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O-mannosylation is an important protein modification in eukaryotes that is initiated by an evolutionarily conserved family of protein O -mannosyltransferases. The first mammalian protein O -mannosyltransferase gene described was the human POMT1 . Mutations in the hPOMT1 gene are responsible for Walker–Warburg syndrome (WWS), a severe recessive congenital muscular dystrophy associated with defects in neuronal migration that produce complex brain and eye abnormalities. During embryogenesis, the murine Pomt1 gene is prominently expressed in the neural tube, the developing eye, and the mesenchyme. These sites of expression correlate with those in which the main tissue alterations are observed in WWS patients. We have inactivated a Pomt1 allele by gene targeting in embryonic stem cells and produced chimeras transmitting the defect allele to offspring. Although heterozygous mice were viable and fertile, the total absence of Pomt1 – / – pups in the progeny of heterozygous intercrosses indicated that this genotype is embryonic lethal. An analysis of the mutant phenotype revealed that homozygous Pomt1 – / – mice suffer developmental arrest around embryonic day (E) 7.5 and die between E7.5 and E9.5. The Pomt1 – / – embryos present defects in the formation of Reichert's membrane, the first basement membrane to form in the embryo. The failure of this membrane to form appears to be the result of abnormal glycosylation and maturation of dystroglycan that may impair recruitment of laminin, a structural component required for the formation of Reichert's membrane in rodents. The targeted disruption of mPomt1 represents an example of an engineered deletion of a known glycosyltransferase involved in O -mannosyl glycan synthesis.

Williams-Beuren syndrome (WBS) is a neurodevelopmental disorder with multisystemic manifestations caused by heterozygosity for a partial deletion of chromosome band 7q11.23. The breakpoints cluster within regions located approximately 1 cM either side of the elastin (ELN) locus. We have characterized a duplicated region near the common deletion breakpoints, which includes a transcribed gene. The centromeric (C) and telomeric (T) copies are almost identical in the duplicated 3[prime] portions but diverge at their 5[prime]-ends. C-specific 4.3 kb mRNA and T-specific 5.4 kb mRNA are widely expressed in embryonic and adult tissues. The telomeric gene gives rise to several alternatively spliced forms and is deleted in all WBS individuals who have documented ELN deletions. Database searches revealed that this gene encodes BAP-135, a protein phosphorylated by Bruton's tyrosine kinase in B cells, as well as the multifunctional transcription factor TFII-I, hence the gene name GTF2I. The centromeric gene is not deleted in WBS and appears to be a partially truncated expressed pseudogene with no protein product (gene name GTF2IP1). Both loci map to different genomic clone contigs that also contain other deleted and non-deleted loci. A probe from the shared region recognizes a>3 Mb Not I junction fragment that is unique to individuals with the WBS deletion. Therefore, the duplicated region containing GTF2I and GTF2IP1 respectively is located close to the deletion breakpoints and may predispose to unequal meiotic recombination between chromosome 7 homologs and/or to intrachromosomal rearrangements. Hemizygosity for GTF2I may also contribute to the WBS phenotype.

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