Immunologic and clinical features of 25 Amish patients with RMRP 70 A-->G cartilage hair hypoplasia

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".

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.

Infections in Inborn Errors of Immunity with Combined Immune Deficiency: A Review

Enhanced susceptibility to microbes, often resulting in severe, intractable and frequent infections due to usually innocuous organisms at uncommon sites, is the most striking feature in individuals with an inborn error of immunity. In this narrative review, based on the International Union of Immunological Societies' 2022 (IUIS 2022) Update on phenotypic classification of human inborn errors of immunity, the focus is on commonly encountered Combined Immunodeficiency Disorders (CIDs) with susceptibility to infections. Combined immune deficiency disorders are usually commensurate with survival beyond infancy unlike Severe Combined Immune Deficiency (SCID) and are often associated with clinical features of a syndromic nature. Defective humoral and cellular immune responses result in susceptibility to a broad range of microbial infections. Although disease onset is usually in early childhood, mild defects may present in late childhood or even in adulthood. A precise diagnosis is imperative not only for determining management strategies, but also for providing accurate genetic counseling, including prenatal diagnosis, and also in deciding empiric treatment of infections upfront before investigation reports are available.

Newborn Screening for Severe Combined Immunodeficiency: 10-Year Experience at a Single Referral Center (2009-2018)

In 2008, newborn screening (NBS) for severe combined immunodeficiency (SCID) began as a pilot study in Wisconsin and has recently been added to every state's newborn screen panel. The incidence of SCID is estimated at 1 per 58,000 births which may suggest infrequent NBS SCID screen positive results in states with low annual birth rates. In this study, we report our center's experience with NBS positive SCID screen referrals over a 10-year period. A total of 68 full-term newborns were referred to our center for confirmatory testing. Of these referrals, 50% were false positives, 12% were SCID diagnoses, 20% syndromic T cell lymphopenia (TCL) disorders, and 18% non-SCID, non-syndromic TCL. Through collaboration with our newborn screening lab, second-tier targeted gene sequencing was performed for newborns with SCID screen positive results from communities with known founder pathogenic variants and provided rapid genetic confirmation of SCID and non-SCID TCL disorders. Despite extensive genetic testing, two of the eight (25%) identified newborns with SCID diagnoses lacked a definable genetic defect. Additionally, our referrals included ten newborns who were otherwise healthy newborns with idiopathic TCL and varied CD3+ T cell number longitudinal trajectories. Collectively, referrals to our single site over a 10-year period describe a broad spectrum of medically actionable and idiopathic TCL disorders which highlight the importance of clinical immunology expertise in all states, demonstrate efficiencies and challenges for second-tier genetic testing, and further emphasize the need to development standardized evaluation algorithms for non-SCID TCL.

The Safety and Efficacy of Live Viral Vaccines in Patients With Cartilage-Hair Hypoplasia

Background

Live viral vaccines are generally contraindicated in patients with combined immunodeficiency including cartilage-hair hypoplasia (CHH); however, they may be tolerated in milder syndromes. We evaluated the safety and efficacy of live viral vaccines in patients with CHH.

Methods

We analyzed hospital and immunization records of 104 patients with CHH and measured serum antibodies to measles, mumps, rubella, and varicella zoster virus (VZV) in all patients who agreed to blood sampling (n = 50). We conducted a clinical trial (ClinicalTrials.gov identifier: NCT02383797) of live VZV vaccine on five subjects with CHH who lacked varicella history, had no clinical symptoms of immunodeficiency, and were seronegative for VZV; humoral and cellular immunologic responses were assessed post-immunization.

Results

A large proportion of patients have been immunized with live viral vaccines, including measles-mumps-rubella (MMR) (n = 40, 38%) and VZV (n = 10, 10%) vaccines, with no serious adverse events. Of the 50 patients tested for antibodies, previous immunization has been documented with MMR (n = 22), rubella (n = 2) and measles (n = 1) vaccines. Patients with CHH demonstrated seropositivity rates of 96%/75%/91% to measles, mumps and rubella, respectively, measured at a medium of 24 years post-immunization. Clinical trial participants developed humoral and cellular responses to VZV vaccine. One trial participant developed post-immunization rash and knee swelling, both resolved without treatment.

