Comparative analysis of the phosphomannomutase genes PMM1, PMM2 and PMM2y: 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