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 years ago) during primate phylogeny. The objective of this study was to determine some of the factors that led to first the emergence of fetal gamma specificity and then the maintenance of different fetal gamma expression patterns in extant simian primates (e.g., human, capuchin monkey). Analyses focused on two platyrrhine (New World monkey) species: the common marmoset (Callithrix 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 jacchus revealed that in addition to its primary epsilon-globin message, considerable amounts of gamma 1 message and just trace levels of gamma 2 message are present. In contrast, analyses of gamma-globin messenger RNAs expressed in a Cebus apella fetal liver indicated that gamma 2 expression is at least 120 times greater than gamma 1 expression. Using a luciferase reporter and a transient assay system, the strengths of gamma 1 and gamma 2 promoter fragments of Cebus apella were compared in erythroid (K562) and non-erythroid (HeLa) cell lines. Due to the lack of chromatin repression in a transient expression system, the results do not fully recapitulate globin expression. However, the results suggest that sequences contained within the Cebus gamma 1 and gamma 2 proximal promoter regions (-200 to +1 bp) can direct gamma 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 that the Cebus gamma 1 promoter contains several mutations, including a proximal CCAAT box mutation (CCAAT-->CCAAc). The epsilon-gamma 1 intergenic distances in these platyrrhines (5.4 kb in Cebus apella and 6.9 kb in Callithrix jacchus) are short, supporting the inference that it was also short in the stem simian primates. The results suggest that immediately following the gamma duplication, the gamma 1 gene of the stem simians was still embryonic and the downstream gamma 2 gene was largely silent. A further inference is that once gamma 2 accumulated regulatory mutations that disrupted binding of fetal repressors, gamma 2 was expressed fetally and, through gene conversion, passed these characteristics to the gamma 1 gene. The fetal expression of gamma 1 is most evident in catarrhines (Old World monkeys and hominoids), which preferentially express the gamma 1 locus during fetal life.