BMS1 is mutated in aplasia cutis congenita

Aplasia Cutis Congenita Pathomechanisms Reveal Key Regulators of Skin and Skin Appendage Morphogenesis

Aplasia cutis congenita (ACC) manifests at birth as a defect of the scalp skin. New findings answer 2 longstanding questions: why ACC forms and why it affects mainly the midline scalp skin. Dominant-negative mutations in the genes KCTD1 or KCTD15 cause ACC owing to loss of function of KCTD1/KCTD15 complexes in cranial neural crest cells (NCCs), which normally form midline cranial suture mesenchymal cells that express keratinocyte growth factors. Loss of KCTD1/KCTD15 function in NCCs impairs the formation of normal midline cranial sutures and, consequently, the overlying skin, resulting in ACC. Moreover, KCTD1/KCTD15 complexes in keratinocytes regulate skin appendage morphogenesis.

Membranous aplasia cutis congenita: A rare case report highlighting clinical presentation, genetic insights, and the need for comprehensive evaluation

Introduction

Membranous aplasia cutis congenita (MACC) is the most common clinical subtype of aplasia cutis congenita (ACC). It is typified by a localized skin lesion devoid of hair and features a membranous surface. While most MACC individuals do not present with concurrent abnormalities, it can sometimes co-occur with additional physical anomalies and various malformation syndromes. Moreover, the underlying causes of MACC remain elusive.

Case presentation

We describe a case of a 6-month-old female infant diagnosed with MACC. The patient presented with a midline skin lesion on the occipital scalp, characterized by a glistening surface and a hair collar sign. Dermoscopic examination revealed specific features, including translucency, telangiectasia, and hypertrichosis. The infant had a history of patent foramen ovale, and further examination uncovered an asymptomatic ventricular septal defect. Whole exome sequencing revealed 20 gene variants relevant to the clinical phenotype of the patient, suggesting a possible association with MACC.

Conclusion

MACC is a rare and underreported condition, primarily diagnosed based on its distinctive clinical features. It is imperative to emphasize the significance of thorough evaluations in MACC patients, encompassing developmental, cardiac, neurological, and genetic assessments to facilitate early detection and the exclusion of potentially life-threatening comorbidities. Importantly, genetic characterization, as demonstrated in this case, contributes to our understanding of MACC's etiology and highlights the need for further research in this field.

An Extensive Case of Aplasia Cutis Congenita

Aplasia cutis congenita (ACC) is a rare connective tissue disorder that affects the epidermis, dermis, and subcutaneous tissue. Lesions can be small and benign or, in some cases, large and extensive. The lesions can be covered by a thin membrane, and depending on their location, they can make the underlying tissue vulnerable to damage or infection. This case report focuses on a term male infant born with extensive scalp defects and later diagnosed with ACC. The study will discuss the pathophysiology and classification of this disorder. This will provide clinicians with a recommended approach for the initial management of infants diagnosed with ACC in their practice.

Aplasia cutis congenita type VII of the lower extremity: a favourable disease course with minimal conservative treatment

Aplasia cutis congenita (ACC) is a group of rare heterogeneous disorders characterised by absent areas of skin at birth. The majority of cases involve the scalp region. ACC limited to one lower limb is extremely rare. We report an usual case of ACC limited to the left thigh of which healing occurred in utero. The case was managed conservatively and the disease course has been favourable with no limitations in limb function and an entirely normal development. Most cases of ACC are self-healing, justifying a conservative approach. This holds further true for ACC limited to one lower limb where the majority of cases reported to date show a favourable disease course with minimal conservative treatment.

Potentials of ribosomopathy gene as pharmaceutical targets for cancer treatment

Ribosomopathies encompass a spectrum of disorders arising from impaired ribosome biogenesis and reduced functionality. Mutation or dysexpression of the genes that disturb any finely regulated steps of ribosome biogenesis can result in different types of ribosomopathies in clinic, collectively known as ribosomopathy genes. Emerging data suggest that ribosomopathy patients exhibit a significantly heightened susceptibility to cancer. Abnormal ribosome biogenesis and dysregulation of some ribosomopathy genes have also been found to be intimately associated with cancer development. The correlation between ribosome biogenesis or ribosomopathy and the development of malignancies has been well established. This work aims to review the recent advances in the research of ribosomopathy genes among human cancers and meanwhile, to excavate the potential role of these genes, which have not or rarely been reported in cancer, in the disease development across cancers. We plan to establish a theoretical framework between the ribosomopathy gene and cancer development, to further facilitate the potential of these genes as diagnostic biomarker as well as pharmaceutical targets for cancer treatment.

