Identification of the TTC26 Splice Variant in a Novel Complex Ciliopathy Syndrome with Biliary, Renal, Neurological, and Skeletal Manifestations.

Ciliopathies constitute heterogeneous disorders that result from mutations in ciliary proteins. These proteins play an important role in the development of organs, physiology, and signaling pathways, and sequence variations in the genes encoding these proteins are associated with multisystem disorders. In this study, we describe a severe ciliopathy disorder that segregates in an autosomal recessive manner in a nonconsanguineous Saudi family. The proband exhibited features such as cholestasis, cystic dilatation of intrahepatic biliary ducts, diabetes insipidus, dysmorphic facial features, optic atrophy, pituitary hypoplasia, hydrocephalus, aqueductal stenosis, hyperextensible knee joints, bilateral knee dislocation, polydactyly, and syndactyly. Whole-genome sequencing and Sanger sequencing revealed a homozygous splice site variant (c.4-1G>C; NM_024926.3) in the tetratricopeptide repeat domain 26 (TTC26) gene located in chromosome 7q34, which cosegregated perfectly with the disease phenotype. qRT-PCR revealed a substantial decrease in the expression of the TTC26 gene as compared to the normal control, suggesting the pathogenicity of the identified variant. This report further strengthens the evidence that homozygous variants in the TTC26 gene cause severe ciliopathies with diverse phenotypes. We named this newly characterized condition as BRENS syndrome, which stands for biliary, renal, neurological, and skeletal features.

The genotypic and phenotypic spectrum of pycnodysostosis in Saudi Arabia: Novel variants and clinical findings.

Pycnodysostosis is characterized by short stature, osteosclerosis, acro-osteolysis, increased tendency of fractures, and distinctive dysmorphic features. It is a rare autosomal recessive disease caused by biallelic CTSK mutations. The clinical details of 18 patients from Saudi Arabia were reviewed. Short stature, osteopetrosis, acro-osteolysis, and distinctive facial dysmorphism were documented in all cases. Our results highlight the significant complications associated with this disease. The large anterior fontanelle is one of the cardinal signs of this disease; however, half of our patients had small fontanelles and a quarter had craniosynostosis, which caused optic nerve compression. Sleep apnea was of the major complications in three patients. Bone fracture can be a presenting symptom, and in our patients it mainly occurred after the age of 3 years. Bone marrow suppression was seen in a single patient of our cohort who was misdiagnosed initially with malignant osteopetrosis. In this study, we also describe two novel (c.5G > A [p.Trp2Ter], c.538G > A [p.Gly180Ser]) and two reported (c.244-29 A > G, c.830C > T [p.Ala277Val]) CTSK mutations. Our results indicate that the recurrent intronic variant, c.244-29 A > G is likely to be a founder mutation, as it was found in 78% (14/18 patients) of our cohort belonging to the same tribe.

Mutations in TOP3A Cause a Bloom Syndrome-like Disorder.

Bloom syndrome, caused by biallelic mutations in BLM, is characterized by prenatal-onset growth deficiency, short stature, an erythematous photosensitive malar rash, and increased cancer predisposition. Diagnostically, a hallmark feature is the presence of increased sister chromatid exchanges (SCEs) on cytogenetic testing. Here, we describe biallelic mutations in TOP3A in ten individuals with prenatal-onset growth restriction and microcephaly. TOP3A encodes topoisomerase III alpha (TopIIIα), which binds to BLM as part of the BTRR complex, and promotes dissolution of double Holliday junctions arising during homologous recombination. We also identify a homozygous truncating variant in RMI1, which encodes another component of the BTRR complex, in two individuals with microcephalic dwarfism. The TOP3A mutations substantially reduce cellular levels of TopIIIα, and consequently subjects' cells demonstrate elevated rates of SCE. Unresolved DNA recombination and/or replication intermediates persist into mitosis, leading to chromosome segregation defects and genome instability that most likely explain the growth restriction seen in these subjects and in Bloom syndrome. Clinical features of mitochondrial dysfunction are evident in several individuals with biallelic TOP3A mutations, consistent with the recently reported additional function of TopIIIα in mitochondrial DNA decatenation. In summary, our findings establish TOP3A mutations as an additional cause of prenatal-onset short stature with increased cytogenetic SCEs and implicate the decatenation activity of the BTRR complex in their pathogenesis.

Utility of whole exome sequencing in the diagnosis of Usher syndrome: Report of novel compound heterozygous MYO7A mutations.

