Type and level of RMRP functional impairment predicts phenotype in the cartilage hair hypoplasia-anauxetic dysplasia spectrum

RMRP variants inhibit the cell cycle checkpoints pathway in cartilage‑hair hypoplasia

Cartilage‑hair hypoplasia (CHH) is an autosomal recessive form of metaphyseal chondrodysplasia caused by RNA component of mitochondrial RNA processing endoribonuclease (RMRP) gene variants; however, its molecular etiology remains unclear. Whole‑exome sequencing was performed to detect possible pathogenic variants in a patient with a typical short stature and sparse hair. A co‑segregation analysis was also conducted and variants in the family members of the patient were confirmed by Sanger sequencing. A novel compound heterozygous variant in RMRP (NR_003051.4: n.‑21_‑2dup and n.197C>T) was identified in the affected patient. Data from 2 years and 4 months of follow‑up showed a positive effect of growth hormone (GH) therapy on height. Subsequently, two gene expression profiles associated with CHH were obtained from the EMBL‑EBI ENA and ArrayExpress databases. Differentially expressed genes between patients with CHH and healthy controls were selected using R software and were subjected to core analysis using ingenuity pathway analysis (IPA) software. IPA core analysis showed that the 'cell cycle checkpoints' was the most prominent canonical pathway, and the top enriched diseases and functions included various types of cancer, immunological diseases, development disorders and respiratory diseases. The integrative analysis displayed that RMRP can regulate the aberrant expression of downstream targets mainly via the transcription factor TP53, which results in the inhibition of 'cell cycle checkpoints'; eventually, functions associated with the CHH phenotype, such as 'growth failure or short stature' are activated. In conclusion, novel disease‑causing genetic variants of RMRP expand the genetic etiology of CHH, which must be clinically differentiated from achondroplasia. The findings of the present study provide new insights into the mechanisms underlying CHH.

Novel phenotype associated with homozygous likely pathogenic variant in the POP1 gene

The biallelic variants of the POP1 gene are associated with the anauxetic dysplasia (AAD OMIM 607095), a rare skeletal dysplasia, characterized by prenatal rhizomelic shortening of limbs and generalized joint hypermobility. Affected individuals usually have normal neurodevelopmental milestones. Here we present three cases from the same family with likely pathogenic homozygous POP1 variant and a completely novel phenotype: a girl with global developmental delay and autism, microcephaly, peculiar dysmorphic features and multiple congenital anomalies. Two subsequent pregnancies were terminated due to multiple congenital malformations. Fetal DNA samples revealed the same homozygous variant in the POP1 gene. Expression of the RMRP was reduced in the proband compared with control and slightly reduced in both heterozygous parents, carriers for this variant. To our knowledge, this is the first report of this new phenotype, associated with a novel likely pathogenic variant in POP1. Our findings expand the phenotypic spectrum of POP1-related disorders.

RMRP-related short stature: A report of six additional Japanese individuals with cartilage hair hypoplasia and literature review

Biallelic pathogenic variants in RMRP, the gene encoding the RNA component of RNase mitochondrial RNA processing enzyme complex, have been reported in individuals with cartilage hair hypoplasia (CHH). CHH is prevalent in Finnish and Amish populations due to a founder pathogenic variant, n.71A > G. Based on the manifestations in the Finnish and Amish individuals, the hallmarks of CHH are prenatal-onset growth failure, metaphyseal dysplasia, hair hypoplasia, immunodeficiency, and other extraskeletal manifestations. Herein, we report six Japanese individuals with CHH from four families. All probands presented with moderate short stature with mild metaphyseal dysplasia or brachydactyly. One of them had hair hypoplasia and the other immunodeficiency. By contrast, the affected siblings of two families showed only mild short stature. We also reviewed all previously reported 13 Japanese individuals. No n.71A > G allele was detected. The proportions of Japanese versus Finnish individuals were 0% versus 70% for birth length < -2.0 SD, 84% versus 100% for metaphyseal dysplasia and 26% versus 88% for hair hypoplasia. Milder manifestations in the Japanese individuals may be related to the difference of genotypes. The mildest form of CHH phenotypes is mild short stature without overt skeletal alteration or extraskeletal manifestation and can be termed "RMRP-related short stature".

Cartilage-hair hypoplasia-anauxetic dysplasia spectrum disorders harboring RMRP mutations in two Korean children: A case report

RATIONALE

Cartilage-hair hypoplasia (CHH, OMIM # 250250) is a rare autosomal recessive disorder, which includes cartilage-hair hypoplasia-anauxetic dysplasia (CHH-AD) spectrum disorders. CHH-AD is caused by homozygous or compound heterozygous mutations in the RNA component of the mitochondrial RNA-processing Endoribonuclease (RMRP) gene.

PATIENT CONCERNS

Here, we report 2 cases of Korean children with CHH-AD.

DIAGNOSES

In the first case, the patient had metaphyseal dysplasia without hypotrichosis, diagnosed by whole exome sequencing (WES), and exhibited only skeletal dysplasia and lacked extraskeletal manifestations, such as hair hypoplasia and immunodeficiency. In the second case, the patient had skeletal dysplasia, hair hypoplasia, and immunodeficiency, which were identified by WES.

INTERVENTIONS

The second case is the first CHH reported in Korea. The patients in both cases received regular immune and lung function checkups.

OUTCOMES

Our cases suggest that children with extremely short stature from birth, with or without extraskeletal manifestations, should include CHH-AD as a differential diagnosis.

LESSONS SUBSECTIONS

Clinical suspicion is the most important and RMRP sequencing should be considered for the diagnosis of CHH-AD.

Diamond-Blackfan anemia, the archetype of ribosomopathy: How distinct is it from the other constitutional ribosomopathies?

Diamond-Blackfan anemia (DBA) was the first ribosomopathy described in humans. DBA is a congenital hypoplastic anemia, characterized by macrocytic aregenerative anemia, manifesting by differentiation blockage between the BFU-e/CFU-e developmental erythroid progenitor stages. In 50 % of the DBA cases, various malformations are noted. Strikingly, for a hematological disease with a relative erythroid tropism, DBA is due to ribosomal haploinsufficiency in 24 different ribosomal protein (RP) genes. A few other genes have been described in DBA-like disorders, but they do not fit into the classical DBA phenotype (Sankaran et al., 2012; van Dooijeweert et al., 2022; Toki et al., 2018; Kim et al., 2017 [1-4]). Haploinsufficiency in a RP gene leads to defective ribosomal RNA (rRNA) maturation, which is a hallmark of DBA. However, the mechanistic understandings of the erythroid tropism defect in DBA are still to be fully defined. Erythroid defect in DBA has been recently been linked in a non-exclusive manner to a number of mechanisms that include: 1) a defect in translation, in particular for the GATA1 erythroid gene; 2) a deficit of HSP70, the GATA1 chaperone, and 3) free heme toxicity. In addition, p53 activation in response to ribosomal stress is involved in DBA pathophysiology. The DBA phenotype may thus result from the combined contributions of various actors, which may explain the heterogenous phenotypes observed in DBA patients, even within the same family.

