Dental issues in lacrimo-auriculo-dento-digital syndrome: An autosomal dominant condition with clinical and genetic variability

BACKGROUND AND OVERVIEW

Lacrimo-auriculo-dento-digital (LADD) syndrome is an autosomal dominant disorder with variable lacrimal and salivary gland hypoplasia and aplasia, auricular anomalies and hearing loss, dental defects and caries, and digital anomalies.

CASE DESCRIPTION

The authors present the cases of 2 unrelated children with enamel defects and history of dry mouth leading to recurrent dental caries. The referring diagnoses were Sjögren disease and hypohidrotic ectodermal dysplasia, respectively. The geneticist suspected LADD syndrome, which was confirmed by means of molecular studies showing mutations of 2 genes: fibroblast growth factor receptor 2 and fibroblast growth factor 10, respectively. Similarly affected relatives indicated an autosomal dominant inheritance. These relatives needed multiple dental rehabilitations during childhood and dentures in adulthood.

CONCLUSIONS AND PRACTICAL IMPLICATIONS

Dry mouth, multiple caries, enamel defects, and abnormal tooth morphology were the reasons for seeking care from dentists. However, clinical evaluation and diagnostic imaging studies helped identify anomalies of the lacrimal and salivary glands, ears, and digits, indicating involvement of different areas of the body, compatible with LADD syndrome. Accordingly, dentists should consider genetic disorders in patients with multiple anomalies. For instance, oculodentodigital syndrome, oral-facial-digital syndrome, and LADD syndrome (among others) may have dental issues as the major clinical manifestation. Accurate identification of a particular syndrome is now commonplace with the use of genetic testing. When a patient has multiple anomalies suggestive of a syndromic condition, appropriate genetic testing can help verify the clinical diagnosis. Keeping genetics in mind helps earlier identification of other affected family members with diagnostic genetic testing and appropriate treatment; the economic advantage is to shorten the diagnostic odyssey and possibly preserve dentition.

Expression of Familial Middle–Ring–Little Finger Syndactyly as Either Simple Syndactyly or Synpolydactyly

A family with middle–ring–little finger syndactyly is reported to demonstrate that the responsible autosomal gene may be expressed as either simple syndactyly or synpolydactyly.

Pro-arrhythmogenic effects of CACNA1C G1911R mutation in human ventricular tachycardia: insights from cardiac multi-scale models

Mutations in the CACNA1C gene are associated with ventricular tachycardia (VT). Although the CACNA1C mutations were well identified in patients with cardiac arrhythmias, mechanisms by which cardiac arrhythmias are generated in such genetic mutation conditions remain unclear. In this study, we identified a novel mechanism of VT resulted from enhanced repolarization dispersion which is a key factor for arrhythmias in the CACNA1C G1911R mutation using multi-scale computational models of the human ventricle. The increased calcium influx in the mutation prolonged action potential duration (APD), produced steepened action potential duration restitution (APDR) curves as well as augmented membrane potential differences among different cell types during repolarization, increasing transmural dispersion of repolarization (DOR) and the spatial and temporal heterogeneity of cardiac electrical activities. Consequentially, the vulnerability to unidirectional conduction block in response to a premature stimulus increased at tissue level in the G1911R mutation. The increased functional repolarization dispersion anchored reentrant excitation waves in tissue and organ models, facilitating the initiation and maintenance of VT due to less meandering rotor tip. Thus, the increased repolarization dispersion caused by the G1911R mutation is a primary factor that may primarily contribute to the genesis of cardiac arrhythmias in Timothy Syndrome.

Modeling psychiatric disorders with patient-derived iPSCs

Psychiatric disorders are heterogeneous disorders characterized by complex genetics, variable symptomatology, and anatomically distributed pathology, all of which present challenges for effective treatment. Current treatments are often blunt tools used to ameliorate the most severe symptoms, often at the risk of disrupting functional neural systems, thus there is a pressing need to develop rational therapeutics. Induced pluripotent stem cells (iPSCs) reprogrammed from patient somatic cells offer an unprecedented opportunity to recapitulate both normal and pathologic human tissue and organ development, and provides new approaches for understanding disease mechanisms and for drug discovery with higher predictability of their effects in humans. Here we review recent progress and challenges in using human iPSCs for modeling neuropsychiatric disorders and developing novel therapeutic strategies.

