HGNC Approved Gene Symbol: PIGY
Cytogenetic location: 4q22.1 Genomic coordinates (GRCh38) : 4:88,520,998-88,523,776 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 4q22.1 | Hyperphosphatasia with impaired intellectual development syndrome 6 | 616809 | Autosomal recessive | 3 |
GPI acts as a membrane anchor for numerous cell surface proteins. It is initially synthesized by the transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to generate GlcNAc-phosphatidylinositol (GlcNAc-PI). This reaction is catalyzed by the enzymatic complex of GPI-acetylglucosaminyltransferase (GPI-GnT), which in mammalian cells has been found to consist of at least 7 proteins, including PIGY (Murakami et al., 2005).
For information on the PIG gene family and the roles of PIG proteins in GPI biosynthesis, see PIGA (311770).
Murakami et al. (2005) determined that Daudi Burkitt lymphoma cells, which are defective in GPI-GnT activity, could not be rescued by any of the 6 known proteins contained in GPI-GnT, suggesting that an additional protein was required for GPI-GnT activity. Using affinity purification followed by SDS-PAGE analysis and peptide sequencing, the authors identified and cloned the gene encoding this protein, phosphatidylinositol glycan class Y (PIGY). PIGY, a 71-amino acid protein with a molecular mass of 6.5 kD, contains 2 putative transmembrane domains. PIGY has homologs in mouse, rice, and yeast, with amino acid identities of 79%, 25%, and 22%, respectively. Permeabilization experiments using tagged PIGY showed that the N terminus of PIGY is cytoplasmic, and Murakami et al. (2005) predicted the C terminus to be cytoplasmic as well based on the hydrophobicity profile. Transfection of PIGY into Daudi cells restored GPI-GnT activity, and RT-PCR demonstrated that Daudi cells contained no PIGY transcript. Northern blot analysis, RT-PCR, and database analysis showed that PIGY is translated from a bicistronic mRNA that encodes both PIGY and an upstream coding sequence for PreY (PYURF; 619956). Expression of mutagenized constructs in HeLa cells showed that separate PIGY translation is initiated by leaky scanning at the PreY initiation site.
Using genomic sequence analysis, Murakami et al. (2005) found that PIGY is encoded by a bicistronic mRNA that contains 2 exons. PIGY is contained within exon 2, and the gene PreY is contained within exon 1 and the 5-prime portion of exon 2.
Gross (2016) mapped the PIGY gene to chromosome 4q22.1 based on an alignment of the PIGY sequence (GenBank BC007876) with the genomic sequence (GRCh38).
Using coimmunoprecipitation experiments, Murakami et al. (2005) showed that PIGY was not required for the other members of the GPI-GnT complex to associate, and that PIGY associated strongly with PIGA and weakly with the other GPI-GnT complex proteins. PreY was not required for biosynthesis of GPI.
In 2 sisters, born of Australian parents, with hyperphosphatasia with impaired intellectual development syndrome-6 (HPMRS6; 616809), Ilkovski et al. (2015) identified a homozygous missense mutation in the PIGY gene (L46P; 610662.0001). The mutation was identified by whole-exome sequencing and segregated with the disorder in the family. Patient fibroblasts showed a 20 to 50% reduction in surface expression of GPI-anchored proteins CD55 (125240) and CD59 (107271), consistent with impaired PIGY function. Two Pakistani sibs, born of consanguineous parents, with a mild form of HPMRS6 were found to have a homozygous mutation in the promoter region of the PIGY gene (610662.0002). Patient blood cells showed significantly decreased PIGY expression (6-10% of controls). Patient fibroblasts were not available for study of GPI-anchored proteins, but granulocytes showed normal CD16 surface expression (see 146740), indicating that PIGY levels in some tissues are sufficient for normal GPI synthesis. These findings were consistent with the less severe phenotype in these patients.
In 2 sisters, born of Australian parents, with hyperphosphatasia with impaired intellectual development syndrome-6 (HPMRS6; 616809), Ilkovski et al. (2015) identified a homozygous c.137T-C transition (c.137T-C, NM_001042616.1) in the PIGY gene, resulting in a leu46-to-pro (L46P) substitution at a conserved residue in the second putative transmembrane domain. The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family and was not found in the 1000 Genomes Project, Exome Variant Server, or ExAC databases. Autozygosity mapping suggested that the parents may be distantly related. Patient fibroblasts showed a 20 to 50% reduction in surface expression of GPI-anchored proteins CD55 (125240) and CD59 (107271). Transfection of the mutation into PIGY-null cells could only partially restore CD55 and CD59 expression under a strong promoter. Western blot analysis showed decreased expression of the mutant protein, consistent with reduced stability. The findings suggested that the mutation disrupts GPI biosynthesis or interferes with GPI anchoring capacity.
In 2 sibs, born of consanguineous parents of Pakistani descent, with a mild variant of hyperphosphatasia with impaired intellectual development syndrome-6 (HPMRS6; 616809), Ilkovski et al. (2015) identified a homozygous c.-540G-A transition (c.-540G-A, NM_001042616.1) in a conserved region of the PIGY 5-prime untranslated region with putative disruption of a consensus SP1 (189906) transcription factor binding site. The mutation, which was found by a combination of autozygosity mapping and whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. It was not found in the 1000 Genomes Project or Exome Variant Server databases, in 274 in-house genomes of mixed ancestry, or in 108 Punjabi individuals. Patient blood cells showed significantly decreased PIGY expression (6-10% of controls). Patient fibroblasts were not available for study of GPI-anchored proteins, but granulocytes showed normal CD16 surface expression (see 146740), indicating that PIGY levels in some tissues are sufficient for normal GPI synthesis. These findings were consistent with the less severe phenotype in these patients compared to other patients (see 610662.0001). Alkaline phosphatase levels in the Pakistani sibs were normal.
Gross, M. B. Personal Communication. Baltimore, Md. 2/16/2016.
Ilkovski, B., Pagnamenta, A. T., O'Grady, G. L., Kinoshita, T., Howard, M. F., Lek, M., Thomas, B., Turner, A., Christodoulou, J., Sillence, D., Knight, S. J. L., and 13 others. Mutations in PIGY: expanding the phenotype of inherited glycosylphosphatidylinositol deficiencies. Hum. Molec. Genet. 24: 6146-6159, 2015. [PubMed: 26293662] [Full Text: https://doi.org/10.1093/hmg/ddv331]
Murakami, Y., Siripanyaphinyo, U., Hong, Y., Tashima, Y., Maeda, Y., Kinoshita, T. The initial enzyme for glycosylphosphatidylinositol biosynthesis requires PIG-Y, a seventh component. Molec. Biol. Cell 16: 5236-5246, 2005. [PubMed: 16162815] [Full Text: https://doi.org/10.1091/mbc.e05-08-0743]