RPGRIP1L mutations are mainly associated with the cerebello-renal phenotype of Joubert syndrome-related disorders

Novel mutation in RPGRIP1L gene causing Joubert syndrome: A case report

INTRODUCTION

Joubert syndrome is a rare disease of genetic origin with autosomal recessive inheritance and extreme genetic heterogeneity with more than 40 causative genes. Joubert syndrome 7 is caused by mutations in the RPGRIP1L gene.

PATIENT CONCERNS

Our report describes a pediatric patient with clinical features compatible with JS type 7 such as hypotonia, developmental delay and aplasia of the cerebellar vermis.

DIAGNOSIS

The clinical features and the MRI of the head and neck which showed alterations at the level of the posterior fossa, with absence of the vermis and horizontal disposition of the cerebellar peduncles, were compatible with Joubert syndrome. Whole exome sequencing detected the variants RPGRIP1L (NM_015272.2) c.697A > T (p. Lys233Ter) and RPGRIP1L (NM_015272.2) c.3545 del (p.Pro1182LeufsTer25).

INTERVENTIONS

Resection was performed to correct the polydactyly. At age 2 years umbilical hernia, adenoid surgery and ventilatory tubes surgery were performed. Renal biopsy confirmed interstitial fibrosis and focally accentuated mild tubular atrophy with focal tubular hypertrophy, compatible with the clinical suspicion of Joubert syndrome. Congenital hip dislocation surgery was performed. The patient underwent surgery for correction of concomitant divergent strabismus and continued with glasses for astigmatism and hyperopia.

OUTCOMES

Sanger sequencing confirmed the patient´s results and the father was found to be a carrier of RPGRIP1L (NM_015272.2) c.697A > T (p. Lys233Ter) and the mother and maternal grandmother as carriers of RPGRIP1L (NM_015272.2) c.3545del (p.Pro1182LeufsTer25). RPGRIP1L:c.3545del novel variant is a deletion which changes the reading frame, altering the RPGR1_C terminal domain and giving rise to an incomplete protein whose functions will be altered.

CONCLUSION

This is the first genetically confirmed case of JS in Colombia, the first carrier of biallelic RPGRIP1L gene mutations with hip dislocation and incomplete glottic closure and the first report of the novel c.3545del likely pathogenic variant causing JS.

A Role for Genetic Modifiers in Tubulointerstitial Kidney Diseases

With the increased availability of genomic sequencing technologies, the molecular bases for kidney diseases such as nephronophthisis and mitochondrially inherited and autosomal-dominant tubulointerstitial kidney diseases (ADTKD) has become increasingly apparent. These tubulointerstitial kidney diseases (TKD) are monogenic diseases of the tubulointerstitium and result in interstitial fibrosis and tubular atrophy (IF/TA). However, monogenic inheritance alone does not adequately explain the highly variable onset of kidney failure and extra-renal manifestations. Phenotypes vary considerably between individuals harbouring the same pathogenic variant in the same putative monogenic gene, even within families sharing common environmental factors. While the extreme end of the disease spectrum may have dramatic syndromic manifestations typically diagnosed in childhood, many patients present a more subtle phenotype with little to differentiate them from many other common forms of non-proteinuric chronic kidney disease (CKD). This review summarises the expanding repertoire of genes underpinning TKD and their known phenotypic manifestations. Furthermore, we collate the growing evidence for a role of modifier genes and discuss the extent to which these data bridge the historical gap between apparently rare monogenic TKD and polygenic non-proteinuric CKD (excluding polycystic kidney disease).

An Overview of Genes Involved in the Pure Joubert Syndrome and in Joubert Syndrome-Related Disorders (JSRD)

Joubert syndrome (JS) is a rare autosomal recessive disease characterized by a peculiar brain malformation, hypotonia, ataxia, developmental delay, abnormal eye movements, and neonatal breathing abnormalities. This picture is often associated with variable multiorgan involvement, mainly of the retina, kidneys and liver, defining a group of conditions termed syndrome and Joubert syndrome-related disorders (JSRD). Currently, more than 30 causative genes have been identified, involved in the development and stability of the primary cilium. Correlations genotype–phenotype are emerging between clinical presentations and mutations in JSRD genes, with implications in terms of molecular diagnosis, prenatal diagnosis, follow-up, and management of mutated patients.

Radiological Features of Joubert's Syndrome

Joubert syndrome (JS) is a rare autosomal recessive disorder. All patients affected by this syndrome presented a characteristic picture of cranial fossa malformations, called “molar tooth sign.” This sign is defined by the presence in axial section at the level of a deck/midbrain, of hypo/dysplasia of the cerebellar vermis, abnormally deep interpeduncular fossa and horizontalized thickened and elongated superior cerebellar peduncles. Although “molar tooth sign” is peculiar of JS, other radiological findings have been also reported in these patients. Here, the authors briefly assumed the principal magnetic resonance imaging findings of JS.

Joubert Syndrome: Diagnostic Evaluation and Follow-up

The follow-up of a child with genetic syndrome is necessarily multidisciplinary because of the multiplicity of problems and calls for close collaboration between different specialists. The primary objective is the total care of the child and his family, regardless of the rarity and complexity of the disease, to obtain the highest possible degree of mental and physical health and autonomy.

Ophthalmological Findings in Joubert Syndrome and Related Disorders

Joubert syndrome (JS) is a rare genetic condition characterized by congenital malformation of the mid-hindbrain, cerebellar ataxia, hypotonia, oculomotor apraxia, hypoplasia of the cerebellar vermis resulting in breathing defects, ataxia, and delayed development. Ophthalmological examination reveals eye involvement with nystagmus and retinal defects. Genetic counseling is important for the prevention of new cases. Great advances have been made in recent years. Management is symptomatic and multidisciplinary. In the present review, we discussed the most frequent ophthalmological anomalies associated with JS and speculated on the role of ciliary physiology in eye development.

Joubert Syndrome and Renal Implication

Twenty-five to 30% of patients with Joubert syndrome (JS) have renal involvement. Two forms of renal disease (RD) have traditionally been described. The less common form is the Dekaban–Arima syndrome, a JS RD that includes congenital blindness and occasional encephalocele. The other, more common RD is juvenile nephronophthisis (NPHP), that presents a progressive interstitial fibrosis, associated with small cysts at the corticomedullary junction. NPHP is the most frequent genetic cause for end-stage RD in the first three decades of life. Symptoms start at approximately 6 years of age with urine concentrating defects, polydipsia, polyuria, and secondary enuresis.

Genetics behind Cerebral Disease with Ocular Comorbidity: Finding Parallels between the Brain and Eye Molecular Pathology

Cerebral visual impairments (CVIs) is an umbrella term that categorizes miscellaneous visual defects with parallel genetic brain disorders. While the manifestations of CVIs are diverse and ambiguous, molecular diagnostics stand out as a powerful approach for understanding pathomechanisms in CVIs. Nevertheless, the characterization of CVI disease cohorts has been fragmented and lacks integration. By revisiting the genome-wide and phenome-wide association studies (GWAS and PheWAS), we clustered a handful of renowned CVIs into five ontology groups, namely ciliopathies (Joubert syndrome, Bardet–Biedl syndrome, Alstrom syndrome), demyelination diseases (multiple sclerosis, Alexander disease, Pelizaeus–Merzbacher disease), transcriptional deregulation diseases (Mowat–Wilson disease, Pitt–Hopkins disease, Rett syndrome, Cockayne syndrome, X-linked alpha-thalassaemia mental retardation), compromised peroxisome disorders (Zellweger spectrum disorder, Refsum disease), and channelopathies (neuromyelitis optica spectrum disorder), and reviewed several mutation hotspots currently found to be associated with the CVIs. Moreover, we discussed the common manifestations in the brain and the eye, and collated animal study findings to discuss plausible gene editing strategies for future CVI correction.