Familial pseudotail, scoliosis and synpolydactyly syndrome

PURPOSE

This case series describes a novel condition characterized by familial pseudotail associated with scoliosis, and synpolydactyly that has not been previously reported in literature.

METHODS

The authors present three siblings and one cousin from the same family living in the northern region of the Arabian Peninsula. All cases presented with pseudotail, scoliosis, and complex synpolydactyly. The authors demonstrated complete clinical and radiological descriptions in addition the detailed performed surgeries.

RESULTS

The histopathological result of the resected pseudotail specimens revealed bony lesion covered with thick fibrous tissue and evidence of mature adipocytes within trabecular spaces.

CONCLUSIONS

The described cases represent a novel condition that has not been previously reported in the literature. Familial pseudotail scoliosis synpolydactyly syndrome is a newly recognized form of familial pseudotail.

Identification and Functional Characterization of a Novel CACNA1C-Mediated Cardiac Disorder Characterized by Prolonged QT Intervals With Hypertrophic Cardiomyopathy, Congenital Heart Defects, and Sudden Cardiac Death

BACKGROUND

A portion of sudden cardiac deaths can be attributed to structural heart diseases, such as hypertrophic cardiomyopathy (HCM) or cardiac channelopathies such as long-QT syndrome (LQTS); however, the underlying molecular mechanisms are distinct. Here, we identify a novel CACNA1C missense mutation with mixed loss-of-function/gain-of-function responsible for a complex phenotype of LQTS, HCM, sudden cardiac death, and congenital heart defects.

METHODS AND RESULTS

Whole exome sequencing in combination with Ingenuity variant analysis was completed on 3 affected individuals and 1 unaffected individual from a large pedigree with concomitant LQTS, HCM, and congenital heart defects and identified a novel CACNA1C mutation, p.Arg518Cys, as the most likely candidate mutation. Mutational analysis of exon 12 of CACNA1C was completed on 5 additional patients with a similar phenotype of LQTS plus a personal or family history of HCM-like phenotypes and identified 2 additional pedigrees with mutations at the same position, p.Arg518Cys/His. Whole cell patch clamp technique was used to assess the electrophysiological effects of the identified mutations in CaV1.2 and revealed a complex phenotype, including loss of current density and inactivation in combination with increased window and late current.

CONCLUSIONS

Through whole exome sequencing and expanded cohort screening, we identified a novel genetic substrate p.Arg518Cys/His-CACNA1C, in patients with a complex phenotype including LQTS, HCM, and congenital heart defects annotated as cardiac-only Timothy syndrome. Our electrophysiological studies, identification of mutations at the same amino acid position in multiple pedigrees, and cosegregation with disease in these pedigrees provide evidence that p.Arg518Cys/His is the pathogenic substrate for the observed phenotype.

Sclerostin and skeletal health

Sclerostin is a cysteine-knot glycoprotein product of the SOST gene, predominately expressed by osteocytes, that is a regulator of osteoblastic bone formation. When sclerostin binds to its low-density lipoprotein receptor-related proteins 5 and 6 on the cell membrane of osteoblasts, it inhibits canonical Wnt/β-catenin signaling and reduces osteoblastic bone formation. Sclerostin was first identified in the study of two rare autosomal recessive disorders, sclerosteosis and van Buchem disease, which are associated with absent or reduced levels of sclerostin. Although homozygote patients with these disorders have serious adverse clinical consequences due to excessive bone growth, heterozygote patients have a normal phenotype, high bone mass, and very low risk of fractures. This has led to the concept that downregulation of sclerostin might be effective in the treatment of osteoporosis. Several humanized monoclonal antibodies to sclerostin, including romosozumab and blosozumab, are now in clinical development. Preliminary data show that these agents result in a transient increase in bone formation markers, a sustained decrease in bone resorption markers, and a robust increase in bone mineral density. If any of these agents are found to reduce fracture risk with a favorable safety profile, it will expand the options for osteoanabolic therapy for patients at high risk for fractures.