Shorter birth length and decreased T-cell production and function predict severe infections in children with non-severe combined immunodeficiency cartilage-hair hypoplasia

Background

Cartilage-hair hypoplasia (CHH) is a syndromic inborn error of immunity caused by variants in the RMRP gene. Disease manifestations vary, and their ability to predict outcome is uncertain. The optimal management of infants with CHH who do not fulfill classical severe combined immunodeficiency (SCID) criteria is unknown.

Objective

We described longitudinal changes in lymphocyte counts during childhood and explored correlations of early childhood clinical and laboratory features with clinical outcomes on long-term follow-up of CHH patients.

Methods

Immunologic laboratory parameters, birth length, the presence of Hirschsprung disease, and severe anemia correlated to the primary end points of respiratory and severe infections. We implemented traditional statistical methods and machine learning techniques.

Results

Thirty-two children with CHH were followed up for 2.7 to 22.1 years (median, 8.2 years, in total 331.3 patient-years). None of the patients had classical SCID. Median lymphocyte subclass counts, apart from CD16+/56+ cells, were subnormal throughout childhood, but did not show age-related decline seen in healthy children. Low immunoglobulin levels were uncommon and often transient. Respiratory and/or severe infections developed in 14 children, 8 of whom had low naive T-cell counts, absent T-cell receptor excision circles, and/or partial "leaky" SCID-level lymphopenia. Shorter birth length correlated with lower lymphocyte counts and the occurrence of infections. Of the laboratory parameters, decreased naive T-cell counts and abnormal lymphocyte proliferation responses contributed most to the development of severe infections. In addition, all participants with absent T-cell receptor excision circles developed severe infections. Opportunistic infections occurred only in children with leaky SCID-level lymphopenia.

Conclusions

Shorter birth length and a combination of laboratory abnormalities can predict the development of severe infections in children with CHH.

Prenatal Cases Reflect the Complexity of the COL1A1/2 Associated Osteogenesis Imperfecta

Introduction: Osteogenesis imperfecta (OI) is a rare mendelian skeletal dysplasia with autosomal dominant or recessive inheritance pattern, and almost the most common primary osteoporosis in prenatal settings. The diversity of clinical presentation and genetic etiology in prenatal OI cases presents a challenge to counseling yet has seldom been discussed in previous studies. Methods: Ten cases with suspected fetal OI were enrolled and submitted to a genetic detection using conventional karyotyping, chromosomal microarray analysis (CMA), and whole-exome sequencing (WES). Sanger sequencing was used as the validation method for potential diagnostic variants. In silico analysis of specific missense variants was also performed. Results: The karyotyping and CMA results of these cases were normal, while WES identified OI-associated variants in the COL1A1/2 genes in all ten cases. Six of these variants were novel. Additionally, four cases here exhibited distinctive clinical and/or genetic characteristics, including the situations of intrafamilial phenotypic variability, parental mosaicism, and “dual nosogenesis” (mutations in collagen I and another gene). Conclusion: Our study not only expands the spectrum of COL1A1/2-related OI, but also highlights the complexity that occurs in prenatal OI and the importance of clarifying its pathogenic mechanisms.

A novel experimental approach for the selective isolation and characterization of human RNase MRP

RNase MRP is a ribonucleoprotein complex involved in the endoribonucleolytic cleavage of different RNAs. Mutations in the RNA component of the RNP are the cause of cartilage hair hypoplasia. Patients with cartilage hair hypoplasia are characterized by skeletal dysplasia. Biochemical purification of RNase MRP is desired to be able to study its biochemical function, composition and activity in both healthy and disease situations. Due to the high similarity with RNase P, a method to specifically isolate the RNase MRP complex is currently lacking. By fusing a streptavidin-binding RNA aptamer, the S1m-aptamer, to the RNase MRP RNA we have been able to compare the relative expression levels of wildtype and mutant MRP RNAs. Moreover, we were able to isolate active RNase MRP complexes. We observed that mutant MRP RNAs are expressed at lower levels and have lower catalytic activity compared to the wildtype RNA. The observation that a single nucleotide substitution at position 40 in the P3 domain but not in other domains of RNase MRP RNA severely reduced the binding of the Rpp25 protein subunit confirmed that the P3 region harbours the main binding site for this protein. Altogether, this study shows that the RNA aptamer tagging approach can be used to identify RNase MRP substrates, but also to study the effect of mutations on MRP RNA expression levels and RNase MRP composition and endoribonuclease activity.

Uncovering pathways regulating chondrogenic differentiation of CHH fibroblasts

Mutations in the non-coding snoRNA component of mitochondrial RNA processing endoribonuclease (RMRP) are the cause of cartilage-hair hypoplasia (CHH). CHH is a rare form of metaphyseal chondrodysplasia characterized by disproportionate short stature and abnormal growth plate development. The process of chondrogenic differentiation within growth plates of long bones is vital for longitudinal bone growth. However, molecular mechanisms behind impaired skeletal development in CHH patients remain unclear. We employed a transdifferentiation model (FDC) combined with whole transcriptome analysis to investigate the chondrogenic transdifferentiation capacity of CHH fibroblasts and to examine pathway regulation in CHH cells during chondrogenic differentiation. We established that the FDC transdifferentiation model is a relevant in vitro model of chondrogenic differentiation, with an emphasis on the terminal differentiation phase, which is crucial for longitudinal bone growth. We demonstrated that CHH fibroblasts are capable of transdifferentiating into chondrocyte-like cells, and show a reduced commitment to terminal differentiation. We also found a number of key factors of BMP, FGF, and IGF-1 signalling axes to be significantly upregulated in CHH cells during the chondrogenic transdifferentiation. Our results support postulated conclusions that RMRP has pleiotropic functions and profoundly affects multiple aspects of cell fate and signalling. Our findings shed light on the consequences of pathological CHH mutations in snoRNA RMRP during chondrogenic differentiation and the relevance and roles of non-coding RNAs in genetic diseases in general.