Attention to renal involvement: report of 17 Joubert syndrome cases in children of a single center in China

Background

Joubert Syndrome (JS) is a rare genetic developmental disorder. We are aiming for increasing awareness of this disease especially kidney involvement in children with JS.

Methods

Clinical and genetic data of 17 cases of JS in Beijing children’s hospital in the past 21 years were collected retrospectively.

Results

Twelve males and 5 females, aged from 12d to 15y8m. The most common involvement was neurological system involvement. The second most common involvement was renal involvement: end stage kidney disease in 6 cases (35%), hematuria in 5 cases (29%), proteinuria in 5 cases (29%), renal diffuse lesions in 4 cases (24%), renal cystic lesions in 2 cases (12%), and echogenic enhancement of parenchyma in 2 cases (12%). 10 cases did genetic tests. 3 cases with renal deficiency all had RPGRIP1L gene mutation.

Conclusions

The most common involvement of JS is neurological involvement, and the second is renal involvement. Pediatricians should improve awareness of JS and conduct systemic evaluation of children. More attention should be paid to renal involvement which may be onset hidden but fatal. Early recognition and diagnosis are the goals to delay the start to dialysis and improve quality of patients’ life. The RPGRIP1L gene mutation maybe the most common gene mutation in JS and may have correlations with renal involvement.

Clinical and genetic spectrums of 413 North African families with inherited retinal dystrophies and optic neuropathies

Background

Inherited retinal dystrophies (IRD) and optic neuropathies (ION) are the two major causes world-wide of early visual impairment, frequently leading to legal blindness. These two groups of pathologies are highly heterogeneous and require combined clinical and molecular diagnoses to be securely identified. Exact epidemiological studies are lacking in North Africa, and genetic studies of IRD and ION individuals are often limited to case reports or to some families that migrated to the rest of the world. In order to improve the knowledge of their clinical and genetic spectrums in North Africa, we reviewed published data, to illustrate the most prevalent pathologies, genes and mutations encountered in this geographical region, extending from Morocco to Egypt, comprising 200 million inhabitants.

Main body

We compiled data from 413 families with IRD or ION together with their available molecular diagnosis. The proportion of IRD represents 82.8% of index cases, while ION accounted for 17.8%. Non-syndromic IRD were more frequent than syndromic ones, with photoreceptor alterations being the main cause of non-syndromic IRD, represented by retinitis pigmentosa, Leber congenital amaurosis, and cone-rod dystrophies, while ciliopathies constitute the major part of syndromic-IRD, in which the Usher and Bardet Biedl syndromes occupy 41.2% and 31.1%, respectively. We identified 71 ION families, 84.5% with a syndromic presentation, while surprisingly, non-syndromic ION are scarcely reported, with only 11 families with autosomal recessive optic atrophies related to OPA7 and OPA10 variants, or with the mitochondrial related Leber ION. Overall, consanguinity is a major cause of these diseases within North African countries, as 76.1% of IRD and 78.8% of ION investigated families were consanguineous, explaining the high rate of autosomal recessive inheritance pattern compared to the dominant one. In addition, we identified many founder mutations in small endogamous communities.

Short conclusion

As both IRD and ION diseases constitute a real public health burden, their under-diagnosis in North Africa due to the absence of physicians trained to the identification of inherited ophthalmologic presentations, together with the scarcity of tools for the molecular diagnosis represent major political, economic and health challenges for the future, to first establish accurate clinical diagnoses and then treat patients with the emergent therapies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02340-7.

Genotype-phenotype correlates in Joubert syndrome: A review

Joubert syndrome (JS) is a genetically heterogeneous primary ciliopathy characterized by a pathognomonic cerebellar and brainstem malformation, the “molar tooth sign,” and variable organ involvement. Over 40 causative genes have been identified to date, explaining up to 94% of cases. To date, gene‐phenotype correlates have been delineated only for a handful of genes, directly translating into improved counseling and clinical care. For instance, JS individuals harboring pathogenic variants in TMEM67 have a significantly higher risk of liver fibrosis, while pathogenic variants in NPHP1, RPGRIP1L, and TMEM237 are frequently associated to JS with renal involvement, requiring a closer monitoring of liver parameters, or renal functioning. On the other hand, individuals with causal variants in the CEP290 or AHI1 need a closer surveillance for retinal dystrophy and, in case of CEP290, also for chronic kidney disease. These examples highlight how an accurate description of the range of clinical symptoms associated with defects in each causative gene, including the rare ones, would better address prognosis and help guiding a personalized management. This review proposes to address this issue by assessing the available literature, to confirm known, as well as to propose rare gene‐phenotype correlates in JS.

Analysis of recent shared ancestry in a familial cohort identifies coding and noncoding autism spectrum disorder variants

Autism spectrum disorder (ASD) is a collection of neurodevelopmental disorders characterized by deficits in social communication and restricted, repetitive patterns of behavior or interests. ASD is highly heritable, but genetically and phenotypically heterogeneous, reducing the power to identify causative genes. We performed whole genome sequencing (WGS) in an ASD cohort of 68 individuals from 22 families enriched for recent shared ancestry. We identified an average of 3.07 million variants per genome, of which an average of 112,512 were rare. We mapped runs of homozygosity (ROHs) in affected individuals and found an average genomic homozygosity of 9.65%, consistent with expectations for multiple generations of consanguineous unions. We identified potentially pathogenic rare exonic or splice site variants in 12 known (including KMT2C, SCN1A, SPTBN1, SYNE1, ZNF292) and 12 candidate (including CHD5, GRB10, PPP1R13B) ASD genes. Furthermore, we annotated noncoding variants in ROHs with brain-specific regulatory elements and identified putative disease-causing variants within brain-specific promoters and enhancers for 5 known ASD and neurodevelopmental disease genes (ACTG1, AUTS2, CTNND2, CNTNAP4, SPTBN4). We also identified copy number variants in two known ASD and neurodevelopmental disease loci in two affected individuals. In total we identified potentially etiological variants in known ASD or neurodevelopmental disease genes for ~61% (14/23) of affected individuals. We combined WGS with homozygosity mapping and regulatory element annotations to identify candidate ASD variants. Our analyses add to the growing number of ASD genes and variants and emphasize the importance of leveraging recent shared ancestry to map disease variants in complex neurodevelopmental disorders.

Nephronophthisis-Pathobiology and Molecular Pathogenesis of a Rare Kidney Genetic Disease

The exponential rise in our understanding of the aetiology and pathophysiology of genetic cystic kidney diseases can be attributed to the identification of cystogenic genes over the last three decades. The foundation of this was laid by positional cloning strategies which gradually shifted towards next-generation sequencing (NGS) based screenings. This shift has enabled the discovery of novel cystogenic genes at an accelerated pace unlike ever before and, most notably, the past decade has seen the largest increase in identification of the genes which cause nephronophthisis (NPHP). NPHP is a monogenic autosomal recessive cystic kidney disease caused by mutations in a diverse clade of over 26 identified genes and is the most common genetic cause of renal failure in children. NPHP gene types present with some common pathophysiological features alongside a diverse range of extra-renal phenotypes associated with specific syndromic presentations. This review provides a timely update on our knowledge of this disease, including epidemiology, pathophysiology, anatomical and molecular features. We delve into the diversity of the NPHP causing genes and discuss known molecular mechanisms and biochemical pathways that may have possible points of intersection with polycystic kidney disease (the most studied renal cystic pathology). We delineate the pathologies arising from extra-renal complications and co-morbidities and their impact on quality of life. Finally, we discuss the current diagnostic and therapeutic modalities available for disease management, outlining possible avenues of research to improve the prognosis for NPHP patients.