A novel homozygous nonsense mutation in the PVRL4 gene and expansion of clinical spectrum of EDSS1

Ectodermal dysplasias (EDs) belong to a group of genetic diseases which result from alterations in ectoderm-derived appendages including hair, nail, teeth and sweat glands. Ectodermal dysplasia syndactyly syndrome (EDSS1) is one of the rare forms of ED caused by mutations in nectin-4, encoded by the PVRL4 gene. The present study described clinical investigation of the EDSS1 identified in a large consanguineous family. Furthermore, DNA sequence analysis revealed a novel homozygous nonsense mutation (181C>T, p.Asp61*) in the PVRL4 gene.

Calcium Channel Mutations in Cardiac Arrhythmia Syndromes

Voltage gated calcium channels are essential for cardiac physiology by serving as sarcolemma- restricted gatekeepers for calcium in cardiac myocytes. Activation of the L-type voltagegated calcium channel provides the calcium entry required for excitation-contraction coupling and contributes to the plateau phase of the cardiac action potential. Given these critical physiological roles, subtle disturbances in L-type channel function can lead to fatal cardiac arrhythmias. Indeed, numerous human arrhythmia syndromes have been linked to mutations in the L-type channel leading to gain-of-function or loss-of-function mutations. In this review, we discuss the current state of knowledge regarding these mutations present in Timothy Syndrome, Long and Short QT Syndromes, Brugada Syndrome and Early Repolarization Syndrome. We discuss the pathological consequences of the mutations, the biophysical effects of the mutations on the channel as well as possible therapeutic considerations and challenges for future studies.

Miscellaneous Bone Disorders

This chapter deals with a few of the important childhood bone disorders associated with high bone mass as well as conditions associated with fragility fractures and limb deformities that have not been addressed in previous chapters. A couple of skeletal dysplasias that can sometimes be confused with rickets are also dealt with in this chapter.

Mutations affecting the BHLHA9 DNA-binding domain cause MSSD, mesoaxial synostotic syndactyly with phalangeal reduction, Malik-Percin type

Mesoaxial synostotic syndactyly, Malik-Percin type (MSSD) (syndactyly type IX) is a rare autosomal-recessive nonsyndromic digit anomaly with only two affected families reported so far. We previously showed that the trait is genetically distinct from other syndactyly types, and through autozygosity mapping we had identified a locus on chromosome 17p13.3 for this unique limb malformation. Here, we extend the number of independent pedigrees from various geographic regions segregating MSSD to a total of six. We demonstrate that three neighboring missense mutations affecting the highly conserved DNA-binding region of the basic helix-loop-helix A9 transcription factor (BHLHA9) are associated with this phenotype. Recombinant BHLHA9 generated by transient gene expression is shown to be located in the cytoplasm and the cell nucleus. Transcription factors 3, 4, and 12, members of the E protein (class I) family of helix-loop-helix transcription factors, are highlighted in yeast two-hybrid analysis as potential dimerization partners for BHLHA9. In the presence of BHLHA9, the potential of these three proteins to activate expression of an E-box-regulated target gene is reduced considerably. BHLHA9 harboring one of the three substitutions detected in MSSD-affected individuals eliminates entirely the transcription activation by these class I bHLH proteins. We conclude that by dimerizing with other bHLH protein monomers, BHLHA9 could fine tune the expression of regulatory factors governing determination of central limb mesenchyme cells, a function made impossible by altering critical amino acids in the DNA binding domain. These findings identify BHLHA9 as an essential player in the regulatory network governing limb morphogenesis in humans.