Conclusion

No serious adverse events have been recorded after immunization with live viral vaccines in Finnish patients with CHH. Patients generate humoral and cellular immune response to live viral vaccines. Immunization with live vaccines may be considered in selected CHH patients with no or clinically mild immunodeficiency.

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.

Gastrointestinal Disorders Associated with Primary Immunodeficiency Diseases

There are now 354 inborn errors of immunity (primary immunodeficiency diseases (PIDDs)) with 344 distinct molecular etiologies reported according to the International Union of Immunological Sciences (IUIS) (Clin Gastroenterol Hepatol 11: p. 1050-63, 2013, Semin Gastrointest Dis 8: p. 22-32, 1997, J Clin Immunol 38: p. 96-128, 2018). Using the IUIS document as a reference and cross-checking PubMed ( www.ncbi.nlm.nih.pubmed.gov ), we found that approximately one third of the 354 diseases of impaired immunity have a gastrointestinal component [J Clin Immunol 38: p. 96-128, 2018]. Often, the gastrointestinal symptomatology and pathology is the heralding sign of a PIDD; therefore, it is important to recognize patterns of disease which may manifest along the gastrointestinal tract as a more global derangement of immune function. As such, holistic consideration of immunity is warranted in patients with clinically significant gastrointestinal disease. Here, we discuss the manifold presentations and GI-specific complications of PIDDs which could lead patients to seek advice from a variety of clinician specialists. Often, patients with these medical problems will engage general pediatricians, surgeons, gastroenterologists, rheumatologists, and clinical immunologists among others. Following delineation of the presenting concern, accurate and often molecular diagnosis is imperative and a multi-disciplinary approach warranted for optimal management. In this review, we will summarize the current state of understanding of PIDD gastrointestinal disease involvement. We will do so by focusing upon gastrointestinal disease categories (i.e., inflammatory, diarrhea, nodular lymphoid hyperplasia, liver/biliary tract, structural disease, and oncologic disease) with an intent to aid the healthcare provider who may encounter a patient with an as-yet undiagnosed PIDD who presents initially with a gastrointestinal symptom, sign, or problem.

Abnormal Newborn Screening Follow-up for Severe Combined Immunodeficiency in an Amish Cohort with Cartilage-Hair Hypoplasia

Cartilage-hair hypoplasia (CHH) is an autosomal recessive, short limb skeletal dysplasia with a variable immunologic phenotype. The spectrum of immune function ranges from clinically normal to severe combined immunodeficiency (SCID). Multiple studies have shown that abnormal immune parameters may not predict severe outcomes. Newborn screening (NBS) using T cell receptor excision circle (TREC) assay can now effectively identify infants with severe T cell deficiency who are at risk for SCID. NBS has allowed for cost-effective identification of patients with SCID and improved outcomes with hematopoietic stem cell transplant (HSCT). Ohio reports two abnormal TREC results: decreased and absent TREC. This study evaluated the laboratory and clinical differences in eight Amish patients with CHH with an abnormal TREC result on the NBS. There were four patients with absent TREC and four patients with decreased TREC. The absent TREC patients had lower CD3, CD4, naïve CD4, CD8 cells, and phytohemagglutinin (PHA)-induced lymphocyte proliferation. Three patients with absent TREC were diagnosed with SCID and two underwent successful HSCT. Patients with absent TREC experienced more CHH-related morbidity including anemia requiring transfusion, Hirschsprung's disease, and failure to thrive. No patients with decreased TREC required HSCT. Our study indicates that CHH patients with absent TREC tend to have more severe immunological and clinical phenotype than patients with decreased TREC. Confirmation of these trends in a larger group would guide providers and parents in a timely referral for HSCT, or cost-effective surveillance monitoring of children with a life-threatening illness.

