Module 5 Β· Diseases of the Ribosome
Ribosomopathies
Mutations in ribosomal proteins or biogenesis factors cause a surprising class of human diseases: tissue-specific developmental disorders in what looks like a universal housekeeping apparatus. Why should a defect in a ribosome cause anaemia, pancreatic insufficiency, and cancer predisposition, rather than uniform loss of translation everywhere? The answer is the emerging story of ribosome specialisation (ModuleΒ 6). This module surveys the canonical ribosomopathies.
1. Diamond-Blackfan Anaemia (DBA)
A congenital pure red-cell aplasia, typically presenting in infancy. ~70% of DBA cases carry heterozygous mutations in a ribosomal protein gene β most commonly RPS19 (~25% of cases) and RPL5, RPL11. Haploinsufficiency is the mechanism. Patients have ~10β15% lifetime risk of AML, MDS, or solid tumours β the paradoxical combination of cytopenia and cancer predisposition.
The erythroid specificity is not fully understood. Leading hypothesis: erythroid precursors have enormous translation demand (2Γ108 haemoglobin molecules per reticulocyte), and their GATA1 transcript in particular has a 5β² UTR whose translation is ribosome-concentration-sensitive. Reduced ribosome output limits GATA1 first, collapsing erythropoiesis while sparing other lineages. Therapeutic: steroids (~40% response), transfusion, eltrombopag (TPO agonist, 2020 approval for DBA), bone-marrow transplantation.
2. 5q- Syndrome (Deletion MDS)
A subtype of myelodysplastic syndrome characterised by deletion of the long arm of chromosome 5, including the RPS14gene. RPS14 haploinsufficiency mimics DBA biology in an acquired context. Patients have macrocytic anaemia, hypolobated megakaryocytes, and a characteristic response to lenalidomide β which selectively kills 5q-deleted clones by enhancing CK1Ξ± (CSNK1A1) degradation, delivering synthetic lethality with the existing RPS14 loss. Lenalidomide was one of the first molecular-glue clinical successes and remains the standard first-line treatment for 5q- MDS.
3. Shwachman-Diamond Syndrome (SDS)
Autosomal recessive disease caused by biallelic mutations in SBDS, a ribosome-biogenesis factor. SBDS + EFL1 GTPase release the assembly factor eIF6 from nascent 60S subunits β a final maturation step. Loss of SBDS traps pre-60S particles and reduces mature ribosome output. Clinical features: pancreatic exocrine insufficiency (leading to malabsorption), marrow failure, skeletal abnormalities, ~30% lifetime risk of AML/MDS. Treatment: enzyme replacement for pancreatic insufficiency, bone-marrow transplantation for marrow failure.
4. Dyskeratosis Congenita & Telomere Biology
DKC1 encodes dyskerin, a pseudouridine synthase that modifies rRNA and also stabilises the telomerase RNA (TERC). X-linked DKC is classified both as a ribosomopathy (rRNA pseudouridylation defect) and a telomeropathy (shortened telomeres). Clinical triad: skin dyspigmentation, oral leukoplakia, nail dystrophy, with severe bone-marrow failure and early-onset cancer. Autosomal forms map to other telomere-maintenance genes (TERC, TERT, TINF2, RTEL1).
5. Treacher Collins Syndrome
Haploinsufficiency for TCOF1 (treacle, an rRNA transcription/processing factor) or POLR1C/D (RNA polymerase I subunits) causes Treacher Collins syndrome β craniofacial malformation (mandibular and midface hypoplasia, downslanting palpebral fissures). Tissue specificity: neural crest cells undergoing craniofacial morphogenesis are exquisitely sensitive to reduced ribosome biogenesis, becoming p53-apoptotic. In mouse models, p53 inhibition partially rescues the phenotype β a direct link from ribosomal stress to p53 and developmental outcome.
6. The p53 Axis: Ribosomal-Protein Surveillance
A unifying mechanism across ribosomopathies: ribosome biogenesis stress activates p53. The mechanism (Lohrum 2003; Zhang 2003; Dai 2004) depends on ribosomal proteins RPL5, RPL11, and RPS7, which under normal conditions assemble into the 60S but under biogenesis stress accumulate as βfreeβ RP that binds MDM2 and inhibits its ubiquitin-ligase activity against p53. Stabilised p53 drives cell-cycle arrest or apoptosis. This is why ribosomopathies produce cytopenia (apoptotic elimination of precursors) and why concurrent p53 loss (in cancer progression) accelerates both proliferation and ribosome biogenesis.
The RP-MDM2-p53 axis also explains why oncoprotein MDM4 amplification (common in cancers with intact p53) provides an alternative route to p53 suppression.
7. Cancer and the Ribosome
Many cancers are ribosome-addicted: c-Myc, mTORC1, and PI3K-AKT signalling converge on upregulation of rRNA transcription, ribosome biogenesis, and cap-dependent translation. Selective targeting of ribosome biogenesis is emerging as an oncology strategy. CX-5461 (Pol I transcription inhibitor) showed activity in haematological cancers with p53 wild-type status; BMH-21 similarly disrupts Pol I. Harringtonine derivatives target translation elongation. Homoharringtonine (omacetaxine) is approved for CML. The ribosome is a legitimate 21st-century oncology target.