Large aplasia cutis congenita of the vertex conservative management

BACKGROUND

Aplasia cutis congenita (ACC) of the vertex with bone defect is a rare and begnin anomaly that can involve the epidermis, dermis, and subcutaneous tissues of the scalp with significant bone defect Bajpai and Pal (J Pediatr Surg 38(2):e4, 2003). When associated with skull defect, this rare malformation carries the risk of severe complications such as rupture of the superior sagittal sinus or infections.

METHODS AND RESULTS

We report a case of aplasia cutis congenita of the scalp with skull defect measuring 9 × 10 cm and an exposed sagittal sinus in a newborn. Both conservative and surgical methods have been proposed to treat this condition. In our case, conservative treatment was planned led to complete epithelization and the patient was healing well at 5 years of follow-up.

CONCLUSIONS

ACC of the vertex with a large scalp defects present a management dilemma Rocha et al. (Clin Case Rep 3(10):841-4, 2015). Based on a review of the literature, we report this case to demonstrate that even for the largest skin and bone defects, an initial conservative approach may allow for complete wound closure without the need for early surgical intervention.

KCTD1/KCTD15 complexes control ectodermal and neural crest cell functions, and their impairment causes aplasia cutis

Aplasia cutis congenita (ACC) is a congenital epidermal defect of the midline scalp and has been proposed to be due to a primary keratinocyte abnormality. Why it forms mainly at this anatomic site has remained a long-standing enigma. KCTD1 mutations cause ACC, ectodermal abnormalities, and kidney fibrosis, whereas KCTD15 mutations cause ACC and cardiac outflow tract abnormalities. Here, we found that KCTD1 and KCTD15 can form multimeric complexes and can compensate for each other's loss and that disease mutations are dominant negative, resulting in lack of KCTD1/KCTD15 function. We demonstrated that KCTD15 is critical for cardiac outflow tract development, whereas KCTD1 regulates distal nephron function. Combined inactivation of KCTD1/KCTD15 in keratinocytes resulted in abnormal skin appendages but not in ACC. Instead, KCTD1/KCTD15 inactivation in neural crest cells resulted in ACC linked to midline skull defects, demonstrating that ACC is not caused by a primary defect in keratinocytes but is a secondary consequence of impaired cranial neural crest cells, giving rise to midline cranial suture cells that express keratinocyte-promoting growth factors. Our findings explain the clinical observations in patients with KCTD1 versus KCTD15 mutations, establish KCTD1/KCTD15 complexes as critical regulators of ectodermal and neural crest cell functions, and define ACC as a neurocristopathy.

SCALP syndrome with a germline heterozygous DOCK6 mutation and somatic mosaic NRAS Q61R mutation

We present a case of SCALP syndrome, which was diagnosed in a male infant with the characteristic findings of sebaceous nevi, central nervous system malformations, aplasia cutis congenita, limbal dermoid, and giant congenital melanocytic nevi, or pigmented nevi. We identified a germline compound heterozygous DOCK6 mutation and a somatic mosaic NRAS Q61R mutation in the giant congenital melanocytic nevus. This report will increase clinician awareness of SCALP syndrome and augment the literature in characterizing this rare syndrome, including its genetic background.

Epidemiology of aplasia cutis congenita: A population-based study in Europe

BACKGROUND

Aplasia cutis congenita (ACC) is a rare congenital anomaly characterized by localized or widespread absence of skin at birth, mainly affecting the scalp. Most information about ACC exists as individual case reports and medium-sized studies.

OBJECTIVES

This study aimed to investigate the epidemiology of ACC, using data from a large European network of population-based registries for congenital anomalies (EUROCAT).

METHODS

Twenty-eight EUROCAT population-based registries in 16 European countries were involved. Poisson regression models were exploited to estimate the overall and live birth prevalence, to test time trends in prevalence between four 5-year periods and to evaluate the impact of the change of coding for ACC from the unspecific ICD9-BPA code to the specific ICD10 code. Proportions of ACC cases associated with other anomalies were reported.

RESULTS

Five hundred cases were identified in the period 1998-2017 (prevalence: 5.10 per 100,000 births). Prevalence across 5-year periods did not differ significantly and no significant differences were evident due to the change from ICD9 to ICD10 in ACC coding. Heterogeneity in prevalence was observed across registries. The scalp was the most common site for ACC (96.4%) and associated congenital anomalies were present in 33.8% of cases. Patau and Adams-Oliver syndromes were the most frequent among the associated chromosomal anomalies (88.3%) and the associated genetic syndromes (57.7%), respectively. 16% of cases were associated with limb anomalies and 15.4% with congenital heart defects. A family history of ACC was found in 2% of cases.

