SIGLEC12, a Human-specific Segregating (Pseudo)gene, Encodes a Signaling Molecule Expressed in Prostate Carcinomas

Nivedita Mitra,Kalyan Banda,Tasha K Altheide,Lana Schaffer,Teresa L Johnson-Pais,Joke Beuten,Robin J Leach,Takashi Angata,Nissi Varki,Ajit Varki

doi:10.1074/jbc.m111.244152

Abstract

The primate SIGLEC12 gene encodes one of the CD33-related Siglec family of signaling molecules in immune cells. We had previously reported that this gene harbors a human-specific missense mutation of the codon for an Arg residue required for sialic acid recognition. Here we show that this R122C mutation of the Siglec-XII protein is fixed in the human population, i.e. it occurred prior to the origin of modern humans. Additional mutations have since completely inactivated the SIGLEC12 gene in some but not all humans. The most common inactivating mutation with a global allele frequency of 58% is a single nucleotide frameshift that markedly shortens the open reading frame. Unlike other CD33-related Siglecs that are primarily found on immune cells, we found that Siglec-XII protein is expressed not only on some macrophages but also on various epithelial cell surfaces in humans and chimpanzees. We also found expression on certain human prostate epithelial carcinomas and carcinoma cell lines. This expression correlates with the presence of the nonframeshifted, intact SIGLEC12 allele. Although SIGLEC12 allele status did not predict prostate carcinoma incidence, restoration of expression in a prostate carcinoma cell line homozygous for the frameshift mutation induced altered regulation of several genes associated with carcinoma progression. These stably transfected Siglec-XII-expressing prostate cancer cells also showed enhanced growth in nude mice. Finally, monoclonal antibodies against the protein were internalized by Siglec-XII-expressing prostate carcinoma cells, allowing targeting of a toxin to such cells. Polymorphic expression of Siglec-XII in humans thus has implications for prostate cancer biology and therapeutics.

Highlights

Sialic acids (Sias)5 are nine-carbon backbone sugar molecules typically found at terminal positions of glycan chains in Deuterostomes, making them potentially important in recognition events [1,2,3]
Mutation of Essential Arginine of Siglec-XII Is Fixed in Humans—In our initial SIGLEC12 paper [15], we reported that the few humans sequenced had a homozygous mutation that changed the Arg residue in the V-set Ig-like domain required for Sia recognition into a cysteine
Evaluating the frequency of this mutation (SIGLEC12P, where P is for pseudogene) in human populations (Fig. 2), we found a global allele frequency of 0.58, with allele frequencies ranging from 0.38 in sub-Saharan Africa to 0.86 in Native American populations

Summary

Introduction

Sialic acids (Sias) are nine-carbon backbone sugar molecules typically found at terminal positions of glycan chains in Deuterostomes (vertebrates and some “higher” invertebrates), making them potentially important in recognition events [1,2,3]. CD33-related Siglecs (CD33rSiglecs) are encoded by a subset of SIGLEC genes clustered on chromosome 19 in humans and chimpanzees They are homologous in sequence and typically expressed on immune cells [10]. Analyses of genomic SIGLEC sequences across humans, chimpanzees, baboons, rats, and mice showed that CD33rSiglecs are evolving rapidly [11]. This is pronounced in the Sia-recognizing V-set domain, suggesting that this domain is under the greatest selection pressure [11,12,13,14]. The protein is referred to as Siglec-XII6 in humans, to differentiate it from Siglec-12 in other primates, where the Sia-binding arginine is present. Because humans and chimpanzees are typically ϳ99% identical in protein coding regions [17, 18] but have significant physiological, anatomical, and biomedical differences

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