A new locus for Seckel syndrome on chromosome 18p11.31-q11.2

Novel Variants of CEP152 in a Case of Compound-Heterozygous Inheritance of Epilepsy

Introduction   CEP152 encodes protein Cep152, which associates with centrosome function. The lack of Cep152 can cause centrosome duplication to fail. CEP152 mutates, causing several diseases such as Seckel syndrome-5 and primary microencephaly-9. Methods  In this study, we reported a patient diagnosed with epilepsy in Tianjin Children's Hospital. We performed clinical examination and laboratory test, and whole-exome sequencing was performed for the proband's and his parents' peripheral blood. The suspected compound-heterozygous variant in the CEP152 gene was verified by Sanger sequencing and quantitative real-time polymerase chain reaction technology. Results  We discovered three variants-two of them from CEP152 and one from HPD . The result showed the variants in CEP152 only. The patient presented with seizures frequently. Sanger sequencing showed two novel variants in CEP152 are in exon26 (NM_014985.3 c.3968C > A p.Ser1323*) and in exon16 (NM_014985.3 c.2034_2036del p.Tyr678*). Conclusions  We reported a novel compound-heterozygous variant in the CEP152 gene in this study. Most of the phenotypes are Seckel syndrome and primary microencephaly, and the novel variant may cause an atypical phenotype that is epilepsy.

Recurrence mutation in RBBP8 gene causing non-syndromic autosomal recessive primary microcephaly; geometric simulation approach for insight into predicted computational models

Primary microcephaly is a rare, congenital, and genetically heterogeneous disorder in which occipitofrontal head circumference is reduced by a minimum of three standard deviations (SDs) from average because of the defect in fetal brain development.

OBJECTIVE

Mapping of RBBP8 gene mutation that produce autosomal recessive primary microcephaly. Insilco RBBP8 protein models prediction and analysis.

METHODS

Consanguineous Pakistani family affected with non-syndromic primary microcephaly was mapped a biallelic sequence variant (c.1807_1808delAT) in the RBBP8 gene via whole-exome sequencing. The deleted variant in the RBBP8 gene in affected siblings (V:4, V:6) of primary microcephaly was confirmed by sanger sequencing.

RESULTS

Identified variant c.1807_1808delAT that truncated the protein translation p. Ile603Lysfs*7 and impaired the functioning of RBBP8 protein. This sequence variant was only reported previously in Atypical Seckel syndrome and Jawad syndrome, while we mapped it in the non-syndromic primary microcephaly family. We predicted 3D protein models by using Insilco tools like I TASSER, Swiss model, and phyre2 of wild RBBP8 protein of 897 amino acids and 608 amino acids of the mutant protein. These models were validated through the online SAVES server and Ramachandran plot and refined by using the Galaxy WEB server. A predicted and refined wild protein 3D model was deposited with accession number PM0083523 in Protein Model Database. A normal mode-based geometric simulation approach was used through the NMSim program, to find out the structural diversity of wild and mutant proteins which were evaluated by RMSD and RMSF. Higher RMSD and RMSF in mutant protein reduced the stability of the protein.

CONCLUSION

The high possibility of this variant results in nonsense-mediated decay of mRNA, leading to the loss of protein functioning which causes primary microcephaly.

Central nervous system vasculopathy and Seckel syndrome: case illustration and systematic review

PURPOSE

To systematically review reported cases of Seckel syndrome (SS) and point out cases associated with central nervous system (CNS) vasculopathy and provide a summary of their clinical presentation, management, and outcomes including our illustrative case.

METHODS

We conducted a search on the MEDLINE, PubMed, Google Scholar, and Cochrane databases using the keywords "Seckel + syndrome." We identified 127 related articles reporting 252 cases of SS apart from our case. Moreover, we searched for SS cases with CNS vasculopathies from the same databases. We identified 7 related articles reporting 7 cases of CNS vasculopathies in SS patients.

RESULTS

The overall rate of CNS vasculopathy in SS patients is 3.16% (n = 8/253), where moyamoya disease (MMD) accounted for 1.97%. The mean age is 13.5 years (6-19 years), with equal gender distribution (M:F, 1:1). The most common presenting symptoms were headache and seizure followed by weakness or coma. Aneurysms were mostly located in the basilar artery, middle cerebral artery, and internal carotid artery, respectively. Regardless of the management approach, 50% of the cases sustained mild-moderate neurological deficit, 37.5% have died, and 12.5% sustained no deficit.

CONCLUSION

A high index of suspicion should be maintained in (SS) patients, and MMD should be part of the differential diagnosis. Prevalence of CNS vasculopathy in SS is 3.16% with a much higher prevalence of MMD compared to the general population. Screening for cerebral vasculopathy in SS is justifiable especially in centers that have good resources. Further data are still needed to identify the most appropriate management plan in these cases.