SLC2A1 and Its Related Epileptic Phenotypes

Glucose transporter type 1 deficiency syndrome (GLUT1DS) is caused by heterozygous, mostly de novo, mutations in SLC2A1 gene encoding the glucose transporter GLUT1, the most relevant energy transporter in the blood–brain barrier. GLUT1DS includes a broad spectrum of neurologic disturbances, from severe encephalopathy with developmental delay, to epilepsy, movement disorders, acquired microcephaly and atypical mild forms. For diagnosis, lumbar puncture and genetic analysis are necessary and complementary; an immediate response to ketogenic diet supports the diagnosis in case of high suspicion of disease and negative exams. The ketogenic diet is the first-line treatment and should be established at the initial stages of disease.

KCNT1-Related Epilepsy: A Review

KCNT1 gene encodes the sodium-dependent potassium channel reported as a causal factor for several different epileptic disorders. The gene has been also linked with cardiac disorders and in a family to sudden unexpected death in epilepsy. KCNT1 mutations, in most cases, result in a gain of function causing a neuronal hyperpolarization with loss of inhibition. Many early-onset epileptic encephalopathies related to gain of function of KCNT1 gene have been described, most often associated with two phenotypes: malignant migrating focal seizures of infancy and familial autosomal-dominant nocturnal frontal lobe epilepsy; however, there is no clear phenotype–genotype correlation, in fact same mutations have been represented in patients with West syndrome, Ohtahara syndrome, and early myoclonic encephalopathy. Additional neurologic features include intellectual disability, psychiatric disorders, hypotonia, microcephaly, strabismus, and movement disorders. Conventional anticonvulsant, vagal stimulation, and ketogenic diet have been used in the absence of clinical benefit in individuals with KCNT1-related epilepsy; in some patients, quinidine therapy off-label has been practiced successfully. This review aims to describe the characteristics of the gene, the phenotypes related to genetic mutations with the possible genotype–phenotype correlations and the treatments proposed to date, discussing the comorbidities reported in the literature.

DNM1 Gene and Its Related Epileptic Phenotypes

The phenotypic variety associated to mutations in dynamin 1 (DNM1), codifying the presynaptic protein DNM1 has been increasingly reported, mainly related to encephalopathy with intractable epilepsy; currently, it is known the phenotype related to DNM1 gene mutations is relatively homogeneous with developmental delay, hypotonia, and epilepsy characterized by infantile spasms and possible progression to Lennox-Gastaut syndrome. By examining all the papers published until 2020 (18 articles), we compared data from 30 patients (extrapolated from 5 papers) with DNM1 mutations, identifying 26 patients with de novo mutations in DNM1. Nine patients (33.3%) reported the recurrent mutation p.Arg237Trp. A usual phenotype observed comprises severe to deep developmental delay and muscular hypotonia in all patients with epilepsy beginning with infantile spasms, which often evolved into Lennox-Gastaut syndrome. Data about GTPase or central domains mutations, and existing structural modeling and functional suggest a dominant negative effect on DMN1 function. Generally genetic epilepsies consist of a wide spectrum of clinical features, unlike that, DNM1-related CNS impairment phenotype is quite uniform. In up to one third of patients it has been found variant p.Arg237Trp, which is one of the most frequent variant detected in epileptic encephalopathies. The understanding of DNM1 function opens up the chance that this gene would become a new therapeutic target for epilepsies.

GRIN2A and GRIN2B and Their Related Phenotypes

Glutamate is the most relevant excitatory neurotransmitter of the central nervous system; it binds with several receptors, including N-methyl-D-aspartate receptors (NMDARs), a subtype of ionotropic glutamate receptor that displays voltage-dependent block by Mg2+ and a high permeability to Ca2+. GRIN2A and GRIN2B genes encode the GluN2A and GluN2B subunits of the NMDARs, which play important roles in synaptogenesis, synaptic transmission, and synaptic plasticity, as well as contributing to neuronal loss and dysfunction in several neurological disorders. Recently, individuals with a range of childhood-onset drug-resistant epilepsies, such as Landau–Kleffner or Lennox–Gastaut syndrome, intellectual disability (ID), and other neurodevelopmental abnormalities have been found to carry mutations in GRIN2A and GRIN2B, with high variable expressivity in phenotype. The first one is found mainly in epilepsy-aphasia syndromes, while the second one mainly in autism, schizophrenia, and ID, such as autism spectrum disorders. Brain magnetic resonance imaging alterations are found in some patients, even if without a clear clinical correlation. At the same time, increasing data on genotype–phenotype correlation have been found, but this is still not fully demonstrated. There are no specific therapies for the treatment of correlated NMDARs epilepsy, although some evidence with memantine, an antagonist of glutamate receptor, is reported in the literature in selected cases with mutation determining a gain of function.

PCDH19-Related Epilepsies

Protocadherin-19 (PCDH19) is considered one of the most relevant genes related to epilepsy. To date, more than 150 mutations have been identified as causative for PCDH19-female epilepsy (also known as early infantile epileptic encephalopathy-9, EIEE9), which is characterized by early onset epilepsy, intellectual disabilities, and behavioral disturbances. More recently, mosaic-males (i.e., exhibiting the variants in less than 25% of their cells) have been described as affected by infant-onset epilepsy associated with intellectual disability, as well as compulsive or aggressive behavior and autistic features. Although little is known about the physiological role of PCDH19 protein and the pathogenic mechanisms that lead to EIEE9, many reports and clinical observation seem to suggest a relevant role of this protein in the development of cellular hyperexcitability. However, a genotype–phenotype correlation is difficult to establish. The main feature of EIEE9 consists in early onset of seizures, which generally occur in clusters lasting 1 to 5 minutes and repeating up to 10 times a day for several days. Seizures tend to present during febrile episodes, similarly to the first phases of Dravet syndrome and PCDH19 variants have been found in ∼25% of females who present with features of Dravet syndrome and testing negative for SCN1A variants. There is no “standardized” treatment for PCDH19-related epilepsy and most of the patients receiving a combination of several drugs. In this review, we focus on the latest researches on these aspects, with regard to protein expression, its known functions, and the mechanisms by which the protein acts. The clinical phenotypes related to PCDH19 mutations are also discussed.

SLC25A22 and Its Related Epileptic Encephalopathies

Epileptic encephalopathy is a condition in which seizures, electroencephalographic epileptiform abnormalities lead to a progressive deterioration of brain functions causing a significant psychomotor delay. One of the typical features of this heterogeneous and large group of severe disorders is the extremely early onset of seizures. The main causes of the epileptic encephalopathies include structural brain defects, inherited metabolic disorders; in this aspect, more than 100 genetic defects, including mutations in the solute carrier family 25 (SLC25A22) gene which encodes a mitochondrial glutamate carrier. To date, the main clinical phenotypes related to mutations of this gene are Ohtahara syndrome (or early infantile epileptic encephalopathy), early myoclonic encephalopathy and migrating partial seizures in infancy. In all the cases, prognosis is poor and no disease-modifying treatment is available in the present days.

Aristaless-Related Homeobox (ARX): Epilepsy Phenotypes beyond Lissencephaly and Brain Malformations

The Aristaless-related homeobox (ARX) transcription factor is involved in the development of GABAergic and cholinergic neurons in the forebrain. ARX mutations have been associated with a wide spectrum of neurodevelopmental disorders in humans and are responsible for both malformation (in particular lissencephaly) and nonmalformation complex phenotypes. The epilepsy phenotypes related to ARX mutations are West syndrome and X-linked infantile spasms, X-linked myoclonic epilepsy with spasticity and intellectual development and Ohtahara and early infantile epileptic encephalopathy syndrome, which are related in most of the cases to intellectual disability and are often drug resistant. In this article, we shortly reviewed current knowledge of the function of ARX with a particular attention on its consequences in the development of epilepsy during early childhood.