The sclerostin story: from human genetics to the development of novel anabolic treatment for osteoporosis

Sclerosteosis and Van Buchem disease are two rare bone sclerosing disorders characterized by increased bone mineral density, tall stature and entrapment of cranial nerves due to overgrowth of a highly dense bone. Recent advances in human genetics have revealed the genetic background of these disorders by cloning the SOST gene, which is localized on chromosome region 17q12-q21 and codes for sclerostin. Sclerostin is a protein produced almost exclusively from osteocytes inhibiting bone formation by both osteoblasts and osteocytes. At the molecular level, sclerostin inhibits the Wnt signaling pathway, which plays a critical role in osteoblast development and function. Induced sclerostin deficiency in mice reproduces the bone sclerosing human diseases, while sclerostin excess leads to bone loss and reduced bone strength. The extracellular nature of sclerostin has rendered it a promising target for the development of novel anti-osteoporotic treatment. Otherwise healthy carriers of the SOST mutation present with increased bone mass and low levels of sclerostin in serum in contrast to patients with sclerosteosis, who exhibit undetectable levels, thus pointing to the possibility of titration of sclerostin levels in the circulation. Based on these unique characteristics, human anti-sclerostin antibodies have been developed and tested in ovariectomized rats and monkeys, demonstrating very promising results in bone formation. Clinical phase II and III trials are currently underway thereby translating human genetics to drug development.

A novel GLI3 mutation affecting the zinc finger domain leads to preaxial-postaxial polydactyly-syndactyly complex

BACKGROUND

Polydactyly is a highly common congenital limb defect. Extra digits may appear as an isolated anomaly or as a part of a syndrome. Mutations in GLI3 have been shown to cause Greig cephalopolysyndactyly, Pallister-Hall syndrome and non-syndromic polydactyly. Genotype-phenotype correlation studies of GLI3 mutations suggest a model by which mutations in the zinc-finger domain (ZFD) of GLI3 likely lead to syndromic polydactyly. Here we describe a rare case of autosomal dominant heterozygous missense mutation in the ZFD of GLI3 leading to a variable polydactyly-syndactyly complex.

CASE PRESENTATION

A large Jewish Moroccan family presented with apparently autosomal dominant heredity of bilateral thumb polydactyly in hands and feet combined with post-axial polydactyly type B or type A. Syndactyly was evident in most patients' hands and feet. Apart from head circumference beyond 90th percentile in some of the affected individuals, none had craniofacial dysmorphism. A novel GLI3 c.1802A > G (p.His601Arg) mutation was found in all affected individuals.

CONCLUSION

We demonstrate that a mutation in the ZFD domain of GLI3 leads to phenotypic variability, including an isolated limb phenotype. Thus, the variability in phenotypes caused by mutations in this master developmental regulator is more profound than has been previously suggested.

A CACNA1C variant associated with reduced voltage-dependent inactivation, increased CaV1.2 channel window current, and arrhythmogenesis

Mutations in CACNA1C that increase current through the Ca_V1.2 L-type Ca²⁺ channel underlie rare forms of long QT syndrome (LQTS), and Timothy syndrome (TS). We identified a variant in CACNA1C in a male child of Filipino descent with arrhythmias and extracardiac features by candidate gene sequencing and performed functional expression studies to electrophysiologically characterize the effects of the variant on Ca_V1.2 channels. As a baby, the subject developed seizures and displayed developmental delays at 30 months of age. At age 5 years, he displayed a QTc of 520 ms and experienced recurrent VT. Physical exam at 17 years of age was notable for microcephaly, short stature, lower extremity weakness and atrophy with hyperreflexia, spastic diplegia, multiple dental caries and episodes of rhabdomyolysis. Candidate gene sequencing identified a G>C transversion at position 5731 of CACNA1C (rs374528680) predicting a glycine>arginine substitution at residue 1911 (p.G1911R) of Ca_V1.2. The allele frequency of this variant is 0.01 in Malays, but absent in 984 Caucasian alleles and in the 1000 genomes project. In electrophysiological analyses, the variant decreased voltage-dependent inactivation, thus causing a gain of function of Ca_V1.2. We also observed a negative shift of V_1/2 of activation and positive shift of V_1/2 of channel inactivation, resulting in an increase of the window current. Together, these suggest a gain-of-function effect on Ca_V1.2 and suggest increased susceptibility for arrhythmias in certain clinical settings. The p.G1911R variant was also identified in a case of sudden unexplained infant death (SUID), for which an increasing number of clinical observations have demonstrated can be associated with arrhythmogenic mutations in cardiac ion channels. In summary, the combined effects of the CACNA1C variant to diminish voltage-dependent inactivation of Ca_V1.2 and increase window current expand our appreciation of mechanisms by which a gain of function of Ca_V1.2 can contribute to QT prolongation.