The pathological consequences of impaired genome integrity in humans; disorders of the DNA replication machinery

Accurate and efficient replication of the human genome occurs in the context of an array of constitutional barriers, including regional topological constraints imposed by chromatin architecture and processes such as transcription, catenation of the helical polymer and spontaneously generated DNA lesions, including base modifications and strand breaks. DNA replication is fundamentally important for tissue development and homeostasis; differentiation programmes are intimately linked with stem cell division. Unsurprisingly, impairments of the DNA replication machinery can have catastrophic consequences for genome stability and cell division. Functional impacts on DNA replication and genome stability have long been known to play roles in malignant transformation through a variety of complex mechanisms, and significant further insights have been gained from studying model organisms in this context. Congenital hypomorphic defects in components of the DNA replication machinery have been and continue to be identified in humans. These disorders present with a wide range of clinical features. Indeed, in some instances, different mutations in the same gene underlie different clinical presentations. Understanding the origin and molecular basis of these features opens a window onto the range of developmental impacts of suboptimal DNA replication and genome instability in humans. Here, I will briefly overview the basic steps involved in DNA replication and the key concepts that have emerged from this area of research, before switching emphasis to the pathological consequences of defects within the DNA replication network; the human disorders. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

RBBP8 syndrome with microcephaly, intellectual disability, short stature and brachydactyly

Primary microcephaly is clinically variable and genetically heterogeneous. Four phenotypically distinct types of autosomal recessive microcephaly syndromes are due to different RBBP8 mutations. We report on a consanguineous Pakistani family with homozygous RBBP8 mutation c.1808_1809delTA (p.Ile603Lysfs*7) manifesting microcephaly and a distinct combination of skeletal, limb and ectodermal defects, mild intellectual disability, minor facial anomalies, anonychia, disproportionate short stature and brachydactyly, and additionally talipes in one patient.

Primordial dwarfism: overview of clinical and genetic aspects

Primordial dwarfism is a group of genetic disorders which include Seckel Syndrome, Silver-Russell Syndrome, Microcephalic Osteodysplastic Primordial Dwarfism types I/III, II and Meier-Gorlin Syndrome. This genetic disorder group is characterized by intra-uterine growth retardation and post-natal growth abnormalities which occur as a result of disorganized molecular and genomic changes in embryonic stage and, thus, it represents a unique area to study growth and developmental abnormalities. Lot of research has been carried out on different aspects; however, a consolidated review that discusses an overall spectrum of this disorder is not accessible. Recent research in this area points toward important molecular and cellular mechanisms in human body that regulate the complexity of growth process. Studies have emerged that have clearly associated with a number of abnormal chromosomal, genetic and epigenetic alterations that can predispose an embryo to develop PD-associated developmental defects. Finding and associating such fundamental changes to its subtypes will help in re-examination of alleged functions at both cellular and developmental levels and thus reveal the intrinsic mechanism that leads to a balanced growth. Although such findings have unraveled a subtle understanding of growth process, we further require active research in terms of identification of reliable biomarkers for different subtypes as an immediate requirement for clinical utilization. It is hoped that further study will advance the understanding of basic mechanisms regulating growth relevant to human health. Therefore, this review has been written with an aim to present an overview of chromosomal, molecular and epigenetic modifications reported to be associated with different subtypes of this heterogenous disorder. Further, latest findings with respect to clinical and molecular genetics research have been summarized to aid the medical fraternity in their clinical utility, for diagnosing disorders where there are overlapping physical attributes and simultaneously inform about the latest developments in PD biology.

Mutations in CDK5RAP2 cause Seckel syndrome

Seckel syndrome is a heterogeneous, autosomal recessive disorder marked by prenatal proportionate short stature, severe microcephaly, intellectual disability, and characteristic facial features. Here, we describe the novel homozygous splice-site mutations c.383+1G>C and c.4005-9A>G in CDK5RAP2 in two consanguineous families with Seckel syndrome. CDK5RAP2 (CEP215) encodes a centrosomal protein which is known to be essential for centrosomal cohesion and proper spindle formation and has been shown to be causally involved in autosomal recessive primary microcephaly. We establish CDK5RAP2 as a disease-causing gene for Seckel syndrome and show that loss of functional CDK5RAP2 leads to severe defects in mitosis and spindle organization, resulting in cells with abnormal nuclei and centrosomal pattern, which underlines the important role of centrosomal and mitotic proteins in the pathogenesis of the disease. Additionally, we present an intriguing case of possible digenic inheritance in Seckel syndrome: A severely affected child of nonconsanguineous German parents was found to carry heterozygous mutations in CDK5RAP2 and CEP152. This finding points toward a potential additive genetic effect of mutations in CDK5RAP2 and CEP152.

Aberrant recombination and repair during immunoglobulin class switching in BRCA1-deficient human B cells

Breast cancer type 1 susceptibility protein (BRCA1) has a multitude of functions that contribute to genome integrity and tumor suppression. Its participation in the repair of DNA double-strand breaks (DSBs) during homologous recombination (HR) is well recognized, whereas its involvement in the second major DSB repair pathway, nonhomologous end-joining (NHEJ), remains controversial. Here we have studied the role of BRCA1 in the repair of DSBs in switch (S) regions during immunoglobulin class switch recombination, a physiological, deletion/recombination process that relies on the classical NHEJ machinery. A shift to the use of microhomology-based, alternative end-joining (A-EJ) and increased frequencies of intra-S region deletions as well as insertions of inverted S sequences were observed at the recombination junctions amplified from BRCA1-deficient human B cells. Furthermore, increased use of long microhomologies was found at recombination junctions derived from E3 ubiquitin-protein ligase RNF168-deficient, Fanconi anemia group J protein (FACJ, BRIP1)-deficient, or DNA endonuclease RBBP8 (CtIP)-compromised cells, whereas an increased frequency of S-region inversions was observed in breast cancer type 2 susceptibility protein (BRCA2)-deficient cells. Thus, BRCA1, together with its interaction partners, seems to play an important role in repairing DSBs generated during class switch recombination by promoting the classical NHEJ pathway. This may not only provide a general mechanism underlying BRCA1's function in maintaining genome stability and tumor suppression but may also point to a previously unrecognized role of BRCA1 in B-cell lymphomagenesis.