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.

Early prenatal presentation of the cartilage-hair hypoplasia / anauxetic dysplasia spectrum of disorders mimicking recurrent thanatophoric dysplasia

Three sibling fetuses identified with limb shortening and thoracic narrowing at twelve weeks' gestation on first trimester ultrasound examination are presented. The parents were non-consanguineous, Caucasian, healthy, of normal stature and had a healthy normal daughter. The radiographic abnormalities were highly suggestive of thanatophoric dysplasia, but molecular analysis failed to identify a pathogenic variant in FGFR3. The three fetuses were found to have identical compound heterozygous mutations in RMRP in trans, one inherited from the mother and one from the father. This represents the early prenatal presentation and fetal findings of metaphyseal dysplasia type McKusick (Cartilage-hair hypoplasia; CHH)/anauxetic dysplasia spectrum of disorders.

How Altered Ribosome Production Can Cause or Contribute to Human Disease: The Spectrum of Ribosomopathies

A number of different defects in the process of ribosome production can lead to a diversified spectrum of disorders that are collectively identified as ribosomopathies. The specific factors involved may either play a role only in ribosome biogenesis or have additional extra-ribosomal functions, making it difficult to ascribe the pathogenesis of the disease specifically to an altered ribosome biogenesis, even if the latter is clearly affected. We reviewed the available literature in the field from this point of view with the aim of distinguishing, among ribosomopathies, the ones due to specific alterations in the process of ribosome production from those characterized by a multifactorial pathogenesis.

The novel R211Q POP1 homozygous mutation causes different pathogenesis and skeletal changes from those of previously reported POP1-associated anauxetic dysplasia

Processing of Precursor RNA 1 (POP1) is a core protein component shared by two essential closely related eukaryotic ribonucleoprotein complexes: RNase MRP (the mitochondrial RNA processing ribonuclease) and RNase P. Recently, five patients harboring mutations in POP1 have been reported with severe spondylo-epi-metaphyseal dysplasia and extremely short stature. We report a unique clinical phenotype resulting from the novel homozygous R211Q POP1 mutation in three patients from one family, presenting with severe short stature but only subtle skeletal dysplastic changes that are merely metaphyseal. The RNA moiety of the RNase-MRP complex quantified in RNA extracted from peripheral lymphocytes was dramatically reduced in affected patients indicating instability of the enzymatic complex. However, pre5.8s rRNA, a substrate of RNase-MRP complex, was not accumulated in patients' RNA unlike in the previously reported POP1 mutations; this may explain the uniquely mild phenotype in our cases, and questions the assumption that alteration in ribosomal biogenesis is the pathophysiological basis for skeletal disorders caused by POP1 mutations. Finally, POP1 mutations should be considered in familial cases with severe short stature even when skeletal dysplasia is not strongly evident.

Rmrp Mutation Disrupts Chondrogenesis and Bone Ossification in Zebrafish Model of Cartilage-Hair Hypoplasia via Enhanced Wnt/β-Catenin Signaling

Cartilage-hair hypoplasia (CHH) is an autosomal recessive metaphyseal chondrodysplasia characterized by bone dysplasia and many other highly variable features. The gene responsible for CHH is the RNA component of the mitochondrial RNA-processing endoribonuclease (RMRP) gene. Currently, the pathogenesis of osteochondrodysplasia and extraskeletal manifestations in CHH patients remains incompletely understood; in addition, there are no viable animal models for CHH. We generated an rmrp KO zebrafish model to study the developmental mechanisms of CHH. We found that rmrp is required for the patterning and shaping of pharyngeal arches. Rmrp mutation inhibits the intramembranous ossification of skull bones and promotes vertebrae ossification. The abnormalities of endochondral bone ossification are variable, depending on the degree of dysregulated chondrogenesis. Moreover, rmrp mutation inhibits cell proliferation and promotes apoptosis through dysregulating the expressions of cell-cycle- and apoptosis-related genes. We also demonstrate that rmrp mutation upregulates canonical Wnt/β-catenin signaling; the pharmacological inhibition of Wnt/β-catenin could partially alleviate the chondrodysplasia and increased vertebrae mineralization in rmrp mutants. Our study, by establishing a novel zebrafish model for CHH, partially reveals the underlying mechanism of CHH, hence deepening our understanding of the role of rmrp in skeleton development.

Identification of Novel and Recurrent RMRP Variants in a Series of Brazilian Patients with Cartilage-Hair Hypoplasia: McKusick Syndrome

Cartilage-hair hypoplasia syndrome (CHH) is an autosomal recessive disorder caused by pathogenic variants of the RMRP gene and characterized by metaphyseal bone dysplasia associated with hypotrichosis, immunodeficiency, and predisposition to malignancy. However, the genotype-phenotype correlation in CHH is not well understood. Here, we report a single country cohort of 23 Brazilian patients with clinical and radiological features consistent with CHH. We found 23 different pathogenic variants in the RMRP gene - 12 novel and 11 previously described in the literature. Interestingly, the most frequent Finnish pathogenic variant related to CHH (g.71A>G) was not found in our cohort. In contrast, more than 50% of the patients carried the rare g.196C>T variant suggesting a possible founder effect in the Brazilian population. In silico analysis showed that pathogenic variants occurred either in the regions conserved in mammalian species or within essential domains for the ribonucleoprotein complex. Pathogenicity prediction studies can improve the understanding of how these variants affect RNA.

'Metaphyseal dysplasia without hypotrichosis' can present with late-onset extraskeletal manifestations

BACKGROUND

Metaphyseal dysplasia without hypotrichosis (MDWH) is a rare form of chondrodysplasia with no extraskeletal manifestations. MDWH is caused by RMRP mutations, but it is differentiated from the allelic condition cartilage-hair hypoplasia (CHH), which in addition to chondrodysplasia is characterised by thin hair, immunodeficiency and increased risk of malignancy. The long-term outcome of MDWH remains unknown.

OBJECTIVE

We diagnosed severe agranulocytosis in a subject with RMRP mutations and normal hair. Based on this observation, we hypothesised that MDWH may, similar to CHH, associate with immune deficiency and malignancy.

METHODS

We collected clinical and laboratory data for a cohort of 80 patients with RMRP mutations followed for over 30 years and analysed outcome data for those with features consistent with MDWH.

