Patients with cartilage-hair hypoplasia have an increased risk for bronchiectasis

Molecular etiology of defective nuclear and mitochondrial ribosome biogenesis: Clinical phenotypes and therapy

Ribosomopathies are rare congenital disorders associated with defective ribosome biogenesis due to pathogenic variations in genes that encode proteins related to ribosome function and biogenesis. Defects in ribosome biogenesis result in a nucleolar stress response involving the TP53 tumor suppressor protein and impaired protein synthesis leading to a deregulated translational output. Despite the accepted notion that ribosomes are omnipresent and essential for all cells, most ribosomopathies show tissue-specific phenotypes affecting blood cells, hair, spleen, or skin. On the other hand, defects in mitochondrial ribosome biogenesis are associated with a range of clinical manifestations affecting more than one organ. Intriguingly, the deregulated ribosomal function is also a feature in several human malignancies with a selective upregulation or downregulation of specific ribosome components. Here, we highlight the clinical conditions associated with defective ribosome biogenesis in the nucleus and mitochondria with a description of the affected genes and the implicated pathways, along with a note on the treatment strategies currently available for these disorders.

Pulmonary Follow-Up Imaging in Cartilage-Hair Hypoplasia: a Prospective Cohort Study

Cartilage-hair hypoplasia is a syndromic immunodeficiency with short stature, chondrodysplasia, and variable degree of immune dysfunction. Patients with cartilage-hair hypoplasia are prone to recurrent respiratory tract infections, and the prevalence of bronchiectasis ranges from 29 to 52%. Pulmonary complications contribute significantly to the mortality; therefore, regular lung imaging is essential. However, the optimal schedule for repeated lung imaging remains unestablished. We determined the rate and correlates of progression of structural lung changes in a prospectively followed cohort of 16 patients with cartilage-hair hypoplasia. We analyzed clinical, laboratory, and pulmonary functional testing data and performed lung magnetic resonance imaging at a median interval of 6.8 years since previous imaging. Imaging findings remained identical or improved due to disappearance of inflammatory changes in all evaluated patients. Patients with subtle signs of bronchiectasis on imaging tended to have low immunoglobulin M levels, as well as suffered from pneumonia during the follow-up. In conclusion, our results suggest slow if any development of bronchiectasis in selected subjects with cartilage-hair hypoplasia.

Immunodeficiency in cartilage-hair hypoplasia: Pathogenesis, clinical course and management

Cartilage-hair hypoplasia (CHH) is an autosomal recessive syndromic immunodeficiency with skeletal dysplasia, short stature, hypotrichosis, variable degree of immune dysfunction and increased incidence of anaemia, Hirschsprung disease and malignancy. CHH is caused by variants in the RMRP gene, encoding the untranslated RNA molecule of the mitochondrial RNA-processing endoribonuclease, which participates in for example cell cycle regulation and telomere maintenance. Recent studies have expanded our understanding of the complex pathogenesis of CHH. Immune dysfunction has a major impact on clinical course and prognosis. Clinical features of immune dysfunction are highly variable, progressive and include infections, lung disease, immune dysregulation and malignancy. Mortality is increased compared with the general population, due to infections, malignancy and pulmonary disease. Several risk factors for early mortality have been reported in the Finnish CHH cohort and can be used to guide management. Newborn screening for severe combined immunodeficiency can possibly be of prognostic value in CHH. Regular follow-up by a multidisciplinary team should be implemented to address immune dysfunction in all patients with CHH, also in asymptomatic cases. Haematopoietic stem cell transplantation can cure immune dysfunction, but its benefits in mildly symptomatic patients with CHH remain debatable. Further research is needed to understand the mechanisms behind the variability of clinical features, to search for potential molecular treatment targets, to examine and validate risk factors for early mortality outside the Finnish CHH cohort and to develop management guidelines. This review focuses on the pathogenesis, clinical course and management of CHH.