A complex microcephaly syndrome in a Pakistani family associated with a novel missense mutation in RBBP8 and a heterozygous deletion in NRXN1

We report on a consanguineous Pakistani family with a severe congenital microcephaly syndrome resembling the Seckel syndrome and Jawad syndrome. The affected individuals in this family were born to consanguineous parents of whom the mother presented with mild intellectual disability (ID), epilepsy and diabetes mellitus. The two living affected brothers presented with microcephaly, white matter disease of the brain, hyponychia, dysmorphic facial features with synophrys, epilepsy, diabetes mellitus and ID. Genotyping with a 250K SNP array in both affected brothers revealed an 18 MB homozygous region on chromosome 18 p11.21-q12.1 encompassing the SCKL2 locus of the Seckel and Jawad syndromes. Sequencing of the RBBP8 gene, underlying the Seckel and Jawad syndromes, identified the novel mutation c.919A>G, p.Arg307Gly, segregating in a recessive manner in the family. In addition, in the two affected brothers and their mother we have also found a heterozygous 607kb deletion, encompassing exons 13-19 of NRXN1. Bidirectional sequencing of the coding exons of NRXN1 did not reveal any other mutation on the other allele. It thus appears that the phenotype of the mildly affected mother can be explained by the NRXN1 deletion, whereas the more severe and complex microcephalic phenotype of the two affected brothers is due to the simultaneous deletion in NRXN1 and the homozygous missense mutation affecting RBBP8.

A nervous predisposition to unrepaired DNA double strand breaks

Ataxia-telangiectasia (A-T) has for a long time stood apart from most other human neurodegenerative syndromes by the characteristic failure of cells derived from these patients to properly repair DNA damage-induced by ionizing radiation. The discovery of mutations in the ATM gene as being the underlying cause for A-T and the demonstration that the ATM protein functions as a DNA damage-responsive kinase has defined current research focusing on decoding how the cell responds to genotoxic stress. Yet, despite significant advances in delineating the cellular DNA damage response pathways coordinated by ATM, very little headway has been made toward understanding how loss of ATM leads to progressive cerebellar ataxia and whether this can be attributed to an underlying defect in DNA double strand break repair (DSBR). Since its identification, A-T has been used as the archetypal model for how a deficiency in DNA repair affects both the development and maintenance of the nervous and immune systems in humans as well as contributing to the process of tumourigenesis. However, following the growing availability and cost effectiveness of next generation sequencing technologies, the increasing recognition of novel human disorders associated with abnormal DNA repair has demonstrated that the neuropathology typified by A-T is an 'exception' rather than the 'rule'. As a consequence, this throws into doubt the longstanding hypothesis that the neurodegeneration seen in A-T is due to the progressive loss of damaged neurons that have acquired toxic levels of unrepaired DNA lesions over time. Therefore, this review aims to address the question: Is defective DNA double strand break repair an underlying cause of neurodegeneration?

Identification of the first ATRIP-deficient patient and novel mutations in ATR define a clinical spectrum for ATR-ATRIP Seckel Syndrome

A homozygous mutational change in the Ataxia-Telangiectasia and RAD3 related (ATR) gene was previously reported in two related families displaying Seckel Syndrome (SS). Here, we provide the first identification of a Seckel Syndrome patient with mutations in ATRIP, the gene encoding ATR-Interacting Protein (ATRIP), the partner protein of ATR required for ATR stability and recruitment to the site of DNA damage. The patient has compound heterozygous mutations in ATRIP resulting in reduced ATRIP and ATR expression. A nonsense mutational change in one ATRIP allele results in a C-terminal truncated protein, which impairs ATR-ATRIP interaction; the other allele is abnormally spliced. We additionally describe two further unrelated patients native to the UK with the same novel, heterozygous mutations in ATR, which cause dramatically reduced ATR expression. All patient-derived cells showed defective DNA damage responses that can be attributed to impaired ATR-ATRIP function. Seckel Syndrome is characterised by microcephaly and growth delay, features also displayed by several related disorders including Majewski (microcephalic) osteodysplastic primordial dwarfism (MOPD) type II and Meier-Gorlin Syndrome (MGS). The identification of an ATRIP-deficient patient provides a novel genetic defect for Seckel Syndrome. Coupled with the identification of further ATR-deficient patients, our findings allow a spectrum of clinical features that can be ascribed to the ATR-ATRIP deficient sub-class of Seckel Syndrome. ATR-ATRIP patients are characterised by extremely severe microcephaly and growth delay, microtia (small ears), micrognathia (small and receding chin), and dental crowding. While aberrant bone development was mild in the original ATR-SS patient, some of the patients described here display skeletal abnormalities including, in one patient, small patellae, a feature characteristically observed in Meier-Gorlin Syndrome. Collectively, our analysis exposes an overlapping clinical manifestation between the disorders but allows an expanded spectrum of clinical features for ATR-ATRIP Seckel Syndrome to be defined.