RESULTS

In our cohort, we identified 10 patients with skeletal but no extraskeletal features during preschool age. Eight of these patients developed malignancy or clinically significant immunodeficiency during follow-up. Two of them died during chemotherapy for malignancy. At the time of the first extraskeletal manifestation, patients were school aged, 20, 43 and 50 years old. Laboratory signs of immunodeficiency (impaired lymphocyte proliferative responses) were demonstrated in four patients before the onset of symptoms. The patient outside this cohort, who had RMRP mutations, skeletal dysplasia, normal hair and severe agranulocytosis at 18 years of age, underwent haematopoietic stem cell transplantation.

CONCLUSIONS

MDWH can present with severe late-onset extraskeletal manifestations and thus should be reclassified and managed as CHH.

A 30-Year Prospective Follow-Up Study Reveals Risk Factors for Early Death in Cartilage-Hair Hypoplasia

Cartilage-hair hypoplasia (CHH) is a skeletal dysplasia with combined immunodeficiency, variable clinical course and increased risk of malignancy. Management of CHH is complicated by a paucity of long-term follow-up data, as well as knowledge on prognostic factors. We assessed clinical course and risk factors for mortality in a prospective cohort study of 80 patients with CHH recruited in 1985-1991 and followed up until 2016. For all patients we collected additional health information from health records and from the national Medical Databases and Cause-of-death Registry. The primary outcome was immunodeficiency-related death, including death from infections, lung disease and malignancy. Standardized mortality ratios (SMRs) were calculated using national mortality rates as reference. Half of the patients (57%, n = 46) manifested no symptoms of immunodeficiency during follow-up while 19% (n = 15) and 24% (n = 19) demonstrated symptoms of humoral or combined immunodeficiency, including six cases of adult-onset immunodeficiency. In a significant proportion of patients (17/79, 22%), clinical features of immunodeficiency progressed over time. Of the 15 patients with non-skin cancer, eight had no preceding clinical symptoms of immunodeficiency. Altogether 20 patients had deceased (SMR = 7.0, 95%CI = 4.3-11); most commonly from malignancy (n = 7, SMR = 10, 95%CI = 4.1-21) and lung disease (n = 4, SMR = 46, 95%CI = 9.5-130). Mortality associated with birth length below -4 standard deviation (compared to normal, SMR/SMR ratio = 5.4, 95%CI = 1.5-20), symptoms of combined immunodeficiency (compared to asymptomatic, SMR/SMR ratio = 3.9, 95%CI = 1.3-11), Hirschsprung disease (odds ratio (OR) 7.2, 95%CI = 1.04-55), pneumonia in the first year of life or recurrently in adulthood (OR = 7.6/19, 95%CI = 1.3-43/2.6-140) and autoimmunity in adulthood (OR = 39, 95%CI = 3.5-430). In conclusion, patients with CHH may develop adult-onset immunodeficiency or malignancy without preceding clinical symptoms of immune defect, warranting careful follow-up. Variable disease course and risk factors for mortality should be acknowledged.

Rare ribosomopathies: insights into mechanisms of cancer

Long thought to be too big and too ubiquitous to fail, we now know that human cells can fail to make sufficient amounts of ribosomes, causing a number of diseases collectively known as ribosomopathies. The best characterized ribosomopathies, with the exception of Treacher Collins syndrome, are inherited bone marrow failure syndromes, each of which has a marked increase in cancer predisposition relative to the general population. Although rare, emerging data reveal that the inherited bone marrow failure syndromes may be underdiagnosed on the basis of classical symptomology, leaving undiagnosed patients with these syndromes at an elevated risk of cancer without adequate counselling and surveillance. The link between the inherited ribosomopathies and cancer has led to greater awareness that somatic mutations in factors involved in ribosome biogenesis may also be drivers in sporadic cancers. Our goal here is to compare and contrast the pathophysiological mechanisms underpinning ribosomopathies to gain a better understanding of the mechanisms that predispose these disorders to cancer.

A Wide Spectrum of Autoimmune Manifestations and Other Symptoms Suggesting Immune Dysregulation in Patients With Cartilage-Hair Hypoplasia

Background: Mutations in RMRP, encoding a non-coding RNA molecule, underlie cartilage-hair hypoplasia (CHH), a syndromic immunodeficiency with multiple pathogenetic mechanisms and variable phenotype. Allergy and asthma have been reported in the CHH population and some patients suffer from autoimmune (AI) diseases.

Objective: We explored AI and allergic manifestations in a large cohort of Finnish patients with CHH and correlated clinical features with laboratory parameters and autoantibodies.

Methods: We collected clinical and laboratory data from patient interviews and hospital records. Serum samples were tested for a range of autoantibodies including celiac, anti-cytokine, and anti-21-hydroxylase antibodies. Nasal cytology samples were analyzed with microscopy.

Results: The study cohort included 104 patients with genetically confirmed CHH; their median age was 39.2 years (range 0.6–73.6). Clinical autoimmunity was common (11/104, 10.6%) and included conditions previously undescribed in subjects with CHH (narcolepsy, psoriasis, idiopathic thrombocytopenic purpura, and multifocal motor axonal neuropathy). Patients with autoimmunity more often had recurrent pneumonia, sepsis, high immunoglobulin (Ig) E and/or undetectable IgA levels. The mortality rates were higher in subjects with AI diseases (χ(2)2 = 14.056, p = 0.0002). Several patients demonstrated serum autoantibody positivity without compatible symptoms. We confirmed the high prevalence of asthma (23%) and allergic rhinoconjunctivitis (39%). Gastrointestinal complaints, mostly persistent diarrhea, were also frequently reported (32/104, 31%). Despite the history of allergic rhinitis, no eosinophils were observed in nasal cytology in five tested patients.

Conclusions: AI diseases are common in Finnish patients with CHH and are associated with higher mortality, recurrent pneumonia, sepsis, high IgE and/or undetectable IgA levels. Serum positivity for some autoantibodies was not associated with clinical autoimmunity. The high prevalence of persistent diarrhea, asthma, and symptoms of inflammation of nasal mucosa may indicate common pathways of immune dysregulation.