[Diploid/triploid mosaicism: a variable but characteristic phenotype]

INTRODUCTION

Diploid/triploid mosaicism is a rare chromosomal abnormality. It is caused by a failure in the postzygotic division during embryonic development. It results in the coexistence of two genetically heterogeneous cell lines (46,XX and 69,XXX) in one individual. His clinical phenotype is characteristic. Pigmentary changes with a distribution pattern following Blaschko's lines abnormalities in other ectoderm-derived tissues are the main diagnostic signs.

CASE REPORTS

Three cases of diploid/triploid mosaicism are described, and compared to the previously reported cases. The most frequently observed symptoms were mental retardation, truncal obesity, short stature, hemihypertrophy, small and narrow hands with clino and camptodactyly. Phenotypic characteristics of our three patients were similar to those of previously reported cases. Although there is no single and specific phenotype associated with mosaicism diploid/triploid, there are some dysmorphic features that shape a recognizable malformative syndrome. Peripheral blood lymphocytes karyotype was normal in our patients. Diagnosis was reached performing a fibroblast karyotype from hypopigmented skin.

CONCLUSIONS

Intellectual disability associated with truncal obesity, short stature, hemihypertrophy or clino/camptodactyly should suggest to clinicians the possible existence of a diploid/triploid mosaicism. In most cases, karyotype from fibroblasts is needed to reach the diagnosis.

Elevated circulating Sclerostin concentrations in individuals with high bone mass, with and without LRP5 mutations

CONTEXT

The role and importance of circulating sclerostin is poorly understood. High bone mass (HBM) caused by activating LRP5 mutations has been reported to be associated with increased plasma sclerostin concentrations; whether the same applies to HBM due to other causes is unknown.

OBJECTIVE

Our objective was to determine circulating sclerostin concentrations in HBM.

DESIGN AND PARTICIPANTS

In this case-control study, 406 HBM index cases were identified by screening dual-energy x-ray absorptiometry (DXA) databases from 4 United Kingdom centers (n = 219 088), excluding significant osteoarthritis/artifact. Controls comprised unaffected relatives and spouses.

MAIN MEASURES

Plasma sclerostin; lumbar spine L1, total hip, and total body DXA; and radial and tibial peripheral quantitative computed tomography (subgroup only) were evaluated.

RESULTS

Sclerostin concentrations were significantly higher in both LRP5 HBM and non-LRP5 HBM cases compared with controls: mean (SD) 130.1 (61.7) and 88.0 (39.3) vs 66.4 (32.3) pmol/L (both P < .001, which persisted after adjustment for a priori confounders). In combined adjusted analyses of cases and controls, sclerostin concentrations were positively related to all bone parameters found to be increased in HBM cases (ie, L1, total hip, and total body DXA bone mineral density and radial/tibial cortical area, cortical bone mineral density, and trabecular density). Although these relationships were broadly equivalent in HBM cases and controls, there was some evidence that associations between sclerostin and trabecular phenotypes were stronger in HBM cases, particularly for radial trabecular density (interaction P < .01).