CtIP Mutations Cause Seckel and Jawad Syndromes

Seckel syndrome is a recessively inherited dwarfism disorder characterized by microcephaly and a unique head profile. Genetically, it constitutes a heterogeneous condition, with several loci mapped (SCKL1-5) but only three disease genes identified: the ATR, CENPJ, and CEP152 genes that control cellular responses to DNA damage. We previously mapped a Seckel syndrome locus to chromosome 18p11.31-q11.2 (SCKL2). Here, we report two mutations in the CtIP (RBBP8) gene within this locus that result in expression of C-terminally truncated forms of CtIP. We propose that these mutations are the molecular cause of the disease observed in the previously described SCKL2 family and in an additional unrelated family diagnosed with a similar form of congenital microcephaly termed Jawad syndrome. While an exonic frameshift mutation was found in the Jawad family, the SCKL2 family carries a splicing mutation that yields a dominant-negative form of CtIP. Further characterization of cell lines derived from the SCKL2 family revealed defective DNA damage induced formation of single-stranded DNA, a critical co-factor for ATR activation. Accordingly, SCKL2 cells present a lowered apoptopic threshold and hypersensitivity to DNA damage. Notably, over-expression of a comparable truncated CtIP variant in non-Seckel cells recapitulates SCKL2 cellular phenotypes in a dose-dependent manner. This work thus identifies CtIP as a disease gene for Seckel and Jawad syndromes and defines a new type of genetic disease mechanism in which a dominant negative mutation yields a recessively inherited disorder.

Cellular DNA is frequently damaged through the actions of exogenous and endogenously arising DNA damaging agents. To maintain genome integrity, cells have evolved complex mechanisms to detect DNA damage, signal its presence, and mediate its repair. The importance of such mechanisms is evident because inherited defects in them can cause embryonic lethality or severe genetically inherited diseases. The clinical manifestations of such diseases are complex and include growth delay, mental retardation, skeletal abnormalities, and predisposition to cancer. While most such syndromes are inherited recessively, in some cases they are inherited dominantly. Here, we show that mutations in CtIP/RBBP8 cause related disorders: Seckel and Jawad syndromes. In addition to revealing how mutated CtIP impairs responses to DNA damage in Seckel cells, we establish that, despite the recessive mode of inheritance for this syndrome, the Seckel mutation has a dominant manifestation at the cellular level. To our knowledge, this represents a new form of molecular mechanism for recessive inheritance of a human disease. Furthermore, the aberrantly spliced mRNA is expressed at very low levels and yet significantly impairs cellular functions and causes severe clinical symptoms. This should provide new awareness that even very subtle splice mutations may have pronounced pathogenic potential.

The shortest of the short: pericentrin mutations and beyond

Microcephalic or Majewski's osteodysplastic primordial dwarfism type II (MOPD II) represents the most common type of primordial dwarfism. Adult height is typically about one meter and short stature is becoming mildly disproportionate over time with mild skeletal anomalies. Mental development is usually borderline or within the low normal range but cerebrovascular events that are common in childhood can result in significant cognitive impairment and cerebral palsy. Despite cerebrovascular insults, cardiomyopathy and early onset type 2 diabetes contribute to early mortality and morbidity. Common minor clinical features are truncal obesity, high pitched voice, microdontia and pigmentary changes. MOPD II is caused by autosomal recessive loss of function mutations in the PCNT gene encoding for a key centrosomal protein. There is clinical overlap with the so called Seckel syndrome, a heterogeneous group of entities with at least four different gene loci known to date.

Growth hormone treatment, final height, insulin-like growth factors, ghrelin, and adiponectin in four siblings with Seckel syndrome

OBJECTIVE

To report on the effect of growth hormone (GH) treatment on final height (FH) and to describe the insulin-like growth factor (IGF) system, ghrelin, and adiponectin (ADPN) in children with Seckel syndrome.

SUBJECTS AND RESULTS

Four severely growth-retarded Iraqi siblings (two girls and two boys) with Seckel syndrome were referred at ages 16.5, 14.4, 12.4, and 10.4 years. They were born at term, but their growth was retarded and birth weight ranged between 1 and 1.5 kg. The children were healthy and had a normal response to GH provocative test. Long-term GH treatment of the youngest brother and sister increased the FH by 7.2 and 3.4 cm, respectively, compared with their older brother and sister. At FH, body mass index standard deviation scores (BMISDS) ranged from -3.0 to -3.9. Serum levels of immunoreactive IGF-1, bioactive IGF-1, and IGF-binding protein 3 were all within normal to high range before GH treatment and increased after GH treatment. Fasting plasma ghrelin remained severely reduced. Despite low BMISDS, plasma ADPN was moderately reduced and showed an almost complete absence of the low-molecular-weight subform.

CONCLUSION

This is the first report on the effect of GH treatment on FH in children with Seckel syndrome. GH may have increased FH. In addition to growth defects and reduced BMISDS, patients with Seckel syndrome are characterized by low fasting ghrelin levels, low total ADPN, and near deficiency of the low-molecular-weight ADPN subform. The possible significance of the hormonal changes requires further investigations.

