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 of N-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 as themis), 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 generated E430004N04Rik-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.