ZSCAN5B and primate-specific paralogs bind RNA polymerase III genes and extra-TFIIIC (ETC) sites to modulate mitotic progression

Mammalian genomes contain hundreds of genes transcribed by RNA Polymerase III (Pol III), encoding noncoding RNAs and especially the tRNAs specialized to carry specific amino acids to the ribosome for protein synthesis. In addition to this well-known function, tRNAs and their genes (tDNAs) serve a variety of other critical cellular functions. For example, tRNAs and other Pol III transcripts can be cleaved to yield small RNAs with potent regulatory activities. Furthermore, from yeast to mammals, active tDNAs and related “extra-TFIIIC” (ETC) loci provide the DNA scaffolds for the most ancient known mechanism of three-dimensional chromatin architecture. Here we identify the ZSCAN5 TF family - including mammalian ZSCAN5B and its primate-specific paralogs - as proteins that occupy mammalian Pol III promoters and ETC sites. We show that ZSCAN5B binds with high specificity to a conserved subset of Pol III genes in human and mouse. Furthermore, primate-specific ZSCAN5A and ZSCAN5D also bind Pol III genes, although ZSCAN5D preferentially localizes to MIR SINE- and LINE2-associated ETC sites. ZSCAN5 genes are expressed in proliferating cell populations and are cell-cycle regulated, and siRNA knockdown experiments suggested a cooperative role in regulation of mitotic progression. Consistent with this prediction, ZSCAN5A knockdown led to increasing numbers of cells in mitosis and the appearance of cells. Together, these data implicate the role of ZSCAN5 genes in regulation of Pol III genes and nearby Pol II loci, ultimately influencing cell cycle progression and differentiation in a variety of tissues.

Non-coding RNAs and disease: the classical ncRNAs make a comeback

Many human diseases have been attributed to mutation in the protein coding regions of the human genome. The protein coding portion of the human genome, however, is very small compared with the non-coding portion of the genome. As such, there are a disproportionate number of diseases attributed to the coding compared with the non-coding portion of the genome. It is now clear that the non-coding portion of the genome produces many functional non-coding RNAs and these RNAs are slowly being linked to human diseases. Here we discuss examples where mutation in classical non-coding RNAs have been attributed to human disease and identify the future potential for the non-coding portion of the genome in disease biology.

Box C/D small nucleolar RNA genes and the Prader-Willi syndrome: a complex interplay

The nucleolus of mammalian cells contains hundreds of box C/D small nucleolar RNAs (SNORDs). Through their ability to base pair with ribosomal RNA precursors, most play important roles in the synthesis and/or activity of ribosomes, either by guiding sequence-specific 2'-O-methylations or by facilitating RNA folding and cleavages. A growing number of SNORD genes with elusive functions have been discovered recently. Intriguingly, the vast majority of them are located in two large, imprinted gene clusters at human chromosome region 15q11q13 (the SNURF-SNRPN domain) and at 14q32 (the DLK1-DIO3 domain) where they are expressed, respectively, only from the paternally and maternally inherited alleles. These placental mammal-specific SNORD genes have many features of the canonical SNORDs that guide 2'-O-methylations, yet they lack obvious complementarity with ribosomal RNAs and, surprisingly, they are processed from large, tandemly repeated genes expressed preferentially in the brain. This review summarizes our understanding of the biology of these peculiar SNORD genes, focusing particularly on SNORD115 and SNORD116 in the SNURF-SNRPN domain. It examines the growing evidence that altered levels of these SNORDs and/or their host-gene transcripts may be a primary cause of Prader-Willi syndrome (PWS; a rare disorder characterized by overeating and obesity) as well as abnormalities in signaling through the 5-HT2C serotonin receptor. Finally, the hypothesis that PWS may be a ribosomopathy (ribosomal disease) is also discussed. WIREs RNA 2017, 8:e1417. doi: 10.1002/wrna.1417 For further resources related to this article, please visit the WIREs website.

Defects in lymphocyte telomere homeostasis contribute to cellular immune phenotype in patients with cartilage-hair hypoplasia

BACKGROUND

Mutations in the long noncoding RNA RNase component of the mitochondrial RNA processing endoribonuclease (RMRP) give rise to the autosomal recessive condition cartilage-hair hypoplasia (CHH). The CHH disease phenotype has some overlap with dyskeratosis congenita, a well-known "telomere disorder." RMRP binds the telomerase reverse transcriptase (catalytic subunit) in some cell lines, raising the possibility that RMRP might play a role in telomere biology.

OBJECTIVE

We sought to determine whether a telomere phenotype is present in immune cells from patients with CHH and explore mechanisms underlying these observations.

METHODS

We assessed proliferative capacity and telomere length using flow-fluorescence in situ hybridization (in situ hybridization and flow cytometry) of primary lymphocytes from patients with CHH, carrier relatives, and control subjects. The role of telomerase holoenzyme components in gene expression and activity were assessed by using quantitative PCR and the telomere repeat amplification protocol from PBMCs and enriched lymphocyte cultures.

RESULTS

Lymphocyte cultures from patients with CHH display growth defects in vitro, which is consistent with an immune deficiency cellular phenotype. Here we show that telomere length and telomerase activity are impaired in primary lymphocyte subsets from patients with CHH. Notably, telomerase activity is affected in a gene dose-dependent manner when comparing heterozygote RMRP carriers with patients with CHH. Telomerase deficiency in patients with CHH is not mediated by abnormal telomerase gene transcript levels relative to those of endogenous genes.

CONCLUSION

These findings suggest that telomere deficiency is implicated in the CHH disease phenotype through an as yet unidentified mechanism.

Decreased telomere length in children with cartilage-hair hypoplasia

BACKGROUND

Cartilage-hair hypoplasia (CHH) is an autosomal recessive chondrodysplasia caused by RMRP (RNA component of mitochondrial RNA processing endoribonuclease) gene mutations. Manifestations include short stature, variable immunodeficiency, anaemia and increased risk of malignancies, all of which have been described also in telomere biology disorders. RMRP interacts with the telomerase RT (TERT) subunit, but the influence of RMRP mutations on telomere length is unknown. We measured relative telomere length (RTL) in patients with CHH, their first-degree relatives and healthy controls and correlated RTL with clinical and laboratory features.

METHODS

The study cohort included 48 patients with CHH with homozygous (n=36) or compound heterozygous RMRP mutations (median age 38.2 years, range 6.0-70.8 years), 86 relatives (74 with a heterozygous RMRP mutation) and 94 unrelated healthy controls. We extracted DNA from peripheral blood, sequenced the RMRP gene and measured RTL by qPCR.