Cilia, ciliopathies and hedgehog-related forebrain developmental disorders

Development of the forebrain critically depends on the Sonic Hedgehog (Shh) signaling pathway, as illustrated in humans by the frequent perturbation of this pathway in holoprosencephaly, a condition defined as a defect in the formation of midline structures of the forebrain and face. The Shh pathway requires functional primary cilia, microtubule-based organelles present on virtually every cell and acting as cellular antennae to receive and transduce diverse chemical, mechanical or light signals. The dysfunction of cilia in humans leads to inherited diseases called ciliopathies, which often affect many organs and show diverse manifestations including forebrain malformations for the most severe forms. The purpose of this review is to provide the reader with a framework to understand the developmental origin of the forebrain defects observed in severe ciliopathies with respect to perturbations of the Shh pathway. We propose that many of these defects can be interpreted as an imbalance in the ratio of activator to repressor forms of the Gli transcription factors, which are effectors of the Shh pathway. We also discuss the complexity of ciliopathies and their relationships with forebrain disorders such as holoprosencephaly or malformations of cortical development, and emphasize the need for a closer examination of forebrain defects in ciliopathies, not only through the lens of animal models but also taking advantage of the increasing potential of the research on human tissues and organoids.

Molecular genetic analysis of 30 families with Joubert syndrome

Joubert syndrome (JS) is rare recessive disorders characterized by the combination of hypoplasia/aplasia of the cerebellar vermis, thickened and elongated superior cerebellar peduncles, and a deep interpeduncular fossa which is defined by neuroimaging and is termed the 'molar tooth sign'. JS is genetically highly heterogeneous, with at least 29 disease genes being involved. To further understand the genetic causes of JS, we performed whole-exome sequencing in 24 newly recruited JS families. Together with six previously reported families, we identified causative mutations in 25 out of 30 (24 + 6) families (83.3%). We identified eight mutated genes in 27 (21 + 6) Japanese families, TMEM67 (7/27, 25.9%) and CEP290 (6/27, 22.2%) were the most commonly mutated. Interestingly, 9 of 12 CEP290 disease alleles were c.6012-12T>A (75.0%), an allele that has not been reported in non-Japanese populations. Therefore c.6012-12T>A is a common allele in the Japanese population. Importantly, one Japanese and one Omani families carried compound biallelic mutations in two distinct genes (TMEM67/RPGRIP1L and TMEM138/BBS1, respectively). BBS1 is the causative gene in Bardet-Biedl syndrome. These concomitant mutations led to severe and/or complex clinical features in the patients, suggesting combined effects of different mutant genes.

Nosological delineation of congenital ocular motor apraxia type Cogan: an observational study

Background

The nosological assignment of congenital ocular motor apraxia type Cogan (COMA) is still controversial. While regarded as a distinct entity by some authorities including the Online Mendelian Inheritance in Man catalog of genetic disorders, others consider COMA merely a clinical symptom.

Methods

We performed a retrospective multicenter data collection study with re-evaluation of clinical and neuroimaging data of 21 previously unreported patients (8 female, 13 male, ages ranging from 2 to 24 years) diagnosed as having COMA.

Results

Ocular motor apraxia (OMA) was recognized during the first year of life and confined to horizontal pursuit in all patients. OMA attenuated over the years in most cases, regressed completely in two siblings, and persisted unimproved in one individual. Accompanying clinical features included early onset ataxia in most patients and cognitive impairment with learning disability (n = 6) or intellectual disability (n = 4). Re-evaluation of MRI data sets revealed a hitherto unrecognized molar tooth sign diagnostic for Joubert syndrome in 11 patients, neuroimaging features of Poretti-Boltshauser syndrome in one case and cerebral malformation suspicious of a tubulinopathy in another subject. In the remainder, MRI showed vermian hypo-/dysplasia in 4 and no abnormalities in another 4 patients. There was a strong trend to more severe cognitive impairment in patients with Joubert syndrome compared to those with inconclusive MRI, but otherwise no significant difference in clinical phenotypes between these two groups.

Conclusions

Systematical renewed analysis of neuroimaging data resulted in a diagnostic reappraisal in the majority of patients with early-onset OMA in the cohort reported here. This finding poses a further challenge to the notion of COMA constituting a separate entity and underlines the need for an expert assessment of neuroimaging in children with COMA, especially if they show cognitive impairment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13023-016-0486-z) contains supplementary material, which is available to authorized users.

Spontaneous malformations of the cerebellar vermis: Prevalence, inheritance, and relationship to lobule/fissure organization in the C57BL/6 lineage

The complex neuronal circuitry of the cerebellum is embedded within its lamina, folia, and lobules, which together play an important role in sensory and motor function. Studies in mouse models have demonstrated that both cerebellar lamination and lobule/fissure development are under genetic control. The cerebellar vermis of C57BL/6 mice exhibits spontaneous malformations of neuronal migration of posterior lobules (VIII-IX; molecular layer heterotopia); however, the extent to which other inbred mice also exhibit these malformations is unknown. Using seven different inbred mouse strains and two first filial generation (F1) hybrids, we show that only the C57BL/6 strain exhibits heterotopia. Furthermore, we observed heterotopia in consomic and recombinant inbred strains. These data indicate that heterotopia formation is a weakly penetrant trait requiring homozygosity of one or more C57BL/6 alleles outside of chromosome 1 and the sex chromosomes. Additional morphological analyses showed no relationship between heterotopia formation and other features of lobule/fissure organization. These data are relevant toward understanding normal cerebellar development and disorders affecting cerebellar foliation and lamination.

A de novo 0.63 Mb 6q25.1 deletion associated with growth failure, congenital heart defect, underdeveloped cerebellar vermis, abnormal cutaneous elasticity and joint laxity

Deletions of the long arm of chromosome 6 are rare and are characterized by great clinical variability according to the deletion breakpoint. We report a on 6-year-old girl with a de novo 0.63 Mb deletion on chromosome 6q25.1 who demonstrated multiple congenital anomalies including a ventricular septal defect and an underdeveloped cerebellar vermis. She presented with severe pre- and post-natal growth failure, hyperextensible small joints (Beighton scores = 8/9; with normal parental scores), and an abnormally elastic, redundant skin. Abnormally high upper/lower segment ratio (i.e., 1.34 = > 3SD), mild dysmorphic facial features and developmental delay were also present. The girl's phenotype was compared with: (i) two girls, each previously reported by Bisgaard et al. and Caselli et al. with similar albeit larger (2.6-7.21 Mb) deletions; (ii) seven additional individuals (6 M; 1 F) harboring deletions within the 6q25.1 region reported in the literature; and (iii) ten further patients (5 M; 4 F; 1 unrecorded sex) recorded in the DECIPHER 6.0 database. We reported on the present girl as her findings could contribute to advance the phenotype of 6q deletions. In addition, the present deletion is the smallest so far recorded in the 6q25 region encompassing eight known genes [vs. 41 of Bisgaard et al., and 23 of Caselli et al.,], including the TAB2 (likely responsible for the girl's congenital heart defect), LATS1 gene, and the UST gene (a regulator of the homeostasis of proteoglycans, which could have played a role in the abnormal dermal and cartilage elasticity).