CONCLUSION

To our knowledge, this is the only population-based study on ACC. The EUROCAT methodologies provide reliable prevalence estimates and proportions of associated anomalies.

Type VII Aplasia Cutis Congenita in Neonates Related to Maternal HBV Infection? Case Report and Literature Review

Aplasia cutis congenita (ACC) is a rare disease with an unclear pathogenic mechanism. ACC has been suggested to result from the disrupted development or degeneration of skin in the uterus. This study describes two cases that may have underlying pathogenic cause that have not been previously reported. Two neonates who were admitted to the neonatal intensive care unit due to "skin lesions on the limbs" without other deformities or complications were diagnosed with type VII ACC by dermatologist. The mothers showed positivity for hepatitis B virus (HBV) surface antigen and elevated level of HBV DNA copies, which may be related to ACC. But this association could be a coincidence. Both neonates were treated with antibacterial dressings and achieved satisfactory healing.

Diagnosis, Management, and Outcome of Bart's Syndrome Observed in a Sub-Saharan African Country (Senegal, Dakar): 2 Case Reports

Introduction

Bart's syndrome is an uncommon inherited congenital disorder associating congenital cutaneous aplasia of the extremities and inherited epidermolysis bullosa. Bilateral and symmetrical involvement of the limbs is exceptionally described on black skin. In most cases, the diagnosis is clinical; however, the management remains very difficult and the extended forms are a real therapeutic challenge. We report 2 cases of Bart's syndrome observed in a sub-Saharan African country (Senegal, Dakar).

Case Presentation

It was about 2 premature female and male newborns. On physical examination, the girl presented with a total absence of skin on the limbs, associated with cutaneous detachment of the trunk representing a detached and detachable skin surface of 46%; the boy underwent a total absence of skin of more than 50% of the skin surface. The diagnosis of Bart's syndrome was set based on the typical clinical aspect. The blood count and CRP were normal for the girl whereas it revealed some disorders for the boy. The 2 newborns were urgently admitted to an incubator, and the intensive care was started with hyperhydration, anti-staphylococcal prophylaxis, and daily dermatological care with antiseptic baths and fatty dressings.

Conclusion

Bart's syndrome is an uncommon genodermatosis characterized by a clinical triad associating congenital cutaneous aplasia of the extremities, inherited epidermolysis bullosa suspected in the presence of bubbles, and areas of cutaneous fragility and nail deformity. All types of which can be associated with this syndrome. The easy clinical diagnosis but the difficult management encumber the vital prognosis of our cases.

A successful case of preimplantation genetic testing for monogenic disorder for aplasia cutis congenita

BACKGROUND

Aplasia cutis congenita (ACC), also called congenital cutaneous hypoplasia, is a serious disease in newborns. Children with ACC often die due to wound infections and bleeding. How the incidence of ACC can be reduced is a question that needs to be solved urgently.

CASE REPORT

We reported a mother who had delivered two children with ACC, both of whom were diagnosed with ACC type VI, skin defects, limb deformities, and congenital heart malformations. One infant died a few days after birth, and another died in utero in the second trimester. Genetic testing in both children showed a heterozygous mutation in the ITGB4 gene [17q25 exon 8, c. 794 dupC, (p. Ala266fs) and exon 15, c. 1860G > A]. The mother later successfully gave birth to a healthy baby using Preimplantation Genetic Testing for Monogenic disorders(PGD-M).

CONCLUSION

The PGD-M technique is highly valuable in reducing the incidence of ACC and improving the prognoses of newborns.

Nucleolar GTPase Bms1 displaces Ttf1 from RFB-sites to balance progression of rDNA transcription and replication

18S, 5.8S, and 28S ribosomal RNAs (rRNAs) are cotranscribed as a pre-ribosomal RNA (pre-rRNA) from the rDNA by RNA polymerase I whose activity is vigorous during the S-phase, leading to a conflict with rDNA replication. This conflict is resolved partly by replication-fork-barrier (RFB)-sites sequences located downstream of the rDNA and RFB-binding proteins such as Ttf1. However, how Ttf1 is displaced from RFB-sites to allow replication fork progression remains elusive. Here, we reported that loss-of-function of Bms1l, a nucleolar GTPase, upregulates rDNA transcription, causes replication-fork stall, and arrests cell cycle at the S-to-G2 transition; however, the G1-to-S transition is constitutively active characterized by persisting DNA synthesis. Concomitantly, ubf, tif-IA, and taf1b marking rDNA transcription, Chk2, Rad51, and p53 marking DNA-damage response, and Rpa2, PCNA, Fen1, and Ttf1 marking replication fork stall are all highly elevated in bms1l mutants. We found that Bms1 interacts with Ttf1 in addition to Rc1l. Finally, we identified RFB-sites for zebrafish Ttf1 through chromatin immunoprecipitation sequencing and showed that Bms1 disassociates the Ttf1‒RFB complex with its GTPase activity. We propose that Bms1 functions to balance rDNA transcription and replication at the S-phase through interaction with Rcl1 and Ttf1, respectively. TTF1 and Bms1 together might impose an S-phase checkpoint at the rDNA loci.

