Diamond Blackfan anemia: A paradigm for a ribosome-based disease

Diamond Blackfan anemia is characterized by a severe hypoplastic anemia and a heterogeneous collection of other clinical features. Approximately 25% of Diamond Blackfan anemia cases are associated with mutations in the gene encoding ribosomal protein S19. The hypothesis presented here ties together molecular and clinical features of the disease, and establishes a conceptual framework for understanding many of the unusual characteristics of a growing number of diseases linked to factors involved in ribosome synthesis. The hypothesis states that ribosomal proteins are expressed in amounts that differ relative to one another in a tissue-specific manner, and that haploinsufficiency for a particular protein may make that protein limiting for ribosome assembly in some tissues, while other tissues remain unaffected. Further, polymorphisms in factors controlling the expression of a particular ribosomal protein gene may alter its expression and expand or contract the number of tissues affected from individual to individual. Support for the hypothesis comes from the observation that promoters in ribosomal protein genes exhibit little conservation and transcription profiling indicates that the absolute amounts of mRNAs for individual ribosomal proteins can vary dramatically relative to one another. Balanced expression of ribosomal proteins is achieved post-translationally, where excess proteins not assembled into ribosomal subunits are often rapidly degraded. The number of ribosomes per cell is therefore determined by the factors that limit assembly. In principle, any essential ribosomal protein could become limiting for assembly if its level of expression falls below a critical threshold. Whether an inactivating mutation in ribosomal protein gene would affect protein synthetic capacity of a tissue would depend on the ratio of the ribosomal protein relative to other ribosomal proteins in that tissue. If the ratio were high, the tissue may not be affected as the level of functional protein may not fall to a point where it becomes limiting for subunit assembly. In contrast, if the ratio were low, an inactivating mutation could make the protein limiting for subunit assembly resulting in a clinical phenotype. Polymorphisms in the myriad of cis- and trans-acting factors, which govern the expression of ribosomal proteins in response to developmental and physiological signals, could act to increase or decrease ribosomal protein expression and thereby impact the profile and severity of clinical phenotypes. Therefore, these factors represent targets for the development of new therapies to treat Diamond Blackfan anemia and other ribosome based diseases.

Biochemical and behavioral characterization of novel methylphenidate analogs

As part of a project to develop treatment agents for cocaine abuse, (+/-)-threo-methylphenidate (TMP) and 11 analogs were characterized biochemically and behaviorally to assess their potential as anti-cocaine medications. The compounds contained aryl and/or nitrogen substitutions, and/or replacement of the ester function by an alcohol or ether. All of the analogs, except for the N-methyl-substituted compounds, showed increased inhibitory potency against (3)H-(-)-2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane 1,5-naphthalenedisulfonate ([(3)H]WIN 35,428) ([(3)H]WIN) binding to the dopamine transporter, compared with TMP. In general, parallel results were obtained for inhibition of [(3)H]dopamine ([(3)H]DA) uptake. Although compounds with N-substitutions were proportionally less potent at blocking DA uptake than WIN binding (compared with the unsubstituted compounds), one such compound that was 6-fold more potent against [(3)H]WIN binding than [(3)H]DA uptake did not attenuate inhibition by cocaine of synaptosomal [(3)H]DA transport. The compounds were significantly less potent in displacing [(3)H]citalopram binding from the serotonin transporter. In cocaine discrimination studies in rats, all but two of the analogs (both N-substituted) completely generalized with the cocaine stimulus. Robust positive correlations were observed between potency in the drug discrimination assay and activity at the dopamine transporter, but not the serotonin transporter. When tested for their ability to alter cocaine discrimination, four of the analogs (three of which had N-substitutions and shallow dose-response curves as cocaine substitutes) actually enhanced cocaine discrimination, often at combined doses of cocaine and test compound that were inactive when given separately. Taken together, the results suggest that TMP analogs may have potential as substitution therapies for the treatment of cocaine abuse.