Mutations affecting thymus organogenesis in Medaka, Oryzias latipes

Abstract

The thymus is an organ for T lymphocyte maturation and is indispensable for the establishment of a highly developed immune system in vertebrates. In order to genetically dissect thymus organogenesis, we carried out a large-scale mutagenesis screening for Medaka mutations affecting recombination activating gene 1 (rag1) expression in the developing thymus. We identified 24 mutations, defining at least 13 genes, which led to a marked reduction of rag1 expression in the thymus. As thymus development depends on pharyngeal arches, we classified those mutations into three classes according to the defects in the pharyngeal arches. Class 1 mutants had no or slight morphological abnormalities in the pharyngeal arches, implying that the mutations may include defects in such thymus-specific events as lymphocyte development and thymic epithelial cell maturation. Class 2 mutants had abnormally shaped pharyngeal arches. Class 3 mutants showed severely attenuated pharyngeal arch development. In Class 2 and Class 3 mutants, the defects in thymus development may be due to abnormal pharyngeal arch development. Those mutations are expected to be useful for identifying the molecular mechanisms underlying thymus organogenesis.

Ethylnitrosourea‐induced thymus‐defective mutants identify roles ofKIAA1440, TRRAP, andSKIV2L2in teleost organ development

“…Thymic rag1 expression in medaka is located bilaterally in the pharyngeal region and is detectable as early as stage 30 (3.5 dpf) [14,22,26]. We found that thymic rag1 expression was completely undetectable in every embryo of awb and awc mutants, whereas it was undetectable only in approximately half of the embryos of hjc embryos and was apparently reduced but slightly detectable in the other half [14] (Fig. 1A).…”

“…Thymus development is differently defective in awabancha, awacha, and houjicha mutants We previously established awabancha (awb), awacha (awc), and houjicha (hjc) lines of medaka by screening for ethylnitrosoureainduced recessive mutations that affected immature-lymphocytespecific rag1 expression in the developing thymus [14]. Thymic rag1 expression in medaka is located bilaterally in the pharyngeal region and is detectable as early as stage 30 (3.5 dpf) [14,22,26].…”

“…For example, mutations in AIRE, which is specifically expressed by a fraction of thymic medullary epithelial cells, cause autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (OMIM 607358) [13]. Nevertheless, the molecular pathways that control thymus development have not been fully elucidated.We previously carried out a large-scale screening for mutations induced by the point mutagen ethylnitrosourea using medaka, Oryzias latipes, to identify mutations affecting thymus organogenesis [14,15]. In order to identify genes that control vertebrate-specific events including thymus development, forward genetic screening for mutations in small teleosts, including zebrafish and medaka, is useful because these animals lay many eggs and their transparent embryos develop in the eggs without placenta [16,17].…”

“…Indeed, T-cell-mediated cellular immune responses, such as allograft rejection, were studied using medaka [22][23][24]. We screened 538 F2 families that presumably covered approximately 60% of medaka genome for recessive mutations affecting immature-lymphocyte-specific recombination activating 1 (rag1) expression in developing thymus and established 22 thymus-defective lines [14]. We recently reported the identification of one of these genes, WDR55, which is responsible for the athymic phenotype and encodes a nucleolar modulator of ribosomal RNA synthesis, cell cycle progression, and the development of various organs including the thymus in medaka and zebrafish [25].…”

“…We previously carried out a large-scale screening for mutations induced by the point mutagen ethylnitrosourea using medaka, Oryzias latipes, to identify mutations affecting thymus organogenesis [14,15]. In order to identify genes that control vertebrate-specific events including thymus development, forward genetic screening for mutations in small teleosts, including zebrafish and medaka, is useful because these animals lay many eggs and their transparent embryos develop in the eggs without placenta [16,17].…”