The molecular basis of the cartilage-hair hypoplasia-anauxetic dysplasia spectrum

Cartilage-hair hypoplasia and anauxetic dysplasia are two autosomal recessive skeletal dysplasias characterized by different degrees from metaphyseal to spondylo-meta-epiphyseal dysplasia and variable additional features including predisposition to cancer, anemia, immunodeficiency, and gastrointestinal malabsorption and Hirschsprung's disease. Both are caused by mutations in the untranslated RMRP gene, which forms the RNA subunit of the RNase MRP complex. This complex is involved in the ribosome assembly by cleavage of 5.8S rRNA, cell cycle control by Cyclin B2 mRNA cleavage at the end of mitosis, processing the mitochondrial RNA, and forming a complex with hTERT suggesting a possible involvement in expression regulation by siRNA synthesis. The degree of skeletal dysplasia correlates mainly with the rRNA cleavage activity, whereas significantly diminished mRNA cleavage activity is a prerequisite for immunodeficiency. Thus, the clinical phenotype emerges in most cases of the combined effect on the respective effect on RNase MRP function.

Functional characterization of the Drosophila MRP (mitochondrial RNA processing) RNA gene

MRP RNA is a noncoding RNA component of RNase mitochondrial RNA processing (MRP), a multi-protein eukaryotic endoribonuclease reported to function in multiple cellular processes, including ribosomal RNA processing, mitochondrial DNA replication, and cell cycle regulation. A recent study predicted a potential Drosophila ortholog of MRP RNA (CR33682) by computer-based genome analysis. We have confirmed the expression of this gene and characterized the phenotype associated with this locus. Flies with mutations that specifically affect MRP RNA show defects in growth and development that begin in the early larval period and end in larval death during the second instar stage. We present several lines of evidence demonstrating a role for Drosophila MRP RNA in rRNA processing. The nuclear fraction of Drosophila MRP RNA localizes to the nucleolus. Further, a mutant strain shows defects in rRNA processing that include a defect in 5.8S rRNA processing, typical of MRP RNA mutants in other species, as well as defects in early stages of rRNA processing.

Viperin mRNA is a novel target for the human RNase MRP/RNase P endoribonuclease

RNase MRP is a conserved endoribonuclease, in humans consisting of a 267-nucleotide RNA associated with 7–10 proteins. Mutations in its RNA component lead to several autosomal recessive skeletal dysplasias, including cartilage-hair hypoplasia (CHH). Because the known substrates of mammalian RNase MRP, pre-ribosomal RNA, and RNA involved in mitochondrial DNA replication are not likely involved in CHH, we analyzed the effects of RNase MRP (and the structurally related RNase P) depletion on mRNAs using DNA microarrays. We confirmed the upregulation of the interferon-inducible viperin mRNA by RNAi experiments and this appeared to be independent of the interferon response. We detected two cleavage sites for RNase MRP/RNase P in the coding sequence of viperin mRNA. This is the first study providing direct evidence for the cleavage of a mRNA by RNase MRP/RNase P in human cells. Implications for the involvement in the pathophysiology of CHH are discussed.

Cartilage-hair hypoplasia: molecular basis and heterogeneity of the immunological phenotype

PURPOSE OF REVIEW

To report on the expanding clinical and immunological spectrum associated with ribonuclease mitochondrial RNA-processing mutations and to review the cellular and molecular mechanisms involved in the pathophysiology of cartilage-hair hypoplasia (CHH) and related disorders in humans.

RECENT FINDINGS

Different types of mutations are associated with skeletal or extraskeletal manifestations of CHH, respectively. In particular, severe immunodeficiency is mostly associated with mutations that alter cyclin B2 mRNA cleavage and thus are likely to reflect disturbances in cell cycle control. The first cases of ribonuclease mitochondrial RNA-processing mutations with severe immunodeficiency, but no skeletal abnormalities, have been identified.

SUMMARY

Abnormalities of ribosome biogenesis have been shown to cause distinct bone marrow failure syndromes, including CHH. However, the specific role of ribosomal and extraribosomal defects in the pathophysiology of the various phenotypic features of CHH remains undefined. Development of suitable animal models is needed to address this important issue.