A 4-Mb BAC/PAC contig and complete genomic structure of the GPC5/GPC6 gene cluster on chromosome 13q32.

The glypicans compose a family of glycosylphosphatidylinositol-anchored heparan sulfate proteoglycans that may play a role in the control of cell division and growth regulation. So far, six members (GPC1-6) of this family are known in vertebrates. We report the construction of a high-resolution 4 Mb sequence-ready BAC/PAC contig of the GPC5/GPC6 gene cluster on chromosome region 13q32. The contig indicates that, like the GPC3/GPC4 genes on Xq26, GPC5 and GPC6 are arranged in tandem array. Both GPC5 and GPC6 are very large genes, with sizes well over 1 Mb. With a size of approximately 2 Mb, GPC5 would be the second largest human gene identified to date. Comparison of the long range gene organisation on 13q and Xq, suggests that these chromosomes share several regions of homology. Mutations and deletions affecting GPC3 are associated with the Simpson-Golabi-Behmel overgrowth syndrome. Mutational analysis of GPC5 and GPC6 in 19 patients with somatic overgrowth failed to reveal pathologic mutations in either of these genes, but identified several coding region polymorphisms.

Mutational analysis of the GPC3/GPC4 glypican gene cluster on Xq26 in patients with Simpson-Golabi-Behmel syndrome: identification of loss-of-function mutations in the GPC3 gene.

Simpson-Golabi-Behmel syndrome (SGBS) is an X-linked syndrome characterized by pre- and postnatal overgrowth (gigantism), which clinically resembles the autosomal Beckwith-Wiedemann syndrome (BWS). Deletions and translocations involving the glypican-3 gene ( GPC3 ) have been shown to be associated with SGBS. Occasionally, these deletions also include the glypican-4 gene ( GPC4 ). Glypicans are heparan sulfate proteoglycans which have a role in the control of cell growth and cell division. We have examined the mutational status of the GPC3 and GPC4 genes in one patient with Perlman syndrome, three patients with overgrowth without syndrome diagnosis, ten unrelated SGBS-patients and 11 BWS patients. We identified one SGBS patient with a deletion of a GPC3 exon. Six SGBS patients showed point mutations in GPC3. One frameshift, three nonsense, and one splice mutation predict a loss-of-function of the glypican-3 protein. One missense mutation, W296R, changes an amino acid that is conserved in all glypicans identified so far. A GPC3 protein that reproduces this mutation is poorly processed and fails to increase the cell surface expression of heparan sulfate, suggesting that this missense mutation is also a loss-of-function mutation. In three SGBS patients and in all non-SGBS patients, no mutations could be identified. We found three single nucleotide polymorphisms in the GPC4 gene but no evidence for loss-of-function mutations in GPC4 associated with SGBS.