Ciliopathies: the trafficking connection

The primary cilium (PC) is a very dynamic hair-like membrane structure that assembles/disassembles in a cell-cycle-dependent manner and is present in almost every cell type. Despite being continuous with the plasma membrane, a diffusion barrier located at the ciliary base confers the PC properties of a separate organelle with very specific characteristics and membrane composition. Therefore, vesicle trafficking is the major process by which components are acquired for cilium formation and maintenance. In fact, a system of specific sorting signals controls the right of cargo admission into the cilia. Disruption to the ciliary structure or its function leads to multiorgan diseases known as ciliopathies. These illnesses arise from a spectrum of mutations in any of the more than 50 loci linked to these conditions. Therefore, it is not surprising that symptom variability (specific manifestations and severity) among and within ciliopathies appears to be an emerging characteristic. Nevertheless, one can speculate that mutations occurring in genes whose products contribute to the overall vesicle trafficking to the PC (i.e. affecting cilia assembly) will lead to more severe symptoms, whereas those involved in the transport of specific cargoes will result in milder phenotypes. In this review, we summarize the trafficking mechanisms to the cilia and also provide a description of the trafficking defects observed in some ciliopathies which can be correlated to the severity of the pathology.

Joubert syndrome: congenital cerebellar ataxia with the molar tooth

Joubert syndrome is a congenital cerebellar ataxia with autosomal recessive or X-linked inheritance, the diagnostic hallmark of which is a unique cerebellar and brainstem malformation recognisable on brain imaging-the so-called molar tooth sign. Neurological signs are present from the neonatal period and include hypotonia progressing to ataxia, global developmental delay, ocular motor apraxia, and breathing dysregulation. These signs are variably associated with multiorgan involvement, mainly of the retina, kidneys, skeleton, and liver. 21 causative genes have been identified so far, all of which encode for proteins of the primary cilium or its apparatus. The primary cilium is a subcellular organelle that has key roles in development and in many cellular functions, making Joubert syndrome part of the expanding family of ciliopathies. Notable clinical and genetic overlap exists between distinct ciliopathies, which can co-occur even within families. Such variability is probably explained by an oligogenic model of inheritance, in which the interplay of mutations, rare variants, and polymorphisms at distinct loci modulate the expressivity of the ciliary phenotype.

Neuronal migration defect of the developing cerebellar vermis in substrains of C57BL/6 mice: cytoarchitecture and prevalence of molecular layer heterotopia

Abnormal development of the cerebellum is often associated with disorders of movement, postural control, and motor learning. Rodent models are widely used to study normal and abnormal cerebellar development and have revealed the roles of many important genetic and environmental factors. In the present report we describe the prevalence and cytoarchitecture of molecular-layer heterotopia, a malformation of neuronal migration, in the cerebellar vermis of C57BL/6 mice and closely-related strains. In particular, we found a diverse number of cell-types affected by these malformations including Purkinje cells, granule cells, inhibitory interneurons (GABAergic and glycinergic), and glia. Heterotopia were not observed in a sample of wild-derived mice, outbred mice, or inbred mice not closely related to C57BL/6 mice. These data are relevant to the use of C57BL/6 mice as models in the study of brain and behavior relationships and provide greater understanding of human cerebellar dysplasia.

Joubert syndrome and related disorders

Joubert syndrome (JS) is a rare autosomal recessive condition characterized by a peculiar midbrain-hindbrain malformation, known as the molar tooth sign (MTS). The neurological presentation of JS includes hypotonia that evolves into ataxia, developmental delay, abnormal eye movements, and neonatal breathing abnormalities. This picture is often associated with variable multiorgan involvement, mainly of the retina, kidneys, and liver, defining a group of conditions termed Joubert syndrome and related disorders (JSRDs), that share the MTS. To date, 16 causative genes have been identified, all encoding for proteins expressed in the primary cilium or its apparatus. Indeed, JSRD present clinical and genetic overlap with a growing field of disorders due to mutations in ciliary proteins, that are collectively known as "ciliopathies." These include isolated nephronophthisis, Senior-Løken syndrome, Bardet-Biedl syndrome and, in particular, Meckel syndrome, which is allelic at JSRD at seven distinct loci. Significant genotype-phenotype correlates are emerging between specific clinical presentations and mutations in JSRD genes, with relevant implications in terms of molecular diagnosis, clinical follow-up, and management of mutated patients. Moreover, the identification of mutations allows early prenatal diagnosis in couples at risk, while fetal neuroimaging may remain uninformative until the late second trimester of pregnancy.

Photoreceptor sensory cilia and ciliopathies: focus on CEP290, RPGR and their interacting proteins

Ciliopathies encompass a broad array of clinical findings associated with genetic defects in biogenesis and/or function of the primary cilium, a ubiquitous organelle involved in the transduction of diverse biological signals. Degeneration or dysfunction of retinal photoreceptors is frequently observed in diverse ciliopathies. The sensory cilium in a photoreceptor elaborates into unique outer segment discs that provide extensive surface area for maximal photon capture and efficient visual transduction. The daily renewal of approximately 10% of outer segments requires a precise control of ciliary transport. Here, we review the ciliopathies with associated retinal degeneration, describe the distinctive structure of the photoreceptor cilium, and discuss mouse models that allow investigations into molecular mechanisms of cilia biogenesis and defects. We have specifically focused on two ciliary proteins - CEP290 and RPGR - that underlie photoreceptor degeneration and syndromic ciliopathies. Mouse models of CEP290 and RPGR disease, and of their multiple interacting partners, have helped unravel new functional insights into cell type-specific phenotypic defects in distinct ciliary proteins. Elucidation of multifaceted ciliary functions and associated protein complexes will require concerted efforts to assimilate diverse datasets from in vivo and in vitro studies. We therefore discuss a possible framework for investigating genetic networks associated with photoreceptor cilia biogenesis and pathology.

Founder mutations and genotype-phenotype correlations in Meckel-Gruber syndrome and associated ciliopathies

Background

Meckel-Gruber syndrome (MKS) is an autosomal recessive lethal condition that is a ciliopathy. MKS has marked phenotypic variability and genetic heterogeneity, with mutations in nine genes identified as causative to date.

Methods

Families diagnosed with Meckel-Gruber syndrome were recruited for research studies following informed consent. DNA samples were analyzed by microsatellite genotyping and direct Sanger sequencing.

Results

We now report the genetic analyses of 87 individuals from 49 consanguineous and 19 non-consanguineous families in an unselected cohort with reported MKS, or an associated severe ciliopathy in a kindred. Linkage and/or direct sequencing were prioritized for seven MKS genes (MKS1, TMEM216, TMEM67/MKS3, RPGRIP1L, CC2D2A, CEP290 and TMEM237) selected on the basis of reported frequency of mutations or ease of analysis. We have identified biallelic mutations in 39 individuals, of which 13 mutations are novel and previously unreported. We also confirm general genotype-phenotype correlations.

Conclusions

TMEM67 was the most frequently mutated gene in this cohort, and we confirm two founder splice-site mutations (c.1546 + 1 G > A and c.870-2A > G) in families of Pakistani ethnic origin. In these families, we have also identified two separate founder mutations for RPGRIP1L (c. 1945 C > T p.R649X) and CC2D2A (c. 3540delA p.R1180SfsX6). Two missense mutations in TMEM67 (c. 755 T > C p.M252T, and c. 1392 C > T p.R441C) are also probable founder mutations. These findings will contribute to improved genetic diagnosis and carrier testing for affected families, and imply the existence of further genetic heterogeneity in this syndrome.

Ciliary signaling cascades in photoreceptors

For being a polarized neuron and having a sensory cilium, photoreceptors attract remarkable attention. This is due their highly polarized structure and active visual signal transduction cascades and for the enrichment of complex networks of proteins in the cilium. Structural and functional maintenance of the photoreceptor sensory cilium, also called outer segment, ensures that light signal is received and relayed appropriately to the brain. Any perturbations in the protein content of the outer segment result in photoreceptor dysfunction, degeneration and eventually, blindness. This review focuses on the importance of photoreceptor sensory cilium to carry out signal transduction cascade for vision.

