A Novel Gene Essential for the Development of Single Positive Thymocytes

A critical step during intrathymic T-cell development is the transition of CD4+ CD8+ double-positive (DP) cells to the major histocompatibility complex class I (MHC-I)-restricted CD4− CD8+ and MHC-II-restricted CD4+ CD8− single-positive (SP) cell stage. Here, we identify a novel gene that is essential for this process. Through the T-cell phenotype-based screening ofN -ethyl-N -nitrosourea (ENU)-induced mutant mice, we established a mouse line in which numbers of CD4 and CD8 SP thymocytes as well as peripheral CD4 and CD8 T cells were dramatically reduced. Using linkage analysis and DNA sequencing, we identified a missense point mutation in a gene,E430004N04Rik (also known asthemis ), that does not belong to any known gene family. This orphan gene is expressed specifically in DP and SP thymocytes and peripheral T cells, whereas in mutant thymocytes the levels of protein encoded by this gene were drastically reduced. We generatedE430004N04Rik -deficient mice, and their phenotype was virtually identical to that of the ENU mutant mice, thereby confirming that this gene is essential for the development of SP thymocytes.