RESULTS

Compared with age-matched and sex-matched healthy controls, median RTL was significantly shorter in patients with CHH (n=40 pairs, 1.05 vs 1.21, p=0.017), but not in mutation carriers (n=48 pairs, 1.16 vs 1.10, p=0.224). RTL correlated significantly with age in RMRP mutation carriers (r=-0.482, p<0.001) and non-carriers (r=-0.498, p<0.001), but not in patients (r=-0.236, p=0.107). In particular children (<18 years) with CHH had shorter telomeres than controls (median RTL 1.12 vs 1.26, p=0.008). In patients with CHH, RTL showed no correlation with genotype, clinical or laboratory characteristics.

CONCLUSIONS

Telomere length was decreased in children with CHH. We found no correlation between RTL and clinical or laboratory parameters.

Dis3l2-Mediated Decay Is a Quality Control Pathway for Noncoding RNAs

Mutations in the 3'-5' exonuclease DIS3L2 are associated with Perlman syndrome and hypersusceptibility to Wilms tumorigenesis. Previously, we found that Dis3l2 specifically recognizes and degrades uridylated pre-let-7 microRNA. However, the widespread relevance of Dis3l2-mediated decay of uridylated substrates remains unknown. Here, we applied an unbiased RNA immunoprecipitation strategy to identify Dis3l2 targets in mouse embryonic stem cells. The disease-associated long noncoding RNA (lncRNA) Rmrp, 7SL, as well as several other Pol III-transcribed noncoding RNAs (ncRNAs) were among the most highly enriched Dis3l2-bound RNAs. 3'-Uridylated Rmrp, 7SL, and small nuclear RNA (snRNA) species were highly stabilized in the cytoplasm of Dis3l2-depleted cells. Deep sequencing analysis of Rmrp 3' ends revealed extensive oligouridylation mainly on transcripts with imprecise ends. We implicate the terminal uridylyl transferases (TUTases) Zcchc6/11 in the uridylation of these ncRNAs, and biochemical reconstitution assays demonstrate the sufficiency of TUTase-Dis3l2 for Rmrp decay. This establishes Dis3l2-mediated decay (DMD) as a quality-control pathway that eliminates aberrant ncRNAs.

Detection of RNA-Protein Interactions in Living Cells with SHAPE

SHAPE-MaP is unique among RNA structure probing strategies in that it both measures flexibility at single-nucleotide resolution and quantifies the uncertainties in these measurements. We report a straightforward analytical framework that incorporates these uncertainties to allow detection of RNA structural differences between any two states, and we use it here to detect RNA-protein interactions in healthy mouse trophoblast stem cells. We validate this approach by analysis of three model cytoplasmic and nuclear ribonucleoprotein complexes, in 2 min in-cell probing experiments. In contrast, data produced by alternative in-cell SHAPE probing methods correlate poorly (r = 0.2) with those generated by SHAPE-MaP and do not yield accurate signals for RNA-protein interactions. We then examine RNA-protein and RNA-substrate interactions in the RNase MRP complex and, by comparing in-cell interaction sites with disease-associated mutations, characterize these noncoding mutations in terms of molecular phenotype. Together, these results reveal that SHAPE-MaP can define true interaction sites and infer RNA functions under native cellular conditions with limited preexisting knowledge of the proteins or RNAs involved.

Cartilage–hair hypoplasia: a spectrum of clinical and radiological findings

Introduction: Cartilage–hair hypoplasia (CHH) is a rare skeletal dysplasia that presents with various degrees of immunodeficiency, short stature, and a susceptibility to malignancies. Individuals with CHH can present with severe combined immunodeficiency or combined immunodeficiency and are at risk for severe and unusual infections irrespective of their laboratory findings. In addition, individuals with CHH can present with variable skeletal abnormalities, mainly involving the metaphysis of long bones. CHH is a rare disease and familiarity with the variable features is crucial for diagnosis.

Methods: We report the clinical, radiological, and genetic findings for 5 patients with proven diagnoses of CHH.

Results: In this study we describe a cohort of patients with CHH and present their clinical findings and progressions. In addition, we present the radiological images and the immunological investigations that were done in these patients. Although all the patients in our cohort had poor cellular immunity, they had a variable clinical course. Three out of 5 patients received a bone marrow transplant (BMT) and 2 out of 5 died at an early age (1 after BMT). Those who had poor humoral function had a worse prognosis compared with those with good humoral function. The skeletal findings were characteristic for CHH.

Conclusion: CHH is a disease with a variable presentation. Clinicians should be aware of the characteristic skeletal and immunological findings to identify the disease as early as possible.

Statement of novelty: We present novel clinical and radiological findings in patients with variable RMRP gene mutations.

Ribosomopathies: Global process, tissue specific defects

Disruptions in ribosomal biogenesis would be expected to have global and in fact lethal effects on a developing organism. However, mutations in ribosomal protein genes have been shown in to exhibit tissue specific defects. This seemingly contradictory finding - that globally expressed genes thought to play fundamental housekeeping functions can in fact exhibit tissue and cell type specific functions - provides new insight into roles for ribosomes, the protein translational machinery of the cell, in regulating normal development and disease. Furthermore it illustrates the surprisingly dynamic nature of processes regulating cell type specific protein translation. In this review, we discuss our current knowledge of a variety of ribosomal protein mutations associated with human disease, and models to better understand the molecular mechanisms associated with each. We use specific examples to emphasize both the similarities and differences between the effects of various human ribosomal protein mutations. Finally, we discuss areas of future study that are needed to further our understanding of the role of ribosome biogenesis in normal development, and possible approaches that can be used to treat debilitating ribosomopathy diseases.

Cartilage hair hypoplasia: characteristics and orthopaedic manifestations

PURPOSE

Cartilage hair hypoplasia (CHH) is a rare metaphyseal chondrodysplasia characterized by short stature and short limbs, found primarily in Amish and Finnish populations. Cartilage hair hypoplasia is caused by mutations in the RMRP gene located on chromosome 9p13.3. The disorder has several characteristic orthopaedic manifestations, including joint laxity, limited elbow extension, ankle varus, and genu varum. Immunodeficiency is of concern in most cases. Although patients exhibit orthopaedic problems, the orthopaedic literature on CHH patients is scant at best. The objective of this study was to characterize the orthopaedic manifestations of CHH based on the authors' unique access to the largest collection of CHH patients ever reported.