Reading ADP-ribosylation signaling using chemical biology and interaction proteomics

ADP-ribose (ADPr) readers are essential components of ADP-ribosylation signaling, which regulates genome maintenance and immunity. The identification and discrimination between monoADPr (MAR) and polyADPr (PAR) readers is difficult because of a lack of suitable affinity-enrichment reagents. We synthesized well-defined ADPr probes and used these for affinity purifications combined with relative and absolute quantitative mass spectrometry to generate proteome-wide MAR and PAR interactomes, including determination of apparent binding affinities. Among the main findings, MAR and PAR readers regulate various common and distinct processes, such as the DNA-damage response, cellular metabolism, RNA trafficking, and transcription. We monitored the dynamics of PAR interactions upon induction of oxidative DNA damage and uncovered the mechanistic connections between ubiquitin signaling and ADP-ribosylation. Taken together, chemical biology enables exploration of MAR and PAR readers using interaction proteomics. Furthermore, the generated MAR and PAR interaction maps significantly expand our current understanding of ADPr signaling.

Nucleolar maturation of the human small subunit processome

[Figure: see text].

Rcl1 depletion impairs 18S pre-rRNA processing at the A1-site and up-regulates a cohort of ribosome biogenesis genes in zebrafish

Yeast Rcl1 is a potential endonuclease that mediates pre-RNA cleavage at the A₂-site to separate 18S rRNA from 5.8S and 25S rRNAs. However, the biological function of Rcl1 in opisthokonta is poorly defined. Moreover, there is no information regarding the exact positions of 18S pre-rRNA processing in zebrafish. Here, we report that zebrafish pre-rRNA harbours three major cleavage sites in the 5′ETS, namely –477nt (A′-site), –97nt (A₀-site) and the 5′ETS and 18S rRNA link (A₁-site), as well as two major cleavage regions within the ITS1, namely 208–218nt (site 2) and 20–33nt (site E). We also demonstrate that depletion of zebrafish Rcl1 mainly impairs cleavage at the A₁-site. Phenotypically, rcl1^–/– mutants exhibit a small liver and exocrine pancreas and die before 15 days post-fertilization. RNA-seq analysis revealed that the most significant event in rcl1^–/– mutants is the up-regulated expression of a cohort of genes related to ribosome biogenesis and tRNA production. Our data demonstrate that Rcl1 is essential for 18S rRNA maturation at the A₁-site and for digestive organogenesis in zebrafish. Rcl1 deficiency, similar to deficiencies in other ribosome biogenesis factors, might trigger a common mechanism to upregulate the expression of genes responsible for ribosome biogenesis.

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.

Genetics of developmental dysplasia of the hip

In the last decade, the advances in the molecular analyses and sequencing techniques allowed researchers to study developmental dysplasia of the hip (DDH) more thoroughly. Certain chromosomes, genes, loci and polymorphisms are being associated with variable severity of this disorder. The wide range of signs and symptoms is dependent either on isolated or systemic manifestation. Phenotypes of isolated cases range from only a mild ligamental laxity, through subluxation, to a complete dislocation of the femoral head. Systemic manifestation is connected to various forms of skeletal dysplasia and other malformations characterized by significant genetic aberrations. To reveal the background of DDH heredity, multiple studies focused on large sample sizes with an emphasis on the correlation between genotype, phenotype and continuous clinical examination. Etiological risk factors that have been observed and documented in patients include genetic, environmental, and mechanical factors, which significantly contribute to the familial or nonfamilial occurrence and phenotypic variability of this disorder. Still, the multifactorial etiology and pathogenesis of DDH are not yet sufficiently clarified, explained, or understood. Formation of connective tissue, osteogenesis, chondrogenesis, and all other affected pathways and variations in the function of their individual elements contribute to the creation of the pathology in a developing human body. This review article presents an up-to-date list of known DDH associated genes, their products, and functional characteristics.

