Comparative gene mapping: order of loci on the X chromosome is different in mice and humans

For comparative studies we have used the somatic cell hybridization approach to regionally map genes on the mouse X chromosome. Fibroblasts from a mouse with the balanced reciprocal translocation T(XD;16B5)16H were fused with a Chinese hamster cell line (V79/380-6) deficient in activity of the enzyme hypoxanthine phosphoribosyltransferase (HPRT). Interpecific cell hybrids were initially selected for retention of the mouse translocation chromosome carrying the Hprt gene. Subsequently, hybrid clones were counterselected to force segregation of this chromosome. Selected and counterselected hybrid clones were analyzed for their chromosome content by trypsin/Giemsa banding and for expression of the mouse forms of the X-linked enzymes HPRT and alpha-galactosidase (GALA) by isoelectric focusing. The results indicate that the breakpoint on the mouse X chromosome (in band XD) has separated the genes for HPRT (Hprt) and for GALA (Ags). Hprt is proximal to the breakpoint in region Xcen-XD and Ags is distal in region XD-Xter. The gene order in the mouse (centromere-Hprt-Ags) is therefore inverted when compared to the order of the homologous loci on the long arm of the human X (centromere-GALA-HPRT).