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.

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.

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.

Vaccine-Preventable Diseases Requiring Hospitalization

BACKGROUND

Plain children often have lower immunization rates than non-Plain children. Penn State Health Children's Hospital is a tertiary medical center with large nearby Plain (Amish and Mennonite) communities. We sought to describe the characteristics of children hospitalized with vaccine-preventable diseases (VPDs). We hypothesized that Amish children would have a higher risk of VPDs than non-Amish children.

METHODS

International Classification of Diseases, Ninth Revision codes were used to identify patients <18 years diagnosed with a VPD from January 1, 2005, to December 31, 2015, at Penn State Children's Hospital. Demographic information, immunization status, and outcomes were obtained from medical records. By using the number of children in our primary service area, we calculated the risk of VPD requiring hospitalization for Amish and non-Amish children. We assessed the relationship between Plain affiliation and vaccination status by using the Pearson correlation coefficient.

RESULTS

There were 215 children with 221 VPDs. Most occurred in non-Plain children: 179 of 221 (81%). Except for pneumococcal infections, VPD occurred mostly in unvaccinated or immunocompromised children, regardless of Plain affiliation. There were 15 Haemophilus influenzae type b and 5 tetanus infections that occurred in children with an unvaccinated or unknown vaccination status. The risk of a VPD requiring hospitalization was greater for Amish than for non-Plain children (risk ratio: 2.67 [95% confidence interval: 1.87-3.82]). There was a strong correlation between Plain affiliation and lack of vaccination (r = -0.63, P < .01).

CONCLUSIONS

Amish children had an increased risk of a VPD requiring hospitalization than non-Plain children. With the exception of those with pneumococcal disease, most vaccinated children hospitalized with a VPD were immunocompromised.

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.

Cartilage Hair Hypoplasia: Two Unrelated Cases with g.70 A > G Mutation in RMRP Gene

Cartilage-hair hypoplasia is an autosomal recessive disorder, characterized by short stature, metaphyseal dysplasia, hypotrichosis and immunodeficiency. More than 90 different biallelic mutations in RMRP gene have been identified to cause this condition. Three cases previously reported from India showed novel mutations in RMRP gene. The authors report two unrelated cases with the more common g.70A > G mutation, stressing the need to screen for this mutation in Indian population having features of cartilage-hair hypoplasia.

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.

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.

Newborn screening for primary immunodeficiencies: beyond SCID and XLA

Primary immunodeficiencies (PID) encompass more than 250 disease entities, including phagocytic disorders, complement deficiencies, T cell defects, and antibody deficiencies. While differing in clinical severity, early diagnosis and treatment is of considerable importance for all forms of PID to prevent organ damage and life-threatening infections. During the past few years, neonatal screening assays have been developed to detect diseases hallmarked by the absence of T or B lymphocytes, classically seen in severe combined immunodeficiencies (SCID) and X-linked agammaglobulinemia (XLA). As described in this review, a reduction or lack of T and B cells in newborns is also frequently found in several other forms of PID, requiring supplemental investigation and involving the development of additional technical platforms in order to help classify abnormal screening results.

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.

Educational paper: syndromic forms of primary immunodeficiency

The syndromic primary immunodeficiencies are disorders in which not only the immune system but also other organ systems are affected. Other features most commonly involve the ectodermal, skeletal, nervous, and gastrointestinal systems. Key in identifying syndromic immunodeficiencies is the awareness that increased susceptibility to infections or immune dysregulation in a patient known to have other symptoms or special features may hint at an underlying genetic syndrome. Because the extraimmune clinical features can be highly variable, it is more difficult establishing the correct diagnosis. Nevertheless, correct diagnosis at an early age is important because of the possible treatment options. Therefore, diagnostic work-up is best performed in a center with extensive expertise in this field, having immunologists and clinical geneticists, as well as adequate support from a specialized laboratory at hand. This paper provides the general pediatrician with the main clinical features that are crucial for the recognition of these syndromes.

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.

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.