The normal phenotype of Pmm1-deficient mice suggests that Pmm1 is not essential for normal mouse development.

Phosphomannomutases (PMMs) are crucial for the glycosylation of glycoproteins. In humans, two highly conserved PMMs exist: PMM1 and PMM2. In vitro both enzymes are able to convert mannose-6-phosphate (mannose-6-P) into mannose-1-P, the key starting compound for glycan biosynthesis. However, only mutations causing a deficiency in PMM2 cause hypoglycosylation, leading to the most frequent type of the congenital disorders of glycosylation (CDG): CDG-Ia. PMM1 is as yet not associated with any disease, and its physiological role has remained unclear. We generated a mouse deficient in Pmm1 activity and documented the expression pattern of murine Pmm1 to unravel its biological role. The expression pattern suggested an involvement of Pmm1 in (neural) development and endocrine regulation. Surprisingly, Pmm1 knockout mice were viable, developed normally, and did not reveal any obvious phenotypic alteration up to adulthood. The macroscopic and microscopic anatomy of all major organs, as well as animal behavior, appeared to be normal. Likewise, lectin histochemistry did not demonstrate an altered glycosylation pattern in tissues. It is especially striking that Pmm1, despite an almost complete overlap of its expression with Pmm2, e.g., in the developing brain, is apparently unable to compensate for deficient Pmm2 activity in CDG-Ia patients. Together, these data point to a (developmental) function independent of mannose-1-P synthesis, whereby the normal knockout phenotype, despite the stringent conservation in phylogeny, could be explained by a critical function under as-yet-unidentified challenge conditions.

Identification and localization of two mouse phosphomannomutase genes, Pmm1 and Pmm2.

Phosphomannomutases catalyze the reversible conversion of mannose 6-phosphate to mannose 1-phosphate. In humans, two different isozymes have recently been identified, PMM1 and PMM2. We have previously shown that mutations in the PMM2 gene cause the most frequent type of the congenital disorders of glycosylation, CDG-Ia. Here, we present data on the two mouse orthologous genes, Pmm1 and Pmm2. The chromosomal localization of the two mouse genes has been determined. We also present the gene structure and the exon-intron organization of Pmm1 and Pmm2. Pmm1 maps to mouse chromosome 15, Pmm2 to chromosome 16. These chromosomal regions are syntenic with regions on human chromosomes 22 and 16, respectively. The Pmm1 gene is composed of eight exons and spans approximately 9.5 kb. The genomic structure is extremely well conserved between the human and mouse gene. The Pmm2 gene consists of eight exons and spans a larger genomic region ( approximately 20 kb). An alignment of the human and mouse protein sequences confirms the conservation among this family of phosphomannomutases. The two mouse genes are expressed in many tissues, but the expression pattern is slightly different between Pmm1 and Pmm2. The most striking difference is the high expression of Pmm1 in brain tissue, whereas Pmm2 is only weakly expressed in this tissue.

Effect of mutations found in carbohydrate-deficient glycoprotein syndrome type IA on the activity of phosphomannomutase 2.

Seven mutant forms of human phosphomannomutase 2 were produced in Escherichia coli and purified. These mutants had a Vmax of 0.2-50% of the wild enzyme and were unstable. The least active protein (R141H) bears a very frequent mutation, which has never been found in the homozygous state whereas the second least active protein (D188G) corresponds to a mutation associated with a particularly severe phenotype. We conclude that total lack of phosphomannomutase 2 is incompatible with life. Another conclusion is that the elevated residual phosphomannomutase activity found in fibroblasts of some patients is contributed by their mutated phosphomannomutase 2.

Comparative analysis of the phosphomannomutase genes PMM1, PMM2 and PMM2psi: the sequence variation in the processed pseudogene is a reflection of the mutations found in the functional gene.

The search for the carbohydrate-deficient glycoprotein syndrome type I (CDG1) gene has revealed the existence of a family of phosphomannomutase (PMM) genes in humans. Two expressed PMM genes, PMM1 and PMM2 , are located on chromosome bands 22q13 and 16p13, respectively, and a processed pseudogene PMM2 psi is located on chromosome 18p. Mutations in PMM2 are the cause of CDG type IA whereas no disorder has been associated with defects in PMM1 as yet. Here, we describe the genomic organization of these paralogous genes. There is a 65% identity of the coding sequence, and all intron/exon boundaries have been conserved. The processed pseudogene is more closely related to PMM2 . Remarkably, several base substitutions in PMM2 that are associated with disease are also present at the corresponding positions in the pseudogene. Thus, mutations that occur at a slow rate in the active gene in the population have also accumulated in the pseudogene.