CONCLUSIONS

Circulating plasma sclerostin concentrations are increased in both LRP5 and non-LRP5 HBM compared with controls. In addition to the general positive relationship between sclerostin and DXA/peripheral quantitative computed tomography parameters, genetic factors predisposing to HBM may contribute to increased sclerostin levels.

Nectin-4 mutations causing ectodermal dysplasia with syndactyly perturb the rac1 pathway and the kinetics of adherens junction formation

Defective nectin-1 and -4 have been implicated in ectodermal dysplasia (ED) syndromes with variably associated features including orofacial and limb defects. In particular, nectin-1 mutations cause cleft lip/palate ED (CLPED1; OMIM#225060), whereas defective nectin-4 is associated with ED-syndactyly syndrome (EDSS1; OMIM#613573). Although the broad phenotypic overlap suggests a common mode of action of nectin-1 and -4, little is known about the pathogenic mechanisms involved. We report the identification of, to our knowledge, a previously undescribed nectin-4 homozygous p.Val242Met missense mutation in a patient with EDSS1. We used patient skin biopsy and primary keratinocytes, as well as nectin-4 ectopic expression in epithelial cell lines, to characterize functional consequences of p.Val242Met and p.Thr185Met mutations, the latter previously identified in compound heterozygosity with a truncating mutation. We show that nectin-4-altered expression perturbs nectin-1 clustering at keratinocyte contact sites and delays, but does not impede cell-cell aggregation and cadherin recruitment at adherens junctions (AJs). Moreover, trans-interaction of nectin-1 and -4 induces the activation of Rac1, a member of the Rho family of small GTPases, and regulates E-cadherin-mediated cell-cell adhesion. These data outline a synergistic action of nectin-1 and -4 in the early steps of AJ formation and implicate this interaction in modulating the Rac1 signaling pathway.

Dentition abnormalities in a Timothy syndrome patient with a novel genetic mutation: a case report

The purpose of this paper was to present the case of a two-year-old male diagnosed with Timothy syndrome who presented with generalized enamel defects in the primary dentition. Timothy syndrome is an autosomal dominant condition characterized by a de novo missense mutation in the Cav1.2 L-type calcium channel CACNA1C. Timothy syndrome patients present with multiple clinical manifestations, including: cardiac arrhythmias; syndactyly; immune deficiency; intermittent hypoglycemia; and neurologic issues, including seizures, mental retardation, hypotonia, and autism. Craniofacial abnormalities reported include: low-set ears; flat nasal bridge; small upper jaw; thin upper lip; round face; and baldness at birth. Abnormalities in the dentition have been reported, including small, misplaced teeth with poor enamel and severe caries. At present, there is no thorough description of the dental abnormalities seen in a patient with Timothy syndrome.

Mutations in the homeodomain of HOXD13 cause syndactyly type 1-c in two Chinese families

Background

Syndactyly type 1 (SD1) is an autosomal dominant limb malformation characterized in its classical form by complete or partial webbing between the third and fourth fingers and/or the second and third toes. Its four subtypes (a, b, c, and d) are defined based on variable phenotypes, but the responsible gene is yet to be identified. SD1-a has been mapped to chromosome 3p21.31 and SD1-b to 2q34–q36. SD1-c and SD1-d are very rare and, to our knowledge, no gene loci have been identified.

Methods and Results

In two Chinese families with SD1-c, linkage and haplotype analyses mapped the disease locus to 2q31-2q32. Copy number variation (CNV) analysis, using array-based comparative genomic hybridization (array CGH), excluded the possibility of microdeletion or microduplication. Sequence analyses of related syndactyly genes in this region identified c.917G>A (p.R306Q) in the homeodomain of HOXD13 in family A. Analysis on family B identified the mutation c.916C>G (p.R306G) and therefore confirmed the genetic homogeneity. Luciferase assays indicated that these two mutations affected the transcriptional activation ability of HOXD13. The spectrum of HOXD13 mutations suggested a close genotype-phenotype correlation between the different types of HOXD13-Syndactyly. Overlaps of the various phenotypes were found both among and within families carrying the HOXD13 mutation.