Seckel syndrome with chromosomal 18 deletion

Four case records of patients with Seckel Syndrome (SS) were retrieved. Typical of bird headed dwarfism was seen in all. Chromosome 18 deletion was seen in one child with SS. MRI abnormalities were detected in 3 patients. Cytogenetic studies and neuroimaging is likely to provide important diagnostic and prognostic information.

Seckel syndrome and moyamoya

Seckel syndrome is an autosomal recessive disorder characterized by intrauterine and postnatal growth delay, microcephaly with mental retardation, and facial dysmorphisms including micrognathia, a recessed forehead, and a large beaked nose. Occurring in 1 in 10,000 children without sex preference, it is the most common primordial microcephalic osteodysplastic dwarfism and has been associated with a variety of congenital brain malformations and intracranial aneurysms. Moyamoya syndrome is an idiopathic, chronic, progressive cerebrovascular disorder marked by stenosis of the intracranial internal carotid arteries and concurrent development of hypertrophied collateral vessels. These tortuous arterial collaterals appear radiographically as "puffs of smoke," giving the syndrome its name. In this report, the authors describe the case of a 16-year-old girl with coincident Seckel and moyamoya syndromes. To their knowledge, this is the first reported case of such an association being treated with surgical revascularization. The patient presented with persistent headaches and a 2-year history of progressive hand, arm, and face numbness. Imaging studies revealed multiple completed cerebral infarcts, global ischemic changes, and vascular anatomy consistent with moyamoya syndrome. Bilateral pial synangioses successfully revascularized each hemisphere with resolution of the patient's symptoms. The patient died 1 year later of complications related to treatment of a rapidly progressing intracranial aneurysm. This report documents the first case associating moyamoya and Seckel syndromes. In addition, the report reveals the rapid development of an intracranial aneurysm in a patient with this syndrome. When coupled with previous reports of other types of cerebrovascular disease in patients with Seckel syndrome or other primordial dwarfisms, the authors' findings are important because they suggest that physicians treating patients with dwarfism should consider the diagnosis of moyamoya syndrome when symptoms suggestive of cerebral ischemia are present. Prompt diagnosis and treatment of moyamoya syndrome, including the use of proven surgical revascularization procedures such as pial synangiosis, may significantly improve the long-term outcomes of these patients.

An anesthetic experience in a patient with Seckel syndrome: A case report

Seckel syndrome is an autosomal recessive, primordial dwarfism. The clinical symptoms and signs include severe intrauterine and postnatal growth retardation, nanocephaly, proportional dwarfism, bird-like faces, beak-like triangular nose, and mental retardation. We report a successful anesthetic management including endotracheal intubation with the GlideScope(R) video laryngoscope in an 18-year old man with Seckel syndrome for curettage of chronic osteomyelitis of pelvic bone.

Identification of EpCAM as the gene for congenital tufting enteropathy

BACKGROUND & AIMS

Congenital tufting enteropathy (CTE) is a rare autosomal recessive diarrheal disorder presenting in the neonatal period. CTE is characterized by intestinal epithelial cell dysplasia leading to severe malabsorption and significant morbidity and mortality. The pathogenesis and genetics of this disorder are not well understood. The objective of this study was to identify the gene responsible for CTE.

METHODS

A family with 2 children affected with CTE was identified. The affected children are double second cousins providing significant statistical power for linkage. Using Affymetrix 50K single nucleotide polymorphism (SNP) chips, genotyping was performed on only 2 patients and 1 unaffected sibling. Direct DNA sequencing of candidate genes, reverse-transcription polymerase chain reaction, immunohistochemistry, and Western blotting were performed on specimens from patients and controls.

RESULTS

SNP homozygosity mapping identified a unique 6.5-Mbp haplotype of homozygous SNPs on chromosome 2p21 where approximately 40 genes are located. Direct sequencing of genes in this region revealed homozygous G>A substitution at the donor splice site of exon 4 in epithelial cell adhesion molecule (EpCAM) of affected patients. Reverse-transcription polymerase chain reaction of duodenal tissue demonstrated a novel alternative splice form with deletion of exon 4 in affected patients. Immunohistochemistry and Western blot of patient intestinal tissue revealed decreased expression of EpCAM. Direct sequencing of EpCAM from 2 additional unrelated patients revealed novel mutations in the gene.

CONCLUSIONS

Mutations in the gene for EpCAM are responsible for CTE. This information will be used to gain further insight into the molecular mechanisms of this disease.

The role of the DNA damage response pathways in brain development and microcephaly: insight from human disorders

A network of DNA damage response (DDR) mechanisms functions co-ordinately to maintain genomic stability and ensure cellular survival in the face of exogenous and endogenous DNA damage. Defects in DDR pathways have been identified in a range of human disorders, collectively classified as DDR-defective syndromes. A common feature of these syndromes is a predisposition to cancer demonstrating the importance of the DDR in cancer avoidance. How the DDR mechanisms serve to maintain genomic stability has been the predominant focus of research into their function. However, many DRR-defective syndromes are also characterised by impaired development demonstrating broader roles for the DDR mechanisms. Microcephaly, representing reduced brain size, is a feature common to a diverse range of DDR-defective disorders. Microcephaly is most likely caused by loss (increased cell death) or failure of the developing neuronal stem cells or their progenitors to divide suggesting a fundamental role for the DDR in maintaining proliferative potential in the developing nervous system. Currently, it is unclear why the DDR proteins should be more important during neuronal development compared with the development of other tissues or why the embryonic brain is more sensitive than the adult brain. Here, we overview the DDR-defective disorders in the context of microcephaly and discuss a model underlying this striking phenotype.