Meckelin is necessary for photoreceptor intraciliary transport and outer segment morphogenesis

PURPOSE

Cilia, complex structures found ubiquitously in most vertebrate cells, serve a variety of functions ranging from cell and fluid movement, cell signaling, tissue homeostasis, to sensory perception. Meckelin is a component of ciliary and cell membranes and is encoded by Tmem67 (Mks3). In this study, the retinal morphology and ciliary function in a mouse model for Meckel Syndrome Type 3 (MKS3) throughout the course of photoreceptor development was examined.

METHODS

To study the effects of a disruption in the Mks3 gene on the retina, the authors introduced a functional allele of Pde6b into B6C3Fe a/a-bpck/J mice and evaluated their retinas by ophthalmoscopic, histologic, and ultrastructural examination. In addition, immunofluorescence microscopy was used to assess protein trafficking through the connecting cilium and to examine the localization of ciliary and synaptic proteins in Tmem67(bpck) mice and controls.

RESULTS

Photoreceptors degenerate early and rapidly in bpck/bpck mutant mice. In addition, phototransduction proteins, such as rhodopsin, arrestin, and transducin, are mislocalized. Ultrastructural examination of photoreceptors reveal morphologically intact connecting cilia but dysmorphic and misoriented outer segment (OS) discs, at the earliest time point examined.

CONCLUSIONS

These findings underscore the important role for meckelin in intraciliary transport of phototransduction molecules and their effects on subsequent OS morphogenesis and maintenance.

Delineation and diagnostic criteria of Oral-Facial-Digital Syndrome type VI

Oral-Facial-Digital Syndrome type VI (OFD VI) represents a rare phenotypic subtype of Joubert syndrome and related disorders (JSRD). In the original report polydactyly, oral findings, intellectual disability, and absence of the cerebellar vermis at post-mortem characterized the syndrome. Subsequently, the molar tooth sign (MTS) has been found in patients with OFD VI, prompting the inclusion of OFD VI in JSRD. We studied the clinical, neurodevelopmental, neuroimaging, and genetic findings in a cohort of 16 patients with OFD VI. We derived the following inclusion criteria from the literature: 1) MTS and one oral finding and polydactyly, or 2) MTS and more than one typical oral finding. The OFD VI neuroimaging pattern was found to be more severe than in other JSRD subgroups and includes severe hypoplasia of the cerebellar vermis, hypoplastic and dysplastic cerebellar hemispheres, marked enlargement of the posterior fossa, increased retrocerebellar collection of cerebrospinal fluid, abnormal brainstem, and frequently supratentorial abnormalities that occasionally include characteristic hypothalamic hamartomas. Additionally, two new JSRD neuroimaging findings (ascending superior cerebellar peduncles and fused thalami) have been identified. Tongue hamartomas, additional frenula, upper lip notch, and mesoaxial polydactyly are specific findings in OFD VI, while cleft lip/palate and other types of polydactyly of hands and feet are not specific. Involvement of other organs may include ocular findings, particularly colobomas. The majority of the patients have absent motor development and profound cognitive impairment. In OFD VI, normal cognitive functions are possible, but exceptional. Sequencing of known JSRD genes in most patients failed to detect pathogenetic mutations, therefore the genetic basis of OFD VI remains unknown. Compared with other JSRD subgroups, the neurological findings and impairment of motor development and cognitive functions in OFD VI are significantly worse, suggesting a correlation with the more severe neuroimaging findings. Based on the literature and this study we suggest as diagnostic criteria for OFD VI: MTS and one or more of the following: 1) tongue hamartoma(s) and/or additional frenula and/or upper lip notch; 2) mesoaxial polydactyly of one or more hands or feet; 3) hypothalamic hamartoma.

The ciliopathies in neuronal development: a clinical approach to investigation of Joubert syndrome and Joubert syndrome-related disorders

A group of disorders with disparate symptomatology, including congenital cerebellar ataxia, retinal blindness, liver fibrosis, polycystic kidney disease, and polydactyly, have recently been united under a single disease mechanism called 'ciliopathies'. The ciliopathies are due to defects of the cellular antenna known as the primary cilium, a microtubule-based extension of cellular membranes found in nearly all cell types. Key among these ciliopathies is Joubert syndrome, displaying ataxia, oculomotor apraxia, and mental retardation* with a pathognomonic 'molar tooth sign' on brain magnetic resonance imaging. The importance of ciliary function in neuronal development has been appreciated only in the last decade with the classification of Joubert syndrome as a ciliopathy. This, together with the identification of many of the clinical features of ciliopathies in individuals with Joubert syndrome and the localization of Joubert syndrome's causative gene products at or near the primary cilium, have defined a new class of neurological disease. Cilia are involved in diverse cellular processes including protein trafficking, photoreception, embryonic axis patterning, and cell cycle regulation. Ciliary dysfunction can affect a single tissue or manifest as multi-organ involvement. Ciliary defects have been described in retinopathies such as retinitis pigmentosa and Leber congenital amaurosis (defects in photoreceptor ciliary protein complexes), renal syndromes with nephronophthisis and cystic dysplastic kidneys, and liver conditions such as fibrosis and biliary cirrhosis. Recognizing the diverse presentations of the ciliopathies and screening strategies following diagnosis is an important part of the treatment plan of children with cilia-related disorders.

B9D1 is revealed as a novel Meckel syndrome (MKS) gene by targeted exon-enriched next-generation sequencing and deletion analysis

Meckel syndrome (MKS) is an embryonic lethal, autosomal recessive disorder characterized by polycystic kidney disease, central nervous system defects, polydactyly and liver fibrosis. This disorder is thought to be associated with defects in primary cilia; therefore, it is classed as a ciliopathy. To date, six genes have been commonly associated with MKS (MKS1, TMEM67, TMEM216, CEP290, CC2D2A and RPGRIP1L). However, mutation screening of these genes revealed two mutated alleles in only just over half of our MKS cohort (46 families), suggesting an even greater level of genetic heterogeneity. To explore the full genetic complexity of MKS, we performed exon-enriched next-generation sequencing of 31 ciliopathy genes in 12 MKS pedigrees using RainDance microdroplet-PCR enrichment and IlluminaGAIIx next-generation sequencing. In family M456, we detected a splice-donor site change in a novel MKS gene, B9D1. The B9D1 protein is structurally similar to MKS1 and has been shown to be of importance for ciliogenesis in Caenorhabditis elegans. Reverse transcriptase-PCR analysis of fetal RNA revealed, hemizygously, a single smaller mRNA product with a frameshifting exclusion of B9D1 exon 4. ArrayCGH showed that the second mutation was a 1.713 Mb de novo deletion completely deleting the B9D1 allele. Immunofluorescence analysis highlighted a significantly lower level of ciliated patient cells compared to controls, confirming a role for B9D1 in ciliogenesis. The fetus inherited an additional likely pathogenic novel missense change to a second MKS gene, CEP290; p.R2210C, suggesting oligogenic inheritance in this disorder.

