Spam1 (PH-20) mutations and sperm dysfunction in mice with the Rb(6.16) orRb(6.15) translocation

In mice bearing the Rb(6.16) or Rb(6.15) Robertsonian translocation. (Rb), sperm dysfunction associated with the Rbs has been shown to lead to transmi ssion ratio distortions (TRDs) in heterozygotes. The severity of the TRDs i s directly related to the severity in the alteration of expression of the g ene for the Sperm Adhesion Molecule 1 (Spam1), which maps to proximal mouse Chromosome 6 (Chr 6) near the translocation. junction and encodes a sperm antigen with hyaluronidase activity. Here we demonstrate that there is a si gnificantly reduced fertility in the Rb homozygotes (P < 0.001), based on l itter size; and that with the Sperm Select Penetration assay Rb-bearing spe rm have significantly decreased (P < 0.02-0.001) rates of penetration of hy aluronic acid. Catalytic kinetics studies indicate that reduced Spam1 (PH-2 0) hyaluronidase activity in the Rb(6.15) mice results from a qualitative d efect, while for Rb(6.16) with the greater TRD both a qualitative and a qua ntitative deficiency (confirmed by Western analysis) of Spam1 exist. Six po int mutations were shown to be clustered in the Spam1 hyaluronic acid-bindi ng domain in Rb(6.15). For Rb(6.16) which has a gross genomic alteration at the Spam1 locus, 11 point mutations are scattered in the 5' and 3' UTRs an d the coding region, where one leads to the replacement of a conserved resi due. Entrapment of spontaneous Spam1 mutations, owing to recombination supp ression near the Rb junctions, is proposed as the major underlying defect o f the sperm dysfunction.