RESUMEN
Bowen-Conradi syndrome (BCS) is a ribosomopathy characterized by severe developmental delay and growth failure that typically leads to death by one year of age. It is caused by a c.257A>G, p.D86G substitution in the ribosomal biogenesis protein, Essential for Mitotic Growth 1 (EMG1). We generated a knock-in of the D86G substitution in mice to characterize the effects of EMG1 deficiency, particularly in the brain, where EMG1 expression is high. Embryos homozygous for the mutation in Emg1 were small for gestational age with neural tube defects, and died between embryonic days 8.5 and 12.5. These embryos exhibited dramatically reduced cell proliferation, which we also detected in autopsy brain tissue and bone marrow of BCS patients, consistent with a requirement for high levels of EMG1 in tissues with rapid cell proliferation. In fibroblasts derived from the BCS mouse embryos, we detected a high proportion of binucleated cells, indicating that a mitotic defect underlies the growth arrest in BCS. These studies add to growing evidence of a link between ribosome biogenesis, mitotic progression, and brain development that is currently unexplored.
Asunto(s)
Proliferación Celular/genética , Retardo del Crecimiento Fetal/genética , Mitosis/genética , Mutación Missense , Proteínas Nucleares/genética , Trastornos Psicomotores/genética , Animales , Apoptosis/genética , Encéfalo/embriología , Encéfalo/crecimiento & desarrollo , Encéfalo/metabolismo , Nucléolo Celular/metabolismo , Células Cultivadas , Niño , Embrión de Mamíferos/citología , Femenino , Retardo del Crecimiento Fetal/patología , Fibroblastos/metabolismo , Técnica del Anticuerpo Fluorescente , Regulación del Desarrollo de la Expresión Génica , Humanos , Immunoblotting , Recién Nacido , Masculino , Ratones Endogámicos C57BL , Ratones Noqueados , Ratones Transgénicos , Persona de Mediana Edad , Proteínas Nucleares/metabolismo , Trastornos Psicomotores/patología , Ribosomas/genética , Ribosomas/metabolismoRESUMEN
Bowen-Conradi syndrome (BCS) is a lethal autosomal recessive disorder caused by a D86G substitution in the protein, Essential for Mitotic Growth 1 (EMG1). EMG1 is essential for 18S rRNA maturation and 40S ribosome biogenesis in yeast, but no studies of its role in ribosome biogenesis have been done in mammals. To assess the effect of the EMG1 mutation on cell growth and ribosomal biogenesis in humans, we employed BCS patient cells. The D86G substitution did not interfere with EMG1 nucleolar localization. In BCS patient lymphoblasts, cells accumulated in G2/M, resulting in reduced proliferation rates; however, patient fibroblasts showed normal proliferation. The rate of 18S rRNA processing was consistently delayed in patient cells, although this did not lead to a difference in the levels of 40S ribosomes, or a change in protein synthesis rates. These results demonstrate that as in yeast, EMG1 in mammals has a role in ribosome biogenesis. The obvious phenotype in lymphoblasts compared to fibroblasts suggests a greater need for EMG1 in rapidly dividing cells. Tissue-specific effects have been seen in other ribosomal biogenesis disorders, and it seems likely that the impact of EMG1 deficiency would be larger in the rapidly proliferating cells of the developing embryo.