Evaluation of risk loci for schizophrenia derived from genome-wide association studies in a German population

In the genome-wide association study (GWAS) on schizophrenia [O'Donovan et al. (2008); Nat Genet 40:1053–1055] a UK-sample of 479 cases with DSM-IV schizophrenia was genotyped in comparison to control subjects with follow up of 12 putative loci in international replication sets of approximately 15,000 cases and controls. In these cohorts and a combined bipolar and schizophrenia UK-sample, six single nucleotide polymorphisms (SNPs) supported association, with the strongest evidence for SNP-marker rs1344706 at the zinc finger ZNF804A locus on chromosome 2q32.1 (P = 1.61 × 10−7). We attempted replication of these findings in a German population of 2,154 individuals (632 with affective disorders, 937 with schizophrenia, and 585 controls), but found none of the GWAS risk alleles significantly associated with psychosis. Particularly rs1344706, initially surpassing the genome-wide significance level in an extended phenotype of schizophrenia and affective disorder, produced consistently negative results. At the ZNF804A locus estimated Odds ratios reached 1.08 (0.93–1.26 95% CI) for the schizophrenia sample and 1.04 (0.90–1.20 95% CI) for the combined set of cases with schizophrenia and affective disorder. The main limitation of our study may be the reduced power of the sample size, but our data may be useful for future meta-analysis of GWA data sets. Although GWAS have proven extraordinary successful in identifying susceptibility genes for complex genetic disorders, the hypothesis of common genetic variants in the complex group of the schizophrenic psychoses with small effect size but relatively high frequency is still put to further scrutiny.

FTO Biology and Obesity: Why Do a Billion of Us Weigh 3 kg More?

Few would dispute that the current obesity epidemic has been driven by lifestyle and environmental changes. However, it is clear that individuals respond differently to these "obesigenic" changes and this variation in response has a strong genetic element. Genome-wide association studies have revealed that single nucleotide polymorphisms in Fat mass and obesity-associated transcript (FTO) are robustly associated with body mass index and obesity. Although the effect of these risk alleles are modest, with heterozygous and homozygous carriers weighing approximately 1.5 and 3 kg more respectively, there are an estimated one billion homozygous carriers in the world, spanning multiple different ethnicities and populations. Yet despite its broad impact, the biological function of FTO, particularly its role in controlling energy balance, remains unknown. Although the study of severe Mendelian obesity has been invaluable in illuminating critical pathways controlling food intake, the major burden of disease is carried by those of us with "common obesity," which to date has resisted yielding meaningful biological insights. FTO has at last given us a handle on a huge, worldwide, common problem. In this review, we focus on the available genetic and in vivo evidence to date that implicates FTO in the control of energy balance.

Joubert Syndrome and related disorders

Joubert syndrome (JS) and related disorders (JSRD) are a group of developmental delay/multiple congenital anomalies syndromes in which the obligatory hallmark is the molar tooth sign (MTS), a complex midbrain-hindbrain malformation visible on brain imaging, first recognized in JS. Estimates of the incidence of JSRD range between 1/80,000 and 1/100,000 live births, although these figures may represent an underestimate. The neurological features of JSRD include hypotonia, ataxia, developmental delay, intellectual disability, abnormal eye movements, and neonatal breathing dysregulation. These may be associated with multiorgan involvement, mainly retinal dystrophy, nephronophthisis, hepatic fibrosis and polydactyly, with both inter- and intra-familial variability. JSRD are classified in six phenotypic subgroups: Pure JS; JS with ocular defect; JS with renal defect; JS with oculorenal defects; JS with hepatic defect; JS with orofaciodigital defects. With the exception of rare X-linked recessive cases, JSRD follow autosomal recessive inheritance and are genetically heterogeneous. Ten causative genes have been identified to date, all encoding for proteins of the primary cilium or the centrosome, making JSRD part of an expanding group of diseases called "ciliopathies". Mutational analysis of causative genes is available in few laboratories worldwide on a diagnostic or research basis. Differential diagnosis must consider in particular the other ciliopathies (such as nephronophthisis and Senior-Loken syndrome), distinct cerebellar and brainstem congenital defects and disorders with cerebro-oculo-renal manifestations. Recurrence risk is 25% in most families, although X-linked inheritance should also be considered. The identification of the molecular defect in couples at risk allows early prenatal genetic testing, whereas fetal brain neuroimaging may remain uninformative until the end of the second trimester of pregnancy. Detection of the MTS should be followed by a diagnostic protocol to assess multiorgan involvement. Optimal management requires a multidisciplinary approach, with particular attention to respiratory and feeding problems in neonates and infants. Cognitive and behavioral assessments are also recommended to provide young patients with adequate neuropsychological support and rehabilitation. After the first months of life, global prognosis varies considerably among JSRD subgroups, depending on the extent and severity of organ involvement.

RPGR-containing protein complexes in syndromic and non-syndromic retinal degeneration due to ciliary dysfunction

Dysfunction of primary cilia due to mutations in cilia-centrosomal proteins is associated with pleiotropic disorders. The primary (or sensory) cilium of photoreceptors mediates polarized trafficking of proteins for efficient phototransduction. Retinitis pigmentosa GTPase regulator (RPGR) is a cilia-centrosomal protein mutated in >70% of X-linked RP cases and 10%-20% of simplex RP males. Accumulating evidence indicates that RPGR may facilitate the orchestration of multiple ciliary protein complexes. Disruption of these complexes due to mutations in component proteins is an underlying cause of associated photoreceptor degeneration. Here, we highlight the recent developments in understanding the mechanism of cilia-dependent photoreceptor degeneration due to mutations in RPGR and PGR-interacting proteins in severe genetic diseases, including retinitis pigmentosa, Leber congenital amaurosis (LCA), Joubert syndrome, and Senior-Loken syndrome, and explore the physiological relevance of photoreceptor ciliary protein complexes.

Posterior fossa imaging in 158 children with ataxia

OBJECTIFS

To propose a MRI cerebellar algorithm that may be applied to guide genetic/malformative or biochemical investigations for patients with cerebellar ataxia.

PATIENTS AND METHODS

Cerebral MRI of 158 patients with cerebellar ataxia and no supratentorial abnormality were examined according to a new categorization system based on posterior fossa imaging. The clinical and radiological findings were confronted to biochemical and/or genetic results using the MR cerebellar algorithm. Seven groups of cerebellar MRI pattern were described: vermian dysgenesis (n=27), cerebellar hypoplasia (n=15), hemispheric cerebellar dysgenesis (n=6), unilateral hemispheric atrophy (n=5), global cerebellar atrophy (n=84), signal abnormalities (n=11) and normal MRI (n=10). Cerebellar hypoplasia, vermian dysgenesis and hemispheric cerebellar dysgenesis groups were classified as malformative disorders. Global atrophy and signal abnormality groups were classified as metabolic disorders.

RESULTS

In the vermian dysgenesis group, a specific genetic diagnosis was obtained in eight children (8/27) and all of the mutated genes (AHI1 (JBS3), CEP290 (JBS5), TMEM67 (JBS6), and RPGRIP1L (JBS7)) are involved in primary cilia function. In the group of pontocerebellar hypoplasia specific genetic diagnosis was obtained in one patient (PCH2) (1/15). Thus, nine of 42 children classified as malformative disorder had a molecular diagnosis. Global atrophy and signal abnormality groups were classified as metabolic disorders, specific biochemical was obtained in 46/95 children. In global atrophy group, respiratory chain deficiency was diagnosed in 18 children (18/84). In 21 children a congenital disorders of glycosylation type 1a (CDG Ia) was diagnosed (21/84) and infantile neuroaxonale dystrophy (INAD) was diagnosed in one child. In signal abnormalities group, specific biochemical diagnosis was obtained in six out of 11 children, five children with respiratory chain deficiency and one child with sulphite oxidase deficiency. In hemispheric cerebellar dysgenesis and normal MRI groups, no biological diagnosis was found for any of the patients. In the group of unilateral hemispheric atrophy, we hypothesized a clastic prenatal injury.

CONCLUSION

The proposed MR cerebellar algorithm was useful to guide genetic/malformative or biochemical investigations, allowing an etiological diagnosis in 55 children.