Conclusions

Mutations (p.R306Q and p.R306G) in the homeodomain of HOXD13 cause SD1-c. There are affinities between SD1-c and synpolydactyly. Different limb malformations due to distinct classes of HOXD13 mutations should be considered as a continuum of phenotypes and further classification of syndactyly should be done based on phenotype and genotype.

Autosomal recessive transmission of a rare KRT74 variant causes hair and nail ectodermal dysplasia: allelism with dominant woolly hair/hypotrichosis

Pure hair and nail ectodermal dysplasia (PHNED) comprises a heterogeneous group of rare heritable disorders characterized by brittle hair, hypotrichosis, onychodystrophy and micronychia. Autosomal recessive (AR) PHNED has previously been associated with mutations in either KRT85 or HOXC13 on chromosome 12p11.1-q14.3. We investigated a consanguineous Pakistani family with AR PHNED linked to the keratin gene cluster on 12p11.1 but without detectable mutations in KRT85 and HOXC13. Whole exome sequencing of affected individuals revealed homozygosity for a rare c.821T>C variant (p.Phe274Ser) in the KRT74 gene that segregates AR PHNED in the family. The transition alters the highly conserved Phe274 residue in the coil 1B domain required for long-range dimerization of keratins, suggesting that the mutation compromises the stability of intermediate filaments. Immunohistochemical (IHC) analyses confirmed a strong keratin-74 expression in the nail matrix, the nail bed and the hyponychium of mouse distal digits, as well as in normal human hair follicles. Furthermore, hair follicles and epidermis of an affected family member stained negative for Keratin-74 suggesting a loss of function mechanism mediated by the Phe274Ser substitution. Our observations show for the first time that homozygosity for a KRT74 missense variant may be associated with AR PHNED. Heterozygous KRT74 mutations have previously been associated with autosomal dominant woolly hair/hypotrichosis simplex (ADWH). Thus, our findings expand the phenotypic spectrum associated with KRT74 mutations and imply that a subtype of AR PHNED is allelic with ADWH.

Megalencephaly syndromes: exome pipeline strategies for detecting low-level mosaic mutations

Two megalencephaly (MEG) syndromes, megalencephaly-capillary malformation (MCAP) and megalencephaly-polymicrogyriapolydactyly-hydrocephalus (MPPH), have recently been defined on the basis of physical and neuroimaging features. Subsequently, exome sequencing of ten MEG cases identified de-novo postzygotic mutations in PIK3CA which cause MCAP and de-novo mutations in AKT and PIK3R2 which cause MPPH. Here we present findings from exome sequencing three unrelated megalencephaly patients which identified a causal PIK3CA mutation in two cases and a causal PIK3R2 mutation in the third case. However, our patient with the PIK3R2 mutation which is considered to cause MPPH has a marked bifrontal band heterotopia which is a feature of MCAP. Furthermore, one of our patients with a PIK3CA mutation lacks syndactyly/polydactyly which is a characteristic of MCAP. These findings suggest that the overlap between MCAP and MPPH may be greater than the available studies suggest. In addition, the PIK3CA mutation in one of our patients could not be detected using standard exome analysis because the mutation was observed at a low frequency consistent with somatic mosaicism. We have therefore investigated several alternative methods of exome analysis and demonstrate that alteration of the initial allele frequency spectrum (AFS), used as a prior for variant calling in samtools, had the greatest power to detect variants with low mutant allele frequencies in our 3 MEG exomes and in simulated data. We therefore recommend non-default settings of the AFS in combination with stringent quality control when searching for causal mutation(s) that could have low levels of mutant reads due to post-zygotic mutation.

