Model for the fetal recruitment of simian gamma-globin genes based on findings from two new world monkeys Cebus apella and Callithrix jacchus (Platyrrhini, primates)

The originally embryonic gamma-globin locus duplicated and acquired a novel (fetal) pattern of expression in a defined time period (55-40 million year s ago) during primate phylogeny. The objective of this study was to determi ne some of the factors that led to first the emergence of fetal gamma speci ficity and then the maintenance of different fetal gamma expression pattern s in extant simian primates (e.g., human, capuchin monkey). Analyses focuse d on two platyrrhine (New World monkey) species: the common marmoset (Calli thrix jacchus) and the brown capuchin monkey (Cebus apella), each of which has paired, non-allelic gamma loci (5'-gamma 1-gamma 2-3'). Quantitation of beta-type globin mRNAs expressed in a 4.5 week old embryo of Callithrix ja cchus revealed that in addition to its primary epsilon- globin message, con siderable amounts of gamma 1 message and just trace levels of gamma 2 messa ge are present. In contrast, analyses of gamma-globin messenger RNAs expres sed in a Cebus apella fetal liver indicated that gamma 2 expression is at l east 120 times greater than gamma 1 expression. Using a luciferase reporter and a transient assay system, the strengths of gamma 1 and gamma 2 promote r fragments of Cebus apella were compared in erythroid (K562) and non-eryth roid (HeLa) cell lines. Due to the lack of chromatin repression in a transi ent expression system, the results do not fully recapitulate globin express ion. However, the results suggest that sequences contained within the Cebus gamma 1 and gamma 2 proximal promoter regions (-200 to +1 bp) can direct g amma transcription in both cell lines. In K562 and, to a lesser extent, in HeLa cells Cebus gamma 2 promoter fragments were significantly stronger (P < 0.01) than gamma 1 promoter fragments. This is consistent with the fact t hat the Cebus gamma 1 promoter contains several mutations, including a prox imal CCAAT box mutation (CCAAT-->CCAAc). The epsilon-gamma 1 intergenic dis tances in these platyrrhines (5.4 kb in Cebus apella and 6.9 kb in Callithr ix jacchus) are short, supporting the inference that it was also short in t he stem simian primates. The results suggest that immediately following the gamma duplication, the gamma 1 gene of the stem simians was still embryoni c and the downstream gamma 2 gene was largely silent. A further inference i s that once gamma 2 accumulated regulatory mutations that disrupted binding of fetal repressors, gamma 2 was expressed fetally and, through gene conve rsion, passed these characteristics to the gamma 1 gene. The fetal expressi on of gamma 1 is most evident in catarrhines (Old World monkeys and hominoi ds), which preferentially express the gamma 1 locus during fetal life. (C) 1999 Wiley-Liss, Inc.