Membranous aplasia cutis congenita in trisomy 18

BACKGROUND

Aplasia cutis congenita (ACC) is a rare congenital condition characterized by the absence of skin layers and sometimes other underlying structures, in a localized or widespread area. The exact etiopathogenesis is not yet completely understood. Membranous ACC (MACC) also described as bullous or cystic ACC is a clinical subtype of ACC, covered with a membranous or glistening surface, and appears as a flat scar. There are less than 20 cases reported in the literature. It has been proposed an abortive form of a defective closure of the neural tube. On the other hand, the trisomy 18 is a chromosomal abnormality characterized by a broad clinical spectrum and the presence of defective closure of the neural tube.

CASE PRESENTATION

We report on an 18-months-old Venezuelan boy, who presented on the parietal scalp a distinctive localized MACC appearing as an oval lesion covered with a membranous surface, characterized by the absence of hairs and the presence of a sharp hair collar. The karyotype in peripheral blood was 47,XY,+ 18.

CONCLUSIONS

This is the second case report of ACC in trisomy 18 and reinforces the interpretation of a non-fortuitous association as well as of a defective closure of the neural tube as pathogenetic mechanism. The case highlights the importance of examining for dermatological alterations such as ACC in cases of chromosomopathy.

Aplasia cutis congenita: a report of two cases from National Hospital Abuja, Nigeria and review of the literature

Aplasia cutis congenita is a rare congenital abnormality first described in 1767 by cordon. It mostly appears as a solitary lesion involving various layers of the skin and sometimes the bone on the scalp, limbs or abdomen. Genetics, environmental and exogenous causes have been implicated as potential causes. Only about 500 cases have been reported globally as of 2013. Two cases of Aplasia Cutis Congenita (ACC) who presented with scalp and bone defects at birth are reported, one in a syndromic child delivered to a consanguineous family, with associated cardiac, skin and nail anomalies (likely Adams Oliver syndrome) and the other as an isolated scalp lesion. Both were large defects managed conservatively by a multidisciplinary team. The challenges of investigating and managing such complex scalp anomalies in sub-Saharan Africa are highlighted.

Ribosomal heterogeneity - A new inroad for pharmacological innovation

The paradigm of ribosome usage in protein translation has shifted from a stance proposed as scientists began to unpick the genetic code that each mRNA was partnered by its own, unique ribosome to a rapid reversal of this view that ribosomes are completely interchangeable and simply recruited to mRNAs from a completely homogenous cellular pool. Evidence that the ribosomal proteome, ribosomal gene transcriptome and ribosome protein and RNA modifications differ between cells and tissues points to the fact that ribosomes are heterogeneous in their composition and have a degree of specialisation in their function. It has also been posited that the tissue-specificity of ribosome diseases provides an indication of functional ribosome heterogeneity, but there are substantial caveats to this interpretation. Only now have proteomic technologies developed to a level enabling accurate stoichiometric comparison of the abundance of specific ribosomal proteins in actively translating ribosomes and to measure protein in non-denatured ribosomes. This poises the field for the provocation that ribosome heterogeneity offers a novel and powerful inroad for the pharmacological targeting of disease. Such ribosome-targeted treatments may extend beyond specific ribosomopathies through strategies such as targeting features of ribosomes that are unique to diseased cells, particularly cancer cells, or to activated immune cells, as well as augmenting the action of other drugs through weakening the production of new proteins in target tissues. We may also be able to harness the potential power in ribosome diversity and specialism to better tune synthetic biology for the production of pharmaceutical proteins.

Recognizable neonatal clinical features of aplasia cutis congenita

Background

Aplasia cutis congenita (ACC), classified in nine groups, is likely to be underreported, since milder isolated lesions in wellbeing newborns could often be undetected, and solitary lesions in the context of polymalformative syndromes could not always be reported. Regardless of form and cause, therapeutic options have in common the aim to restore the deficient mechanical and immunological cutaneous protection and to limit the risk of fluid leakage or rupture of the exposed organs. We aimed to review our institutional prevalence, comorbidities, treatment and outcome of newborns with ACC.

Methods

We conducted a retrospective study including all newborns affected by ACC and admitted at the University Mother-Child Department from October 2010 to October 2019. Anthropometric and clinical characteristics of ACC1 versus a non-isolated ACC group were analyzed.

Results

We encountered 37 newborns, 16 with ACC1 versus 21 with non-isolated ACC. The incidence rate of 0.1% in ACC1 was higher than expected, while 19% of cases showed intrafamilial autosomal dominant transmission. Higher birth weight centile, though lower than reference population, being adequate for gestational age, normal Apgar score and euglycemia characterizing ACC1 resulted associated to a rapid tissue regeneration. Non-isolated ACC, in relation to concomitant congenital anomalies and higher prematurity rate, showed more surgical and medical complications along with the risk of neonatal death. Specifically, newborns with ACC4 were characterized by the frequent necessity of abdominal wall defect repair, responsible for the occurrence of an abdominal compartment syndrome.