A trans-acting protein effect causes severe eye malformation in the Mp mouse

Mp is an irradiation-induced mouse mutation associated with microphthalmia, micropinna and hind limb syndactyly. We show that Mp is caused by a 660 kb balanced inversion on chromosome 18 producing reciprocal 3-prime gene fusion events involving Fbn2 and Isoc1. The Isoc1-Fbn2 fusion gene (Isoc1^Mp) mRNA has a frameshift and early stop codon resulting in nonsense mediated decay. Homozygous deletions of Isoc1 do not support a significant developmental role for this gene. The Fbn2-Isoc1 fusion gene (Fbn2^Mp) predicted protein consists of the N-terminal Fibrillin-2 (amino acids 1–2646, exons 1–62) lacking the C-terminal furin-cleavage site with a short out-of-frame extension encoded by the final exon of Isoc1. The Mp limb phenotype is consistent with that reported in Fbn2 null embryos. However, severe eye malformations, a defining feature of Mp, are not seen in Fbn2 null animals. Fibrillin-2^Mp forms large fibrillar structures within the rough endoplasmic reticulum (rER) associated with an unfolded protein response and quantitative mass spectrometry shows a generalised defect in protein secretion in conditioned media from mutant cells. In the embryonic eye Fbn2 is expressed within the peripheral ciliary margin (CM). Mp embryos show reduced canonical Wnt-signalling in the CM – known to be essential for ciliary body development - and show subsequent aplasia of CM-derived structures. We propose that the Mp “worse-than-null” eye phenotype plausibly results from a failure in normal trafficking of proteins that are co-expressed with Fbn2 within the CM. The prediction of similar trans-acting protein effects will be an important challenge in the medical interpretation of human mutations from whole exome sequencing.

With the current increase in large-scale sequencing efforts, correct interpretation of mutation consequences has never been more important. Here, we present evidence for a trans-acting protein effect in a novel mutation of Fbn2, associated with severe developmental eye defects not found in loss of function Fibrillin-2 alleles. The mutant protein is expressed in the developing eye but is unable to exit the cells, instead forming large protein aggregates within the endoplasmic reticulum. We observed ER-stress in mutant eyes, and detected a general reduction to secretion of co-expressed proteins in cell cultures. We propose that similar effects could be caused by mutations to other proteins that are trafficked through the ER, highlighting a disease mechanism that results in different clinical outcomes than observed, or predicted, from loss-off-function alleles.

Cellular mechanisms of ventricular arrhythmias in a mouse model of Timothy syndrome (long QT syndrome 8)

Ca(2+) flux through l-type CaV1.2 channels shapes the waveform of the ventricular action potential (AP) and is essential for excitation-contraction (EC) coupling. Timothy syndrome (TS) is a disease caused by a gain-of-function mutation in the CaV1.2 channel (CaV1.2-TS) that decreases inactivation of the channel, which increases Ca(2+) influx, prolongs APs, and causes lethal arrhythmias. Although many details of the CaV1.2-TS channels are known, the cellular mechanisms by which they induce arrhythmogenic changes in intracellular Ca(2+) remain unclear. We found that expression of CaV1.2-TS channels increased sarcolemmal Ca(2+) "leak" in resting TS ventricular myocytes. This resulted in higher diastolic [Ca(2+)]i in TS ventricular myocytes compared to WT. Accordingly, TS myocytes had higher sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+) spark activity, larger amplitude [Ca(2+)]i transients, and augmented frequency of Ca(2+) waves. The large SR Ca(2+) release in TS myocytes had a profound effect on the kinetics of CaV1.2 current in these cells, increasing the rate of inactivation to a high, persistent level. This limited the amount of influx during EC coupling in TS myocytes. The relationship between the level of expression of CaV1.2-TS channels and the probability of Ca(2+) wave occurrence was non-linear, suggesting that even low levels of these channels were sufficient to induce maximal changes in [Ca(2+)]i. Depolarization of WT cardiomyocytes with a TS AP waveform increased, but did not equalize [Ca(2+)]i, compared to depolarization of TS myocytes with the same waveform. We propose that CaV1.2-TS channels increase [Ca(2+)] in the cytosol and the SR, creating a Ca(2+)overloaded state that increases the probability of arrhythmogenic spontaneous SR Ca(2+) release.