Codanin-1, the Protein Encoded by the Gene Mutated in Congenital Dyserythropoietic Anemia Type I (CDAN1), Is Cell Cycle Regulated

Congenital dyserythropoietic anemia (CDA) type I is an inherited autosomal recessive macrocytic anemia associated with ineffective erythropoiesis and the development of secondary hemochromatosis. Distinct erythroid precursors with inter-nuclear chromatin bridges and spongy heterochromatin are pathognomonic for the disease. The mutated gene (CDAN1) encodes a ubiquitously expressed protein of unknown function, codanin-1. Based on the morphological features of CDA type I erythroblasts and the preliminary data on the Drosophila homolog, dlt, which was found to be required for cell survival and cell cycle progression, we investigated the location and the behavior of codanin-1 during the cell cycle. Using immunofluorescence and immune electron microscopy, we localized codanin-1 to the heterochromatin in interphase cells. During the cell cycle, high levels of codanin-1 were observed in S phase. At mitosis, codanin-1 underwent phosphorylation, which coincided with exclusion from condensed chromosomes. The proximal CDAN1 gene promoter region, never before characterized, was found to contain 5 putative E2F1 binding sites. E2F transcription factors are the main regulators of G1/S transition. Cotransfection of an E2F1 expression plasmid increased luciferase activity, confirming that E2F1 activates the transcription of CDAN1, and chromatin immunoprecipitation identified the codanin-1 promoter as a direct target of E2F1. Taken together, these data suggest that codanin-1 is a cell cycle-regulated protein active in S-phase. Based on the localization of codanin-1 to the heterochromatin and the spongy appearance of heterochromatin in CDA I, we suggest that codanin-1 may be involved in heterochromatin organization during DNA replication. This represents the first work towards understanding the function of the proteins involved in CDAs.