Fine mapping of a locus for nonsyndromic mental retardation on chromosome 19p13

Mental retardation (MR) occurs in approximately 3% of the population and therefore significantly impacts public health. Despite this relatively high prevalence, the specific causes of MR remain unknown in most cases, although both genetic and environmental factors are known to contribute. We describe a consanguineous family with autosomal recessive (AR) nonsyndromic MR (NSMR). Because the consanguinity of this family is complex, we explore alternative approaches for generating accurate estimates of the evidence for linkage in this family, and demonstrate evidence for linkage to chromosome 19p13 (lod score ranging from 1.2 to 3.5, depending on assumptions of allele frequencies). Fine mapping of the linked region defined a critical region of 3.6 Mb, which overlaps with a previously reported gene (CC2D1A) for MR. However, no mutations in the coding region of this gene are present in the family we describe. These results suggest that another gene causing autosomal recessive nonsyndromic MR (ARNSMR) is located within this genomic region.

The genetic basis of inherited anomalies of the teeth. Part 2: syndromes with significant dental involvement

Teeth are specialized structural components of the craniofacial skeleton. Developmental defects occur either alone or in combination with other birth defects. In this paper, we review the dental anomalies in several multiple congenital anomaly (MCA) syndromes, in which the dental component is pivotal in the recognition of the phenotype and/or the molecular basis of the disorder is known. We will consider successively syndromic forms of amelogenesis imperfecta or enamel defects, dentinogenesis imperfecta (i.e. osteogenesis imperfecta) and other dentine anomalies. Focusing on dental aspects, we will review a selection of MCA syndromes associated with teeth number and/or shape anomalies. A better knowledge of the dental phenotype may contribute to an earlier diagnosis of some MCA syndromes involving teeth anomalies. They may serve as a diagnostic indicator or help confirm a syndrome diagnosis.

Haploinsufficiency of DNA Damage Response Genes and their Potential Influence in Human Genomic Disorders

Genomic disorders are a clinically diverse group of conditions caused by gain, loss or re-orientation of a genomic region containing dosage-sensitive genes. One class of genomic disorder is caused by hemizygous deletions resulting in haploinsufficiency of a single or, more usually, several genes. For example, the heterozygous contiguous gene deletion on chromosome 22q11.2 causing DiGeorge syndrome involves at least 20-30 genes. Determining how the copy number variation (CNV) affects human variation and contributes to the aetiology and progression of various genomic disorders represents important questions for the future. Here, I will discuss the functional significance of one form of CNV, haploinsufficiency (i.e. loss of a gene copy), of DNA damage response components and its association with certain genomic disorders. There is increasing evidence that haploinsufficiency for certain genes encoding key players in the cells response to DNA damage, particularly those of the Ataxia Telangiectasia and Rad3-related (ATR)-pathway, has a functional impact. I will review this evidence and present examples of some well known clinically similar genomic disorders that have recently been shown to be defective in the ATR-dependent DNA damage response. Finally, I will discuss the potential implications of a haploinsufficiency-induced defective DNA damage response for the clinical management of certain human genomic disorders.

A syndromic form of autosomal recessive congenital microcephaly (Jawad syndrome) maps to chromosome 18p11.22-q11.2

We report a consanguineous Pakistani family with seven affected individuals showing a syndromic form of congenital microcephaly. Clinical features of affected individuals include congenital microcephaly with sharply slopping forehead, moderate to severe mental retardation, anonychia congenita, and digital malformations. By screening human genome with microsatellite markers, this autosomal recessive condition was mapped to a 25.2 cM interval between markers D18S1150 and D18S1100 on chromosome 18p11.22-q12.3. However, the region of continuous homozygosity between markers D18S1150 and D18S997 spanning 15.33 cM, probably define the most likely candidate region for this condition. This region encompasses a physical distance of 12.03 Mb. The highest two-point LOD score of 3.03 was obtained with a marker D18S1104 and multipoint score reached a maximum of 3.43 with several markers. Six candidate genes, CEP76, ESCO1, SEH1L, TUBB6, ZNF519, and PTPN2 were sequenced, and were found to be negative for functional sequence variants.

Pentoxifylline suppresses transduction by HIV-1-based vectors

Pentoxifylline, a caffeine-related compound, was shown to suppress human immunodeficiency virus type 1 (HIV-1) replication. This effect is thought to be mediated by inhibition of tumor necrosis factor-alpha (TNFalpha)-mediated long-terminal repeat (LTR)-driven expression. We now demonstrate that pentoxifylline efficiently inhibits transduction by HIV-1-based vectors. This latter effect is independent of LTR-driven expression, and correlates with a reduced efficiency of the completion of the integration process in infected cells. Finally, the effect of pentoxifylline is dramatically reduced in cells expressing a dominant negative ATR protein, and in primary human cells that exhibit low level of ATR activity, suggesting that the effect of pentoxifylline on HIV-1 transduction and replication is at least partly mediated by suppression of the ATR kinase.