Conclusion

Prompt carefully assessment of the newborn with ACC in order to exclude concomitant other congenital malformations, provides clues to the underlying pathophysiology, and to the short-term prognosis. Family should be oriented toward identification of other family members affected by similar pathology, while obstetric history should exclude initial multiple pregnancy with death of a co-twin, placental anomalies and drug assumption. Molecular-genetic diagnosis and genetic counseling are integrative in individualized disease approach.

Ribosomopathies: Old Concepts, New Controversies

Ribosomopathies are a diverse subset of diseases caused by reduced expression of, or mutations in, factors necessary for making ribosomes, the protein translation machinery in the cell. Despite the ubiquitous need for ribosomes in all cell types, ribosomopathies manifest with tissue-specific defects and sometimes increased cancer susceptibility, but few treatments target the underlying cause. By highlighting new research in the field, we review current hypotheses for the basis of this tissue specificity. Based on new work, we broaden our understanding of the role of ribosome biogenesis in diverse tissue types throughout embryonic development. We also pose the question of whether previously described human conditions such as aging can be at least partially attributed to defects in making ribosomes.

Translation deregulation in human disease

Advances in sequencing and high-throughput techniques have provided an unprecedented opportunity to interrogate human diseases on a genome-wide scale. The list of disease-causing mutations is expanding rapidly, and mutations affecting mRNA translation are no exception. Translation (protein synthesis) is one of the most complex processes in the cell. The orchestrated action of ribosomes, tRNAs and numerous translation factors decodes the information contained in mRNA into a polypeptide chain. The intricate nature of this process renders it susceptible to deregulation at multiple levels. In this Review, we summarize current evidence of translation deregulation in human diseases other than cancer. We discuss translation-related diseases on the basis of the molecular aberration that underpins their pathogenesis (including tRNA dysfunction, ribosomopathies, deregulation of the integrated stress response and deregulation of the mTOR pathway) and describe how deregulation of translation generates the phenotypic variability observed in these disorders.

Uncovering the assembly pathway of human ribosomes and its emerging links to disease

The essential cellular process of ribosome biogenesis is at the nexus of various signalling pathways that coordinate protein synthesis with cellular growth and proliferation. The fact that numerous diseases are caused by defects in ribosome assembly underscores the importance of obtaining a detailed understanding of this pathway. Studies in yeast have provided a wealth of information about the fundamental principles of ribosome assembly, and although many features are conserved throughout eukaryotes, the larger size of human (pre-)ribosomes, as well as the evolution of additional regulatory networks that can modulate ribosome assembly and function, have resulted in a more complex assembly pathway in humans. Notably, many ribosome biogenesis factors conserved from yeast appear to have subtly different or additional functions in humans. In addition, recent genome-wide, RNAi-based screens have identified a plethora of novel factors required for human ribosome biogenesis. In this review, we discuss key aspects of human ribosome production, highlighting differences to yeast, links to disease, as well as emerging concepts such as extra-ribosomal functions of ribosomal proteins and ribosome heterogeneity.

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.

Pre-Ribosomal RNA Processing in Human Cells: From Mechanisms to Congenital Diseases

Ribosomal RNAs, the most abundant cellular RNA species, have evolved as the structural scaffold and the catalytic center of protein synthesis in every living organism. In eukaryotes, they are produced from a long primary transcript through an intricate sequence of processing steps that include RNA cleavage and folding and nucleotide modification. The mechanisms underlying this process in human cells have long been investigated, but technological advances have accelerated their study in the past decade. In addition, the association of congenital diseases to defects in ribosome synthesis has highlighted the central place of ribosomal RNA maturation in cell physiology regulation and broadened the interest in these mechanisms. Here, we give an overview of the current knowledge of pre-ribosomal RNA processing in human cells in light of recent progress and discuss how dysfunction of this pathway may contribute to the physiopathology of congenital diseases.

Molecular basis of the human ribosomopathy Shwachman-Diamond syndrome

Mutations that target the ubiquitous process of ribosome assembly paradoxically cause diverse tissue-specific disorders (ribosomopathies) that are often associated with an increased risk of cancer. Ribosomes are the essential macromolecular machines that read the genetic code in all cells in all kingdoms of life. Following pre-assembly in the nucleus, precursors of the large 60S and small 40S ribosomal subunits are exported to the cytoplasm where the final steps in maturation are completed. Here, I review the recent insights into the conserved mechanisms of ribosome assembly that have come from functional characterisation of the genes mutated in human ribosomopathies. In particular, recent advances in cryo-electron microscopy, coupled with genetic, biochemical and prior structural data, have revealed that the SBDS protein that is deficient in the inherited leukaemia predisposition disorder Shwachman-Diamond syndrome couples the final step in cytoplasmic 60S ribosomal subunit maturation to a quality control assessment of the structural and functional integrity of the nascent particle. Thus, study of this fascinating disorder is providing remarkable insights into how the large ribosomal subunit is functionally activated in the cytoplasm to enter the actively translating pool of ribosomes.

