Although Hox gene expression has been linked to motoneuron identity, a role of these genes in development of the spinal sensory system remained undocumented. Hoxb genes are expressed at high levels in the dorsal horn of the spinal cord. Hoxb8 null mutants manifest a striking phenotype of excessive grooming and hairless lesions on the lower back. Applying local anesthesia underneath the hairless skin suppressed excessive grooming, indicating that this behavior depends on peripheral nerve activity. Functional ablation of mouse Hoxb8 also leads to attenuated response to nociceptive and thermal stimuli. Although spinal ganglia were normal, a lower postmitotic neural count was found in the dorsalmost laminae at lumbar levels around birth, leading to a smaller dorsal horn and a correspondingly narrowed projection field of nociceptive and thermoceptive afferents. The distribution of the dorsal neuronal cell types that we assayed, including neurons expressing the itch-specific gastrin-releasing peptide receptor, was disorganized in the lumbar region of the mutant. BrdU labeling experiments and gene-expression studies at stages around the birth of these neurons suggest that loss of Hoxb8 starts impairing development of the upper laminae of the lumbar spinal cord at approximately embryonic day (E)15.5. Because none of the neuronal markers used was unexpressed in the adult dorsal horn, absence of Hoxb8 does not impair neuronal differentiation. The data therefore suggest that a lower number of neurons in the upper spinal laminae and neuronal disorganization in the dorsal horn underlie the sensory defects including the excessive grooming of the Hoxb8 mutant.anteroposterior patterning ͉ mouse Hox genes ͉ spinal cord ͉ reaction to heat pain and itch

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This introductory paper gathers general considerations on the biosafety of virus-derived vectors that are used in human gene therapy and/or vaccination. The importance to assess the potential risks for human health and the environment related to the use of genetically modified organisms (GMO) in this case genetically modified viral vectors is highlighted by several examples. This environmental risk assessment is one of the requirements within the European regulatory framework covering the conduct of clinical trials using GMO. Risk assessment methodologies for the environmental risk assessment of genetically modified virus-derived vectors have been developed.

Hoxb8 mutant mice were generated by inserting the lacZ coding sequence in frame with the first exon of Hoxb8. These mice express a fusion protein with a functional beta-galactosidase activity instead of Hoxb8. Mutant embryos were analyzed for anatomical changes. The results indicate that Hoxb8 is not an indispensable regulator of A-P patterning in the forelimb, unlike suggested by our Hoxb8 gain of function experiments (Charité J, DeGraaff W, Shen S, Deschamps J. Cell 1994;78:589-601). The null mutant phenotypic traits include degeneration of the second spinal ganglion (C2), an abnormality opposite to the alteration in the gain of function transgenic mice. Subtle changes in the thoracic part of the vertebral column were observed as well. Adult homozygous mutants exhibit an abnormal clasping reflex of the limbs.

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Mouse Cdx and Hox genes presumably evolved from genes on a common ancestor cluster involved in anteroposterior patterning. Drosophila caudal (cad) is involved in specifying the posterior end of the early embryo, and is essential for patterning tissues derived from the most caudal segment, the analia. Two of the three mouse Cdx paralogues, Cdx 1 and Cdx2, are expressed early in a Hox-like manner in the three germ layers. In the nascent paraxial mesoderm, both genes are expressed in cells contributing first to the most rostral, and then to progressively more caudal parts of the vertebral column. Later, expression regresses from the anterior sclerotomes, and is only maintained for Cdx1 in the dorsal part of the somites, and for both genes in the tail bud. Cdx1 null mutants show anterior homeosis of upper cervical and thoracic vertebrae. Cdx2-null embryos die before gastrulation, and Cdx2 heterozygotes display anterior transformations of lower cervical and thoracic vertebrae. We have analysed the genetic interactions between Cdx1 and Cdx2 in compound mutants. Combining mutant alleles for both genes gives rise to anterior homeotic transformations along a more extensive length of the vertebral column than do single mutations. The most severely affected Cdx1 null/Cdx2 heterozygous mice display a posterior shift of their cranio-cervical, cervico-thoracic, thoraco-lumbar, lumbo-sacral and sacro-caudal transitions. The effects of the mutations in Cdx1 and Cdx2 were co-operative in severity, and a more extensive posterior shift of the expression of three Hox genes was observed in double mutants. The alteration in Hox expression boundaries occurred early. We conclude that both Cdx genes cooperate at early stages in instructing the vertebral progenitors all along the axis, at least in part by setting the rostral expression boundaries of Hox genes. In addition, Cdx mutants transiently exhibit alterations in the extent of Hox expression domains in the spinal cord, reminding of the strong effects of overexpressing Cdx genes on Hox gene expression in the neurectoderm. Phenotypical alterations in the peripheral nervous system were observed at mid-gestation stages. Strikingly, the altered phenotype at caudal levels included a posterior truncation of the tail, mildly affecting Cdx2 heterozygotes, but more severely affecting Cdx1/Cdx2 double heterozygotes and Cdx1 null/Cdx2 heterozygotes. Mutations in Cdx1 and Cdx2 therefore also interfere with axis elongation in a cooperative way. The function of Cdx genes in morphogenetic processes during gastrulation and tail bud extension, and their relationship with the Hox genes are discussed in the light of available data in Amphioxus, C. elegans, Drosophila and mice.