Phylogenetic Distribution of CMP-Neu5Ac Hydroxylase (CMAH), the Enzyme Synthetizing the Proinflammatory Human Xenoantigen Neu5Gc

Peri, Sateesh; Kulkarni, Asmita; Feyertag, Felix; Berninsone, Patricia M; Alvarez-Ponce, David

Some deuterostomes, including humans, have undergone inactivation or loss of the gene CMAH, and therefore have lost the ability of synthesizing Neu5Gc. In an ancestor of humans, an Alu-mediated deletion removed a region of the genome encompassing a 92-bp exon of CMAH (Irie and Suzuki 1998; Hayakawa et al. 2001; Chou et al. 2002) . Deletion of this exon resulted in a frameshift mutation, as a result of which the human protein is only 72 amino acids long and nonfunctional (the full ancestral CMAH protein was 590 amino acids long). Therefore, human tissues exhibit very low levels of Neu5Gc (Muchmore et al. 1998; Tangvoranuntakul et al. 2003; Diaz et al. 2009 ), which are probably the result of incorporation from animal foods. It has been estimated that this inactivation took place 2.5-3 Ma (Chou et al. 2002) and was fixed rapidly in the population, probably with the intervention of positive selection (Hayakawa et al. 2006 ). Inactivation of CMAH may have affected human biology in multiple ways (for a comprehensive review, see Varki 2009; Okerblom and Varki 2017) . First, it may have freed our ancestors from pathogens that require attaching to Neu5Gc for infection, such as Plasmodium reichenowi (responsible for malaria in chimpanzees and gorillas; Martin et al. 2005) , E. coli K99 (Kyogashima et al. 1989) , transmissible gastroenteritis coronavirus (Schwegmann-Wessels and Herrler 2006), and simian virus 40 (Campanero-Rhodes et al. 2007 ). Nonetheless, inactivation of CMAH probably made humans susceptible to pathogens preferentially recognizing Neu5Ac, such as Plasmodium falciparum (Martin et al. 2005) and Streptococcus pneumoniae (Hentrich et al. 2016) . Remarkably, P. falciparum emerged from P. reichenowi after inactivation of CMAH in humans (Rich et al. 2009; Varki and Gagneux 2009) . Second, the CMAH pseudogene may have been driven to fixation via sexual selection. In the time in which the presence of a functional CMAH gene was polymorphic in the ancestral hominin population, anti-Neu5Gc antibodies in the reproductive tract of Neu5Gc-negative females may have targeted Neu5Gccontaining sperm or fetal tissues, thus reducing reproductive compatibility (Ghaderi et al. 2011) . Third, loss of Neu5Gc may have unchained a series of changes in human sialic acid biology and its controlling genes. Out of the <60 genes known to be involved in sialic acid biology, at least 10 have undergone human-specific changes, some of which have been linked directly to Neu5Gc loss (Altheide et al. 2006; Varki and Varki 2007; Varki 2009 ). Fourth, the phenotypes of CMAH À/À mice suggest that loss of Neu5Gc may have contributed to a number of human-specific diseases (Hedlund et al. 2007; Chandrasekharan et al. 2010; Kavaler et al. 2011) .