The zebrafish is amenable to forward genetic screens on a scale that is infeasible in any other vertebrate models that have the same low cost and space requirements (
Patton and Zon, 2001). The embryos can survive without red blood cells by passive diffusion of oxygen for the first week, allowing the identification of mutations that would be embryonic lethal in mice. Screens using the chemical mutagen ENU (N-ethyl-N-nitrosourea) or retroviral mutagenesis have successfully yielded a plethora of blood mutants (
Amsterdam et al., 2004;
Ransom et al., 1996;
Weinstein et al., 1996). These mutants have helped elucidate the genetic pathways that regulate HSCs as well as different hematopoietic lineages, and many of them are also excellent models of human blood diseases (
Carradice and Lieschke, 2008;
North and Zon, 2003;
Shafizadeh and Paw, 2004). In some instances, zebrafish mutants have revealed genes that have later been shown to be involved in human disease. For example, cloning of the hypochromic anemic mutant
weissherbst (
weh) identified a previously unknown iron transporter,
ferroportin1 (
Donovan et al., 2000).
ferroportin1 is required for iron transport from the yolk store to the circulation. Subsequent to the zebrafish findings, mutations in this gene were found in patients with type IV hemochromatosis (iron overload) (
Gordeuk et al., 2003;
Montosi et al., 2001).
Genetic modifier screens that are routinely used in invertebrates such as flies and worms are effective for identifying genes that suppress or enhance the function of the gene of interest, and thus have the potential to reveal new therapeutic targets. The first zebrafish suppressor screen was recently carried out in
moonshine mutants, which are deficient for
transcriptional intermediary factor 1-γ (
tif1γ; also known as
trim33) (
Bai et al., 2010). These embryos have severely impaired erythropoiesis because they fail to appropriately express Gata1, a crucial factor for erythropoiesis (
Ransom et al., 2004). Tif1γ also promotes erythropoiesis in mammals (
He et al., 2006), but its molecular mechanism remained elusive. Bai et al. created a viable homozygous
tif1γ−/− line by rescuing zebrafish by using a green fluorescent protein (GFP)-labeled wild-type
tif1γ transgene, and subsequently performed a haploid genetic suppressor screen (see ). The screen identified that mutation of the
cdc73 gene [which encodes a component of the PAF (Pol-II-associated factor) complex], restores erythropoiesis in
tif1γ−/− embryos. Knocking down the expression of other PAF subunits or components of the 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole (DRB) sensitivity-inducing factor (DSIF) complex restored red blood cells to wild-type levels in
tif1γ−/− embryos. Mechanistically, PAF and DSIF stall RNA polymerase II (Pol II) at target gene promoters.
tif1γ recruits transcription elongation factors such as positive transcription elongation factor b (p-TEFb) and facilitates chromatin transcription (FACT) complex to erythroid genes to release the paused Pol II, thereby promoting transcription. Importantly,
tif1γ-regulated elongation is conserved in human cells. The suppressor screen thus uncovered an unexpected role of
tif1γ in transcriptional elongation during erythroid cell differentiation.