Aplasia cutis congenita: Two case reports and discussion of the literature

Background:

Aplasia cutis congenita (ACC) is a part of a heterogeneous group of conditions characterized by the congenital absence of epidermis, dermis, and in some cases, subcutaneous tissues or bone usually involving the scalp vertex. There is an estimated incidence of 3 in 10,000 births resulting in a total number of 500 reported cases to date. The lesions may occur on every body surface although localized scalp lesions form the most frequent pattern (70%). Complete aplasia involving bone defects occurs in approximately 20% of cases. ACC can occur as an isolated defect or can be associated with a number of other congenital anomalies such as limb anomalies or embryologic malformations. In patients with large scalp and skull defects, there is increased risk of infection and bleeding along with increased mortality and therefore prompt and effective management is advised.

Case Description:

We describe two cases of ACC, involving a 4 × 3 cm defect managed conservatively and a larger 10 × 5 cm defect managed surgically with the use of a temporo-occipital scalp flap. Both cases had an excellent outcome.

Conclusions:

Multiple treatment regimens exist for ACC, but there is no consensus on treatment strategies. Conservative treatment has been described and advocated, but many authors have emphasized the disadvantages of this treatment modality. Decision between conservative and surgical management must be individualized according to lesion size and location.

The complete structure of the small-subunit processome

The small-subunit processome represents the earliest stable precursor of the eukaryotic small ribosomal subunit. Here we present the cryo-EM structure of the Saccharomyces cerevisiae small-subunit processome at an overall resolution of 3.8 Å, which provides an essentially complete near-atomic model of this assembly. In this nucleolar superstructure, 51 ribosome-assembly factors and two RNAs encapsulate the 18S rRNA precursor and 15 ribosomal proteins in a state that precedes pre-rRNA cleavage at site A1. Extended flexible proteins are employed to connect distant sites in this particle. Molecular mimicry and steric hindrance, as well as protein- and RNA-mediated RNA remodeling, are used in a concerted fashion to prevent the premature formation of the central pseudoknot and its surrounding elements within the small ribosomal subunit.

Missense variant in UBA2 associated with aplasia cutis congenita, duane anomaly, hip dysplasia and other anomalies: A possible new disorder involving the SUMOylation pathway

We report a patient with aplasia cutis congenita, Duane anomaly, hip dysplasia, and other anomalies who had a de novo missense variant in UBA2, which encodes for a protein involved in the SUMOylation pathway. It has previously been suggested that UBA2 haploinsufficiency underlies scalp defects in the 19q13.11 deletion syndrome. We propose that disturbance of the SUMOylation pathway, mediated by pathogenic variants in UBA2, is a novel mechanism for aplasia cutis congenita and other phenotypic abnormalities. © 2017 Wiley Periodicals, Inc.

Temporal lobe proteins implicated in synaptic failure exhibit differential expression and deamidation in vascular dementia

Progressive synaptic failure precedes the loss of neurons and decline in cognitive function in neurodegenerative disorders, but the specific proteins and posttranslational modifications that promote synaptic failure in vascular dementia (VaD) remain largely unknown. We therefore used an isobaric tag for relative and absolute proteomic quantitation (iTRAQ) to profile the synapse-associated proteome of post-mortem human cortex from vascular dementia patients and age-matched controls. Brain tissue from VaD patients exhibited significant down-regulation of critical synaptic proteins including clathrin (0.29; p < 1.0⋅10(-3)) and GDI1 (0.51; p = 3.0⋅10(-3)), whereas SNAP25 (1.6; p = 5.5⋅10(-3)), bassoon (1.4; p = 1.3⋅10(-3)), excitatory amino acid transporter 2 (2.6; p = 9.2⋅10(-3)) and Ca(2+)/calmodulin dependent kinase II (1.6; p = 3.0⋅10(-2)) were substantially up-regulated. Our analyses further revealed divergent patterns of protein modification in the dementia patient samples, including a specific deamidation of synapsin1 predicted to compromise protein structure. Our results reveal potential molecular targets for intervention in synaptic failure and prevention of cognitive decline in VaD.