Pigmentary changes and atopic dermatitis in a patient with Seckel syndrome

Seckel syndrome is a very rare form of primordial dwarfism characterized by antenatal and postnatal growth delay, proportionate extreme short stature, a prominent beak-like nose, hypoplasia of the malar area, small chin, microcephaly, deformed ears lacking lobules, skeletal malformations, mental retardation, and developmental delay. This syndrome has been described with associated disorders of orthopedic, neurologic, hematologic, cardiac, and ocular systems; however, only a few reports mention dermatologic involvement. We describe a 5-year-old girl with classic Seckel syndrome who presented with moderately severe atopic dermatitis and diffuse hypopigmented macules and papules.

3-M syndrome: a report of three Egyptian cases with review of the literature

The 3-M syndrome is a rare autosomal recessive disorder. It is characterized by prenatal and postnatal growth retardation associated with characteristic features. In this study, we report on three patients from two unrelated families, including two male sibs, with the characteristic features and radiological findings of the 3-M syndrome. The main features in our cases were low birth weight, short stature, malar hypoplasia, anteverted nostrils with a fleshy nasal tip, long philtrum, pointed full chin, short broad neck, broad chest with transverse grooves of anterior thorax and hyperlordosis. An orodental examination revealed characteristic findings, some of which were not reported before. Prominent premaxilla, hypoplastic maxilla, thick patulous lips, high-arched palate, median fissured tongue, delayed eruption of teeth with enamel hypocalcification and malocclusion were present in our three studied cases. Radiographic studies showed slender long bones and ribs, a narrow pelvis and foreshortened vertebral bodies. Our reported cases are the offspring of healthy consanguineous parents, confirming the autosomal recessive pattern of inheritance in the syndrome. Cases were reported from different countries all over the world. To our knowledge, these are the first reported Egyptian patients with this rare disorder. This syndrome may be underreported because of the phenotypic overlap with other low birth dwarfism syndromes. Recent identification of a gene mutated in some cases of 3-M syndrome will aid diagnosis.

Disparate roles of ATR and ATM in immunoglobulin class switch recombination and somatic hypermutation

Class switch recombination (CSR) and somatic hypermutation (SHM) are mechanistically related processes initiated by activation-induced cytidine deaminase. Here, we have studied the role of ataxia telangiectasia and Rad3-related protein (ATR) in CSR by analyzing the recombinational junctions, resulting from in vivo switching, in cells from patients with mutations in the ATR gene. The proportion of cells that have switched to immunoglobulin (Ig)A and IgG in the peripheral blood seems to be normal in ATR-deficient (ATRD) patients and the recombined S regions show a normal "blunt end-joining," but impaired end joining with partially complementary (1-3 bp) DNA ends. There was also an increased usage of microhomology at the mu-alpha switch junctions, but only up to 9 bp, suggesting that the end-joining pathway requiring longer microhomologies (> or =10 bp) may be ATR dependent. The SHM pattern in the Ig variable heavy chain genes is altered, with fewer mutations occurring at A and more mutations at T residues and thus a loss of strand bias in targeting A/T pairs within certain hotspots. These data suggest that the role of ATR is partially overlapping with that of ataxia telangiectasia-mutated protein, but that the former is also endowed with unique functional properties in the repair processes during CSR and SHM.

Seckel syndrome exhibits cellular features demonstrating defects in the ATR-signalling pathway

To date, the only reported genetic defect identified in the developmental disorder, Seckel syndrome, is a mutation in ataxia telangiectasia and Rad3-related protein (ATR). Seckel syndrome is clinically and genetically heterogeneous and whether defects in ATR significantly contribute to Seckel syndrome is unclear. Firstly, we characterize ATR-Seckel cells for their response to DNA damage. ATR-Seckel cells display impaired phosphorylation of ATR-dependent substrates, impaired G2/M checkpoint arrest and elevated micronucleus (MN) formation following exposure to UV and agents that cause replication stalling. We describe a novel phenotype, designated nuclear fragmentation (NF), that occurs following replication arrest. Finally, we report that ATR-Seckel cells have an endogenously increased number of centrosomes in mitotic cells demonstrating a novel role for ATR in regulating centrosome stability. We exploit these phenotypes to examine cell lines derived from additional unrelated Seckel syndrome patients. We show that impaired phosphorylation of ATR-dependent substrates is a common but not invariant feature of Seckel syndrome cell lines. In contrast, all cell lines displayed defective G2/M arrest, increased levels of NF and MN formation following exposure to agents that cause replication stalling. All the Seckel syndrome cell lines examined showed increased endogenous centrosome numbers. Though ATR cDNA can complement the defects in ATR-Seckel cells, it failed to complement any of the additional cell lines. We conclude that Seckel syndrome represents a further damage response disorder that is uniquely associated with defects in the ATR-signalling pathway resulting in failed checkpoint arrest following exposure to replication fork stalling.