The molecular basis of human retinal and vitreoretinal diseases

During the last two to three decades, a large body of work has revealed the molecular basis of many human disorders, including retinal and vitreoretinal degenerations and dysfunctions. Although belonging to the group of orphan diseases, they affect probably more than two million people worldwide. Most excitingly, treatment of a particular form of congenital retinal degeneration is now possible. A major advantage for treatment is the unique structure and accessibility of the eye and its different components, including the vitreous and retina. Knowledge of the many different eye diseases affecting retinal structure and function (night and colour blindness, retinitis pigmentosa, cone and cone rod dystrophies, photoreceptor dysfunctions, as well as vitreoretinal traits) is critical for future therapeutic development. We have attempted to present a comprehensive picture of these disorders, including biological, clinical, genetic and molecular information. The structural organization of the review leads the reader through non-syndromic and syndromic forms of (i) rod dominated diseases, (ii) cone dominated diseases, (iii) generalized retinal degenerations and (iv) vitreoretinal disorders, caused by mutations in more than 165 genes. Clinical variability and genetic heterogeneity have an important impact on genetic testing and counselling of affected families. As phenotypes do not always correlate with the respective genotypes, it is of utmost importance that clinicians, geneticists, counsellors, diagnostic laboratories and basic researchers understand the relationships between phenotypic manifestations and specific genes, as well as mutations and pathophysiologic mechanisms. We discuss future perspectives.

Novel VLDLR microdeletion identified in two Turkish siblings with pachygyria and pontocerebellar atrophy

Congenital ataxia with cerebellar hypoplasia is a heterogeneous group of disorders that presents with motor disability, hypotonia, incoordination, and impaired motor development. Among these, disequilibrium syndrome describes a constellation of findings including non-progressive cerebellar ataxia, mental retardation, and cerebellar hypoplasia following an autosomal recessive pattern of inheritance and can be caused by mutations in the Very Low Density Lipoprotein Receptor (VLDLR). Interestingly, while the majority of patients with VLDL-associated cerebellar hypoplasia in the literature use bipedal gait, the previously reported patients of Turkish decent have demonstrated similar neurological sequelae, but rely on quadrupedal gait. We present a consanguinous Turkish family with two siblings with cerebellar atrophy, predominantly frontal pachygyria and ataxic bipedal gait, who were found to have a novel homozygous deletion in the VLDLR gene identified by using high-density single nucleotide polymorphism microarrays for homozygosity mapping and identification of CNVs within these regions. Discovery of disease causing homozygous deletions in the present Turkish family capable of maintaining bipedal movement exemplifies the phenotypic heterogeneity of VLDLR-associated cerebellar hypoplasia and ataxia.

Clinical and molecular features of Joubert syndrome and related disorders

Joubert syndrome (JBTS; OMIM 213300) is a rare, autosomal recessive disorder characterized by a specific congenital malformation of the hindbrain and a broad spectrum of other phenotypic findings that is now known to be caused by defects in the structure and/or function of the primary cilium. The complex hindbrain malformation that is characteristic of JBTS can be identified on axial magnetic resonance imaging and is known as the molar tooth sign (MTS); other diagnostic criteria include intellectual disability, hypotonia, and often, abnormal respiratory pattern and/or abnormal eye movements. In addition, a broad spectrum of other anomalies characterize Joubert syndrome and related disorders (JSRD), and may include retinal dystrophy, ocular coloboma, oral frenulae and tongue tumors, polydactyly, cystic renal disease (including cystic dysplasia or juvenile nephronophthisis), and congenital hepatic fibrosis. The clinical course can be variable, but most children with this condition survive infancy to reach adulthood. At least eight genes cause JSRD, with some genotype-phenotype correlations emerging, including the association between mutations in the MKS3 gene and hepatic fibrosis characteristic of the JSRD subtype known as COACH syndrome. Several of the causative genes for JSRD are implicated in other ciliary disorders, such as juvenile nephronophthisis and Meckel syndrome, illustrating the close association between these conditions and their overlapping clinical features that reflect a shared etiology involving the primary cilium.

A developmental and genetic classification for midbrain-hindbrain malformations

Advances in neuroimaging, developmental biology and molecular genetics have increased the understanding of developmental disorders affecting the midbrain and hindbrain, both as isolated anomalies and as part of larger malformation syndromes. However, the understanding of these malformations and their relationships with other malformations, within the central nervous system and in the rest of the body, remains limited. A new classification system is proposed, based wherever possible, upon embryology and genetics. Proposed categories include: (i) malformations secondary to early anteroposterior and dorsoventral patterning defects, or to misspecification of mid-hindbrain germinal zones; (ii) malformations associated with later generalized developmental disorders that significantly affect the brainstem and cerebellum (and have a pathogenesis that is at least partly understood); (iii) localized brain malformations that significantly affect the brain stem and cerebellum (pathogenesis partly or largely understood, includes local proliferation, cell specification, migration and axonal guidance); and (iv) combined hypoplasia and atrophy of putative prenatal onset degenerative disorders. Pertinent embryology is discussed and the classification is justified. This classification will prove useful for both physicians who diagnose and treat patients with these disorders and for clinical scientists who wish to understand better the perturbations of developmental processes that produce them. Importantly, both the classification and its framework remain flexible enough to be easily modified when new embryologic processes are described or new malformations discovered.

Combination of cor triatriatum sinistrum and hypoplastic left heart syndrome in Meckel-Gruber syndrome: a case report

Meckel-Gruber syndrome (MKS) is a fatal, autosomal recessive disorder characterized by malformation of central nervous system, particularly occipital encephalocele, bilateral renal dysplasia, and polydactyly. However, the clinical findings of this syndrome encompass various organ abnormalities as a result of genetic heterogeneity. The associated heart anomaly in MKS is inconstant. Its prevalence is rare and no striking or specific cardiac defects have been documented. We present a case of MKS with combined cor triatriatum sinistrum (left atrium divided into upper and lower compartment by a thin membrane) and hypoplastic left heart syndrome (underdeveloped mitral valve, left ventricle, and aorta) in a 33-week male fetus that was ultrasonographically detected and confirmed by autopsy. In addition to the cardiac defects, the patient was found to have postaxial polydactyly of 4 extremities, Dandy-Walker malformation, bilateral renal cystic dysplasia, and hepatic plate malformation. To the best of our knowledge, this is the first time that a combination of cor triatriatum sinistrum and hypoplastic left heart syndrome in MKS has been reported in the literature.

Cholangiociliopathies: genetics, molecular mechanisms and potential therapies

PURPOSE OF REVIEW

The present review summarizes recent knowledge on polycystic liver diseases (PCLDs), mechanisms of hepatic cystogenesis and potential therapies for these conditions.

RECENT FINDINGS

PCLD may be classified as cholangiociliopathies. In PCLD associated with polycystic kidney disease, cell proliferation is one of the major mechanisms of cystogenesis, whereas in isolated PCLD (autosomal dominant polycystic liver disease), disrupted cell adhesion may be more important in cyst progression. In cystic cholangiocytes, overexpression of ion transporters and water channels facilitates fluid secretion into the cystic lumen, and growth factors, estrogens and cytokines promote cholangiocyte proliferation. With age, cholangiocytes lining liver cysts acquire features of mesenchymal cells contributing to hepatic fibrocystogenesis. A novel mechanism of liver cyst expansion in PCLD involves microRNA regulatory pathways. Hyperproliferation of cystic cholangiocytes is linked to abnormalities in cell cycle progression and microRNA expression. Decreased levels of miR-15a are coupled to upregulation of its target--the cell cycle regulator, Cdc25A. Cholangiocyte cilia in liver cysts are structurally abnormal. Somatostatin analogues and sirolimus reduce liver cyst volume in PCLD patients.

SUMMARY

Clarification of molecular mechanisms of hepatic cystogenesis provides an opportunity for the development of targeted therapeutic options in PCLD.