Comment on Hirashima et al, page 711
This article describes a gene-trap expression screening leading to identification and follow-up of endothelial-specific genes during murine embryonic stem cell differentiation and embryogenesis.
Decoding of the human and murine genomes and parallel development in ingenious new techniques of molecular biology have opened the novel field of functional genomics during the last few years. Microarrays enable expression profiling of thousands of genes within a short range of time. These new methods allow random genome-based screenings that, compared with conventional methods, are less susceptible to investigator-related bias. However, elaboration of the biologic relevance of databases obtained by these methods requires a lot of subsequent laboratory work.
In this issue of Blood, Hirashima and colleagues use another very interesting approach that allows random screening for genes that are relevant for specific cellular systems; in this study, endothelial cells. They developed a protocol for gene-trap expression screening for endothelial-specific genes in murine embryonic stem (ES) cells. Using this technique, the insertion of distinct vector constructs into the genome results in more or less random targeting of murine genes in ES cells.1 Reporter gene expression then allows for follow-up of such trapped genes during the differentiation of ES cells into endothelial cells in vitro or during embryogenesis in vivo. Depending on the specific vector construct, the gene-trap technique either leads to expression of functional fusion products of the target and reporter genes or results in destruction of target genes. Thus, this technique can be used to either follow up the expression pattern of functional genes in vivo or to generate a knockout mouse of specific gene products.1
Analyzing differentiation of ES cells to endothelial cells, Hirashima et al identified 3 genes by gene-trap expression analysis that are specifically expressed during early endothelial development: Endoglin, an endothelial-specific transforming growth factor β (TGF-β) type III receptor; the transcription factor Hes1; and the p53-binding protein ASPP1 (apoptosis-stimulating protein of p53). These in vitro data were confirmed by lacZ reporter gene expression during early mouse embryogenesis. Endoglin has a well-known function in endothelial biology as well as in angiogenesis,2 and its identification is therefore a nice confirmation of the reliability of this method. The second gene, Hes1, has recently been described by other groups to be expressed by endothelial cells in vitro where it seems to be involved in the process of angiogenesis.3 The most interesting candidate, however, is ASPP1, which has been proposed to show no endothelium-specific expression pattern, at least in adult mice.4 By virtue of their elegant approach, Hirashima and colleagues now demonstrate that ASPP1 shows a specific expression in endothelial cells during early embryogenesis, in contrast to adult animals. This finding points to a novel role of this molecule during vascular development inasmuch as the role of ASPP1 in p53-dependent apoptosis is likely to be relevant for processes of vascular reconstruction and formation of new vessels.
The combination of a random genome-based screening technique with the facility of functional analysis in vivo makes gene-trap expression analysis a promising candidate for future scientific approaches. It may lead to identification not only of further endothelial-specific genes but also of decisive genes for the development of the different myeloid cell lineages.
