Reversed genetics: a new approach to the elucidation of structure--function relationship

Synthetic viruses: a new opportunity to understand and prevent viral disease

Rapid progress in DNA synthesis and sequencing is spearheading the deliberate, large-scale genetic alteration of organisms. These new advances in DNA manipulation have been extended to the level of whole-genome synthesis, as evident from the synthesis of poliovirus, from the resurrection of the extinct 1918 strain of influenza virus and of human endogenous retroviruses and from the restructuring of the phage T7 genome. The largest DNA synthesized so far is the 582,970 base pair genome of Mycoplasma genitalium, although, as yet, this synthetic DNA has not been 'booted' to life. As genome synthesis is independent of a natural template, it allows modification of the structure and function of a virus's genetic information to an extent that was hitherto impossible. The common goal of this new strategy is to further our understanding of an organism's properties, particularly its pathogenic armory if it causes disease in humans, and to make use of this new information to protect from, or treat, human viral disease. Although only a few applications of virus synthesis have been described as yet, key recent findings have been the resurrection of the 1918 influenza virus and the generation of codon- and codon pair-deoptimized polioviruses.

Ten years of gene targeting: targeted mouse mutants, from vector design to phenotype analysis

Gene targeting, defined as the introduction of site-specific modifications into the genome by homologous recombination, has revolutionarized the field of mouse genetics and allowed the analysis of diverse aspects of gene function in vivo. It is now possible to engineer specific genetic alterations ranging from subtle mutations to chromosomal rearrangements and more recently, even tissue-specific inducible gene targeting with temporo-spatial control has become feasible. This review tries to recapitulate what we have learned in this extremely rapidly expanding field during the past decade. Diverse aspects of the technique will be discussed starting from basic construct design to the analysis of complex phenotypes, including recent advances on inducible expression system. Many examples from different areas of biomedical research are given to illustrate the purpose and limitations of the employed experimental approaches.

Gene clusters and the HLA system

The major human histocompatibility, or HLA system, is now known to include a number of closely linked loci controlling cell surface specificities, components of the complement system and immune response. Three loci, HLA-A, B and C, control serological determinants found on nearly all cells; a fourth locus, HLA-D, codes for determinants controlling the mixed lymphocyte culture reaction that are present only on B lymphocytes and monocytes. The products of the HLA-A, B and C loci are very similar to each other, as expected if they have arisen by duplication from a common ancestor. The HLA-D products however, show no homology with these and each in turn is different from the complement components (C2, C4 and Bf) coded for in the HLA region. If this complex genetic region has arisen by a series of duplications, one must explain how four different structural gene products can arise from a single duplicated nucleic acid sequence. Different proteins could be translated from the same nucleotide sequence through the use either of different reading frames or of different, not necessarily adjacent, parts of a sequence in different combinations. It seems likely that the general organization of the genetic material of higher organisms is into clusters of duplicated genes like the HLA region and that the numbers of such clusters, rather than the number of individual genes, reflects the genetic complexity of higher organisms.