METHODS

The authors examined charts and/or radiographs in 135 cases of CHH. We analyzed the orthopaedic manifestations to better characterize and further understand the orthopaedic surgeon's role in this disorder. In addition to describing the clinical characteristics, we report on our surgical experience in caring for CHH patients.

RESULTS

Genu varum, with or without knee pain, is the most common reason a patient with CHH will seek orthopaedic consultation. Of the cases reviewed, 32 patients had undergone surgery, most commonly to correct genu varum.

CONCLUSION

This paper characterizes the orthopaedic manifestations of CHH. Characterizing this condition in the orthopaedic literature will likely assist orthopaedic surgeons in establishing a correct diagnosis and appreciating the orthopaedic manifestations. It is important that the accompanying medical conditions are appreciated and evaluated.

Variable phenotype of severe immunodeficiencies associated with RMRP gene mutations

PURPOSE

Mutations in RMRP primarily give rise to Cartilage Hair Hypoplasia (CHH), a highly diverse skeletal disorder which can be associated with severe immunodeficiency. Increased availability of RMRP mutation screening has uncovered a number of infants with significant immunodeficiency but only mild or absent skeletal features. We surveyed the clinical and immunological phenotype of children who have undergone allogeneic haematopoietic stem cell transplantation for this condition in the UK.

METHODS

Thirteen patients with confirmed RMRP mutations underwent allogeneic stem cell transplantation (SCT) at two nationally commissioned centres using a variety of donors and conditioning regimens. Records were retrospectively reviewed.

RESULTS

Median time from clinical presentation to diagnosis was 12 months (range 1 to 276 months), with three infants diagnosed with severe combined immunodeficiency (SCID) without radiographical manifestations of CHH. A total of 17 allogeneic procedures were performed on 13 patients including two stem-cell top-ups. The median age at transplant was 32.4 months (range 1.5 to 125 months). Of the eleven surviving patients, median follow-up was 50 months (range 21.6 to 168 months).

CONCLUSIONS

RMRP mutations can cause short stature and significant immunodeficiency which can be corrected by allogeneic SCT and the diagnosis should be considered even in the absence of skeletal manifestations.

RNase MRP cleaves pre-tRNASer-Met in the tRNA maturation pathway

Ribonuclease mitochondrial RNA processing (RNase MRP) is a multifunctional ribonucleoprotein (RNP) complex that is involved in the maturation of various types of RNA including ribosomal RNA. RNase MRP consists of a potential catalytic RNA and several protein components, all of which are required for cell viability. We show here that the temperature-sensitive mutant of rmp1, the gene for a unique protein component of RNase MRP, accumulates the dimeric tRNA precursor, pre-tRNA^Ser-Met. To examine whether RNase MRP mediates tRNA maturation, we purified the RNase MRP holoenzyme from the fission yeast Schizosaccharomyces pombe and found that the enzyme directly and selectively cleaves pre-tRNA^Ser-Met, suggesting that RNase MRP participates in the maturation of specific tRNA in vivo. In addition, mass spectrometry–based ribonucleoproteomic analysis demonstrated that this RNase MRP consists of one RNA molecule and 11 protein components, including a previously unknown component Rpl701. Notably, limited nucleolysis of RNase MRP generated an active catalytic core consisting of partial mrp1 RNA fragments, which constitute “Domain 1” in the secondary structure of RNase MRP, and 8 proteins. Thus, the present study provides new insight into the structure and function of RNase MRP.

Diverse diseases from a ubiquitous process: the ribosomopathy paradox

Collectively, the ribosomopathies are caused by defects in ribosome biogenesis. Although these disorders encompass deficiencies in a ubiquitous and fundamental process, the clinical manifestations are extremely variable and typically display tissue specificity. Research into this paradox has offered fascinating new insights into the role of the ribosome in the regulation of mRNA translation, cell cycle control, and signaling pathways involving TP53, MYC and mTOR. Several common features of ribosomopathies such as small stature, cancer predisposition, and hematological defects, point to how these diverse diseases may be related at a molecular level.

Phenotypic variations of cartilage hair hypoplasia: granulomatous skin inflammation and severe T cell immunodeficiency as initial clinical presentation in otherwise well child with short stature

We report a child with short stature since birth who was otherwise well, presenting at 2.8 years with progressive granulomatous skin lesions when diagnosed with severe T cell immunodeficiency. When previously investigated for short stature, and at the time of current investigations, she had no radiological skeletal features characteristics for cartilage hair hypoplasia, but we found a disease causing RMRP (RNase mitochondrial RNA processing endoribonuclease) gene mutation. Whilst search for HLA matched unrelated donor for haematopoietic stem cell transplantation (HSCT) was underway, she developed rapidly progressive EBV-related lymphoproliferative disorder requiring laparotomy and small bowel resection, and was treated with anti-B cell monoclonal antibody and eventually curative allogeneic HSCT. Screening for RMRP gene mutations should be part of immunological evaluation of patients with ‘severe and/or combined’ T cell immunodeficiency of unknown origin, especially when associated with short stature and regardless of presence or absence of radiological skeletal features.

Small RNAs derived from lncRNA RNase MRP have gene-silencing activity relevant to human cartilage-hair hypoplasia

Post-transcriptional processing of some long non-coding RNAs (lncRNAs) reveals that they are a source of miRNAs. We show that the 268-nt non-coding RNA component of mitochondrial RNA processing endoribonuclease, (RNase MRP), is the source of at least two short (∼20 nt) RNAs designated RMRP-S1 and RMRP-S2, which function as miRNAs. Point mutations in RNase MRP cause human cartilage-hair hypoplasia (CHH), and several disease-causing mutations map to RMRP-S1 and -S2. SHAPE chemical probing identified two alternative secondary structures altered by disease mutations. RMRP-S1 and -S2 are significantly reduced in two fibroblast cell lines and a B-cell line derived from CHH patients. Tests of gene regulatory activity of RMRP-S1 and -S2 identified over 900 genes that were significantly regulated, of which over 75% were down-regulated, and 90% contained target sites with seed complements of RMRP-S1 and -S2 predominantly in their 3' UTRs. Pathway analysis identified regulated genes that function in skeletal development, hair development and hematopoietic cell differentiation including PTCH2 and SOX4 among others, linked to major CHH phenotypes. Also, genes associated with alternative RNA splicing, cell proliferation and differentiation were highly targeted. Therefore, alterations RMRP-S1 and -S2, caused by point mutations in RMRP, are strongly implicated in the molecular mechanism of CHH.

