, mediated by recognition of host sialic acids as “self”. Nine of the 13 known primate Siglec genes along with 14 Siglec pseudogenes comprise the CD33-related Siglec gene cluster on human chromosome 19. Gene conversion is a mechanism for copying part of a genomic sequence into another, contributing to genetic diversity. Pseudogenes are known to play role in generating functional diversity of related genes (e.g., antibody diversity via gene conversion in chickens). We recently analyzed genomic sequences of the CD33-related Siglec gene cluster in three primates (human, chimpanzee and baboon) and found evidence for rapid evolution in this gene family (Angata et al., PNAS, in press). Additional evolutionary studies using distance-based phylogenetic trees shows evidence for three partial gene conversions between Siglec genes and adjacent Siglec pseudogenes. All three involve the coding regions for extracellular domains that mediate sialic acid recognition, and two involve a pseudogene converting a known Siglec gene. Functional analyses using recombinant proteins show marked differences in sialic acid-binding properties between the converted Siglec and its non-converted ortholog. These findings suggest that gene conversion with pseudogenes has contributed to the rapid functional evolution of the Siglecs, and provides a novel mechanism for changing sialic acid binding specificity. We hypothesize that this mechanism allows for rapid evolutionary adjustments in the recognition of endogenous sialic acids as “self”, a potential factor in controlling the innate immune response.