A KINASE-NEGATIVE MUTATION OF DNA-PKCS IN EQUINE SCID RESULTS IN DEFECTIVE CODING AND SIGNAL JOINT FORMATION

The equine SCID defect is more severe than its murine counterpart in t hat SCID foals are incapable of forming either coding or signal joints , whereas SCID mice manifest normal signal joint formation. To determi ne the basis of this difference and whether DNA-dependent kinase, cata lytic subunit (DNA-PKCS), is involved in signal joint formation, equin e DNA-PKCS transcripts were cloned and sequenced from normal and SCID cell lines, In the mutant allele, a frame-shift mutation truncates the protein N terminal of the domain with homology to the phosphatidylino sitol 3-kinase family resulting in complete absence of full length DNA -PKCS and accounting for the kinase-negative phenotype of these cells; the mutation in SCID mice allows for some DNA-PKCS expression. The di fference in DNA-PKCS expression in SCID mice and foals explains the mo re severe phenotype of equine SCID, and definition of DNA-PKCS as the defect in equine SCID demonstrates that DNA-PKCS is required for both coding and signal joint formation.