The spontaneous mouse mutation Bst (belly spot and tail) causes midventral spotting, vertebral anomalies (tail kinks), preaxial polydactyly, a reduction in retinal ganglion cell (RGC) number, optic nerve aplasia, colobomata, and subretinal neovascularization (Rice et al. 1997; Smith et al. 2000). Bst homozygotes die before implantation. We have identified Bst as a deletion within Rpl24, the gene encoding the large subunit ribosomal protein L24. The mutation causes aberrant splicing in 80% of transcripts, leading to truncation of the L24 polypeptide. Mouse BAC and human cDNA transgenes correct the Bst/+ phenotype. Almost nothing is known about ribosomal protein (RP) gene mutations in vertebrates. In Drosophila, numerous ribosomal gene mutations, termed Minutes, are dispersed across the genome (Lambertsson 1998). They are homozygous lethal and have similar heterozygous phenotypes consisting of smaller, thinner bristles and profoundly delayed development (Schultz 1929). Their effects are not additive when combined. Minutes were instrumental in pioneering studies that defined concepts of cell autonomy, clonal lineage and developmental compartmentation, as Minute cells compete poorly with wild type clones in somatic mosaics (Simpson and Morata 1981). In humans, Diamond-Blackfan anemia (hereditary red blood cell aplasia with variable congenital anomalies) is caused by mutations in RPS19, which encodes a small subunit riboprotein (Willig et al. 1999). In view of the universal cellular requirement for ribosomes, the tissue specificity of mouse Rpl24 and human RPS19 phenotypes is puzzling. Our findings suggest that riboprotein defects may underlie common human malformation syndromes that have a suspected genetic etiology but no single target locus (Lalani et al. 2003). Fibroblasts from Bst/+ embryos grow slower than wild-type MEFs, remain longer in G1 phase, and exhibit decreased rates of protein synthesis. Although complete phenotypic surveys are lacking, the majority of tissues compensate for this cellular growth defect, so that organogenesis proceeds and Bst/+ mice approach normal adult size. While extraribosomal functions for L24 are possible, unique Bst phenotypes can also be explained by a dyschronic mechanism, in tissues such as the retina where proliferation and the sequence of cell fate determination are relatively uncoupled. In this proposal, we aim to: (1) complete characterization of Bst molecular, cellular and tissue developmental effects; (2) investigate the selective growth disadvantage of Rpl24 deficient cells in ROSA26 lacZ <-> Bst/+ blastocyst chimeras and somatic mosaics, as predicted by classical Minute studies; and (3) define the spectrum of riboprotein phenotypes by comparing Bst/+ mice to selected knockout mice with other RP mutations, including several that are available as targeted ES cells.
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