An overview of pre-ribosomal RNA processing in eukaryotes

Ribosomal RNAs are the most abundant and universal noncoding RNAs in living organisms. In eukaryotes, three of the four ribosomal RNAs forming the 40S and 60S subunits are borne by a long polycistronic pre-ribosomal RNA. A complex sequence of processing steps is required to gradually release the mature RNAs from this precursor, concomitant with the assembly of the 79 ribosomal proteins. A large set of trans-acting factors chaperone this process, including small nucleolar ribonucleoparticles. While yeast has been the gold standard for studying the molecular basis of this process, recent technical advances have allowed to further define the mechanisms of ribosome biogenesis in animals and plants. This renewed interest for a long-lasting question has been fueled by the association of several genetic diseases with mutations in genes encoding both ribosomal proteins and ribosome biogenesis factors, and by the perspective of new anticancer treatments targeting the mechanisms of ribosome synthesis. A consensus scheme of pre-ribosomal RNA maturation is emerging from studies in various kinds of eukaryotic organisms. However, major differences between mammalian and yeast pre-ribosomal RNA processing have recently come to light. WIREs RNA 2015, 6:225–242. doi: 10.1002/wrna.1269

Crucial role of the Rcl1p-Bms1p interaction for yeast pre-ribosomal RNA processing

The essential Rcl1p and Bms1p proteins form a complex required for 40S ribosomal subunit maturation. Bms1p is a GTPase and Rcl1p has been proposed to catalyse the endonucleolytic cleavage at site A₂ separating the pre-40S and pre-60S maturation pathways. We determined the 2.0 Å crystal structure of Bms1p associated with Rcl1p. We demonstrate that Rcl1p nuclear import depends on Bms1p and that the two proteins are loaded into pre-ribosomes at a similar stage of the maturation pathway and remain present within pre-ribosomes after cleavage at A₂. Importantly, GTP binding to Bms1p is not required for the import in the nucleus nor for the incorporation of Rcl1p into pre-ribosomes, but is essential for early pre-rRNA processing. We propose that GTP binding to Bms1p and/or GTP hydrolysis may induce conformational rearrangements within the Bms1p-Rcl1p complex allowing the interaction of Rcl1p with its RNA substrate.

Hypoxia-inducible factor 2 alpha is essential for hepatic outgrowth and functions via the regulation of leg1 transcription in the zebrafish embryo

The liver plays a vital role in metabolism, detoxification, digestion, and the maintenance of homeostasis. During development, the vertebrate embryonic liver undergoes a series of morphogenic processes known as hepatogenesis. Hepatogenesis can be separated into three interrelated processes: endoderm specification, hepatoblast differentiation, and hepatic outgrowth. Throughout this process, signaling molecules and transcription factors initiate and regulate the coordination of cell proliferation, apoptosis, differentiation, intercellular adhesion, and cell migration. Hifs are already recognized to be essential in embryonic development, but their role in hepatogenesis remains unknown. Using the zebrafish embryo as a model organism, we report that the lack of Hif2-alpha but not Hif1-alpha blocks hepatic outgrowth. While Hif2-alpha is not involved in hepatoblast specification, this transcription factor regulates hepatocyte cell proliferation during hepatic outgrowth. Furthermore, we demonstrated that the lack of Hif2-alpha can reduce the expression of liver-enriched gene 1 (leg1), which encodes a secretory protein essential for hepatic outgrowth. Additionally, exogenous mRNA expression of leg1 can rescue the small liver phenotype of hif2-alpha morphants. We also showed that Hif2-alpha directly binds to the promoter region of leg1 to control leg1 expression. Interestingly, we discovered overrepresented, high-density Hif-binding sites in the potential upstream regulatory sequences of leg1 in teleosts but not in terrestrial mammals. We concluded that hif2-alpha is a key factor required for hepatic outgrowth and regulates leg1 expression in zebrafish embryos. We also proposed that the hif2-alpha-leg1 axis in liver development may have resulted from the adaptation of teleosts to their environment.

Eukaryotic ribosome biogenesis at a glance

Ribosomes play a pivotal role in the molecular life of every cell. Moreover, synthesis of ribosomes is one of the most energetically demanding of all cellular processes. In eukaryotic cells, ribosome biogenesis requires the coordinated activity of all three RNA polymerases and the orchestrated work of many (>200) transiently associated ribosome assembly factors. The biogenesis of ribosomes is a tightly regulated activity and it is inextricably linked to other fundamental cellular processes, including growth and cell division. Furthermore, recent studies have demonstrated that defects in ribosome biogenesis are associated with several hereditary diseases. In this Cell Science at a Glance article and the accompanying poster, we summarise the current knowledge on eukaryotic ribosome biogenesis, with an emphasis on the yeast model system.

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.