Reduced thymic output, cell cycle abnormalities, and increased apoptosis of T lymphocytes in patients with cartilage-hair hypoplasia

BACKGROUND

Cartilage-hair hypoplasia (CHH) is characterized by metaphyseal dysplasia, bone marrow failure, increased risk of malignancies, and a variable degree of immunodeficiency. CHH is caused by mutations in the RNA component of the mitochondrial RNA processing (RMRP) endoribonuclease gene, which is involved in ribosomal assembly, telomere function, and cell cycle control.

OBJECTIVES

We aimed to define thymic output and characterize immune function in a cohort of patients with molecularly defined CHH with and without associated clinical immunodeficiency.

METHODS

We studied the distribution of B and T lymphocytes (including recent thymic emigrants), in vitro lymphocyte proliferation, cell cycle, and apoptosis in 18 patients with CHH compared with controls.

RESULTS

Patients with CHH have a markedly reduced number of recent thymic emigrants, and their peripheral T cells show defects in cell cycle control and display increased apoptosis, resulting in poor proliferation on activation.

CONCLUSION

These data confirm that RMRP mutations result in significant defects of cell-mediated immunity and provide a link between the cellular phenotype and the immunodeficiency in CHH.

Whole-exome re-sequencing in a family quartet identifies POP1 mutations as the cause of a novel skeletal dysplasia

Recent advances in DNA sequencing have enabled mapping of genes for monogenic traits in families with small pedigrees and even in unrelated cases. We report the identification of disease-causing mutations in a rare, severe, skeletal dysplasia, studying a family of two healthy unrelated parents and two affected children using whole-exome sequencing. The two affected daughters have clinical and radiographic features suggestive of anauxetic dysplasia (OMIM 607095), a rare form of dwarfism caused by mutations of RMRP. However, mutations of RMRP were excluded in this family by direct sequencing. Our studies identified two novel compound heterozygous loss-of-function mutations in POP1, which encodes a core component of the RNase mitochondrial RNA processing (RNase MRP) complex that directly interacts with the RMRP RNA domains that are affected in anauxetic dysplasia. We demonstrate that these mutations impair the integrity and activity of this complex and that they impair cell proliferation, providing likely molecular and cellular mechanisms by which POP1 mutations cause this severe skeletal dysplasia.

Skeletal dysplasias are a group of genetic disorders affecting skeletal development that cause deficiencies and deformities of the limbs and spine, dwarfism, or abnormal bone strength. Skeletal dysplasias are usually inherited as monogenic Mendelian traits or occur as a result of de novo mutations. We report identification of mutations in human POP1 gene as the cause of a severe novel skeletal dysplasia. Molecular analyses presented in our work provide an important link between the pathogenesis of the disease and basic cellular processes including RNA processing and the cell cycle. We posit that our work will also have an immediate impact on assessment and counselling of novel cases of severe short stature.

An infant with cartilage-hair hypoplasia due to a novel homozygous mutation in the promoter region of the RMRP gene associated with chondrodysplasia and severe immunodeficiency

Cartilage-hair hypoplasia (CHH) is a rare autosomal-recessive disorder characterized by short-limbed dwarfism, sparse hair, and immune deficiency. It is caused by mutations in the RMRP gene, which encodes the RNA component of the mitochondrial RNA-processing ribonuclease (RNase MRP). Several mutations have been identified in its promoter region or transcribed sequence. However, homozygous mutations in the promoter region have been only reported in a patient with primary immunodeficiency without other features of CHH. We report on a Thai girl who first presented with chronic diarrhea, recurrent pneumonia, and severe failure to thrive, without apparently disproportionate dwarfism. The diagnosis of CHH was made after the severe wasting was corrected, and disproportionate growth became noticeable. The patient had the typical features of CHH, including sparse hair and metaphyseal abnormalities. The immunologic profiles were consistent with combined immune deficiency. Mutation analysis identified a novel homozygous mutation, g.-19_-25 dupACTACTC, in the promoter region of the RMRP gene. Identification of the mutation enabled us to provide a prenatal diagnosis in the subsequent pregnancy. This patient is the first CHH case with the characteristic features due to the homozygous mutation in the promoter region of the RMRP gene. The finding of severe immunodeficiency supports that promoter mutations markedly disrupt mRNA cleavage function, which causes cell-cycle impairment.

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.

Diamond Blackfan anemia

Mutations affecting genes encoding ribosomal proteins cause Diamond Blackfan anemia (DBA), a rare congenital syndrome associated with physical anomalies, short stature, red cell aplasia, and an increased risk of malignancy. p53 activation has been identified as a key component in the pathophysiology of DBA after cellular and molecular studies of knockdown cellular and animal models of DBA and other disorders affecting ribosomal assembly or function. Other potential mechanisms that warrant further investigation include impaired translation as the result of ribosomal insufficiency, which may be ameliorated by leucine supplementation, and alternative splicing leading to reduced expression of a cytoplasmic heme exporter, the human homolog of the receptor for feline leukemia virus C (FVLCR). However, the molecular basis for the characteristic steroid responsiveness of the erythroid failure in DBA remains unknown. This review explores the clinical and therapeutic implications of the current state of knowledge and delineates important but as-yet-unanswered questions.

Do ribosomopathies explain some cases of common variable immunodeficiency?

The considerable clinical heterogeneity of patients with common variable immunodeficiency disorders (CVID) shares some similarity with bone-marrow failure disorders such as Diamond-Blackfan anaemia (DBA) and Shwachman-Diamond syndrome (SDS), now recognized as defects in ribosome biogenesis or ribosomopathies. The recognition of a patient with DBA who subsequently developed CVID lends support to our previous finding of a heterozygous mutation in the SBDS gene of SBDS in another CVID patient, suggesting that ribosome biogenesis defects are responsible for a subset of CVID. Genetic defects in the ribosomal translational machinery responsible for various bone marrow failure syndromes are recognized readily when they manifest in children, but diagnosing these in adults presenting with complex phenotypes and hypogammaglobulinaemia can be a challenge. In this perspective paper, we discuss our clinical experience in CVID patients with ribosomopathies, and review the immunological abnormalities in other conditions associated with ribosomal dysfunction. With genetic testing available for various bone marrow failure syndromes, our hypothesis that ribosomal abnormalities may be present in patients with CVID could be proved in future studies by testing for mutations in specific ribosomal genes. New knowledge might then be translated into novel therapeutic strategies for patients in this group of immunodeficiency disorders.

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.