Chromosomal instability at common fragile sites in Seckel syndrome

Seckel syndrome (SCKL) is a rare, genetically heterogeneous disorder, with dysmorphic facial appearance, growth retardation, microcephaly, mental retardation, variable chromosomal instability, and hematological disorders. To date, three loci have been linked to this syndrome, and recently, the gene encoding ataxia-telangiectasia and Rad3-related protein (ATR) was identified as the gene mutated at the SCKL1 locus. The ATR mutation affects splicing efficiency, resulting in low levels of ATR in affected individuals. Elsewhere, we reported increased instability at common chromosomal fragile sites in cells lacking the replication checkpoint gene ATR. Here, we tested whether cells from patients carrying the SCKL1 mutation would show increased chromosome breakage following replication stress. We found that, compared with controls, there is greater chromosomal instability, particularly at fragile sites, in SCKL1-affected patient cells after treatment with aphidicolin, an inhibitor of DNA polymerase alpha and other polymerases. The difference in chromosomal instability between control and patient cells increases at higher levels of aphidicolin treatment, suggesting that the low level of ATR present in these patients is not sufficient to respond appropriately to replication stress. This is the first human genetic syndrome associated with increased chromosome instability at fragile sites following replication stress, and these findings may be related to the phenotypic findings in patients with SCKL1.

An overview of three new disorders associated with genetic instability: LIG4 syndrome, RS-SCID and ATR-Seckel syndrome

Around 15-20 hereditary disorders associated with impaired DNA damage response mechanisms have been previously described. The range of clinical features associated with these disorders attests to the significant role that these pathways play during development. Recently, three new such disorders have been reported extending the importance of the damage response pathways to human health. LIG4 syndrome is conferred by hypomorphic mutations in DNA ligase IV, an essential component of DNA non-homologous end-joining (NHEJ), and is associated with pancytopaenia, developmental and growth delay and dysmorphic facial features. Radiosensitive severe combined immunodeficiency (RS-SCID) is caused by mutations in Artemis, a protein that plays a subsidiary role in non-homologous end-joining although it is not an essential component. RS-SCID is characterised by severe combined immunodeficiency but patients have no overt developmental abnormalities. ATR-Seckel syndrome is caused by mutations in ataxia telangiectasia and Rad3 related protein (ATR), a component of a DNA damage signalling pathway. ATR-Seckel syndrome patients have dramatic microcephaly and marked growth and developmental delay. The clinical features of these patients are considered in the light of the function of the defective protein.

Chromosome instability induced in vitro with mitomycin C in five Seckel syndrome patients

Seckel syndrome (SS) is an autosomal recessive entity characterized by proportionate pre- and post-natal growth retardation, microcephaly, typical facial appearance with beak-like protrusion, and severe mental retardation. A heterogeneous basis for SS was proposed since around 25% of SS patients have hematological anomalies, suggesting a subgroup of SS with chromosome instability and hematological disorders. Chromosome instability induced by mitomycin C (MMC) has been observed in previous reports. The purpose of this study is to report cytogenetic features in five patients with SS. The patients had low birth weight (mean 1,870 g), short stature (SD = 6.36), microcephaly (OFC, SD = 8.1), typical facial appearance, and multiple articular dislocations. None of them had anemia at the time of examination. In all cases their parents were healthy and non-consanguineous. Lymphocytes of SS patients and a control group (n = 9) matched by age and sex were cultured with and without MMC, and harvested at 72 and 96 hr. Chromosomal aberrations (chromatid and chromosomal gaps and breaks, deletions, fragments, and exchanges) were scored in 100 metaphases per culture. A statistical increase of chromosomal aberrations was observed in 96 hr MMC cultures in all patients (40.2% vs. 2.8%). Sister chromatid exchanges were also performed with no differences between groups. Clinical and cytogenetic findings support the idea that SS may correspond to a chromosome instability syndrome.

Is the novel SCKL3 at 14q23 the predominant Seckel locus?

Seckel syndrome (SCKL) is a rare disease with wide phenotypic heterogeneity. A locus (SCKL1) has been identified at 3q and another (SCKL2) at 18p, both in single kindreds afflicted with the syndrome. We report here a novel locus (SCKL3) at 14q by linkage analysis in 13 Turkish families. In total, 18 affected and 10 unaffected sibs were included in the study. Of the 10 informative families, nine with parental consanguinity and one reportedly nonconsanguineous but with two affected sibs, five were indicative of linkage to the novel locus. One of those families also linked to the SCKL1 locus. A consanguineous family with one affected sib was indicative of linkage to SCKL2. The novel gene locus SCKL3 is 1.18 cM and harbors ménage a trois 1, a gene with a role in DNA repair.

Clinical and genetic heterogeneity of Seckel syndrome

Seckel syndrome is a rare autosomal recessive condition belonging to the group of osteodysplastic primordial "dwarfism" and characterized by the association of 1) severe pre- and postnatal growth retardation, 2) microcephaly with mental retardation, and 3) specific dysmorphic features. Recently, two disease loci have been mapped to chromosomes 3q22.1-q24 and 18p11.31-q11.2, respectively, by homozygosity mapping in consanguineous families. Here, we report on the exclusion of these loci in five consanguineous and one multiplex nonconsanguineous Seckel syndrome families and in two consanguineous families presenting type II osteodysplastic primordial dwarfism. These results support the view that Seckel syndrome is a clinically and genetically heterogeneous condition.