Next generation sequencing (NGS), such as targeted panel sequencing, whole-exome sequencing and whole-genome sequencing has led to an exponential increase of elucidated genetic causes in both rare diseases, and common but heterogeneous disorders. NGS is applied in both research and clinical settings, and the clinical exome sequencing (CES), which provides not only the sequence variation data but also clinical interpretation, aids in reaching a final conclusion with regards to a genetic diagnosis. Usher syndrome is a group of disorders, characterized by bilateral sensorineural hearing loss, with or without vestibular dysfunction and retinitis pigmentosa. The index patient, a 2-year-old child was initially diagnosed with nonsyndromic hearing impairment. Homozygosity mapping followed by CES was utilized as a diagnostic tool to identify the genetic basis of his hearing loss. A paternally inherited novel insertion, c.198_199insA (p.Val67Serfs*73) and a maternally inherited novel deletion, c.1219_1226del (p.Phe407Aspfs*33) in gene MYO7A were found in compound heterozygous state in the index patient. The result expands the mutational spectrum of MYO7A. In addition it helped in early diagnosis of the syndrome, for planning and adjustments for the patient, and as well as for future family planning. This study highlights the clinical effectiveness of CES for Usher syndrome diagnosis in a child presented with congenital hearing loss.

Spectrum of bone marrow pathology and hematological abnormalities in methylmalonic acidemia.

Patients with isolated methylmalonic acidemia (MMA) may present with a wide range of hematological complications including anemia, leukopenia, thrombocytopenia, and pancytopenia. However, there are very limited data on the development of hemophagocytosis or myelodysplasia in these patients. We report three patients with isolated MUT related MMA who presented with severe refractory pancytopenia during acute illness. Their bone marrow examination revealed a wide spectrum of pathology varying from bone marrow hypoplasia, hemophagocytosis to myelodysplasia with ring sideroblasts. We discuss their management and outcome. This report emphasizes the need for bone marrow examination in these patients with refractory or unexplained severe cytopenia, to confirm bone marrow pathology, and to rule out other diseases with similar clinical presentation for a better clinical outcome.

Mutations of KIF14 cause primary microcephaly by impairing cytokinesis.

OBJECTIVE: Autosomal recessive primary microcephaly (MCPH) is a rare condition characterized by a reduced cerebral cortex accompanied with intellectual disability. Mutations in 17 genes have been shown to cause this phenotype. Recently, mutations in CIT, encoding CRIK (citron rho-interacting kinase)-a component of the central spindle matrix-were added. We aimed at identifying novel MCPH-associated genes and exploring their functional role in pathogenesis. METHODS: Linkage analysis and whole exome sequencing were performed in consanguineous and nonconsanguineous MCPH families to identify disease-causing variants. Functional consequences were investigated by RNA studies and on the cellular level using immunofluorescence and microscopy. RESULTS: We identified homozygous mutations in KIF14 (NM_014875.2;c.263T>A;pLeu88*, c.2480_2482delTTG; p.Val827del, and c.4071G>A;p.Gln1357=) as the likely cause in 3 MCPH families. Furthermore, in a patient presenting with a severe form of primary microcephaly and short stature, we identified compound heterozygous missense mutations in KIF14 (NM_014875.2;c.2545C>G;p.His849Asp and c.3662G>T;p.Gly1221Val). Three of the 5 identified mutations impaired splicing, and 2 resulted in a truncated protein. Intriguingly, Kif14 knockout mice also showed primary microcephaly. Human kinesin-like protein KIF14, a microtubule motor protein, localizes at the midbody to finalize cytokinesis by interacting with CRIK. We found impaired localization of both KIF14 and CRIK at the midbody in patient-derived fibroblasts. Furthermore, we observed a large number of binucleated and apoptotic cells-signs of failed cytokinesis that we also observed in experimentally KIF14-depleted cells. INTERPRETATION: Our data corroborate the role of an impaired cytokinesis in the etiology of primary and syndromic microcephaly, as has been proposed by recent findings on CIT mutations. Ann Neurol 2017;82:562-577.

Long-term Outcome of 4 Patients With Transcobalamin Deficiency Caused by 2 Novel TCN2 Mutations.

Cobalamin (vitamin B12 [Cbl]) is an essential cofactor for many biochemical pathways. Transcobalamin (TC) is required to internalize Cbl into the cells through membrane receptor-mediated endocytosis. Cbl is then processed in the cytoplasm and mitochondria by complementation factors leading to its active metabolites; methylcobalamin and 5-deoxyadenosyl-cobalamin. Deficiency of TC results in an elevation in methylmalonic acid and homocysteine. Patients usually present with macrocytic anemia, pancytopenia, failure to thrive, gastrointestinal symptoms, and neurological dysfunction. In this study, we report 4 patients from 2 unrelated families, with confirmed diagnosis of TC deficiency. Patients initially had a typical presentation of TC deficiency: severe diarrhea and vomiting, recurrent infections, stomatitis, macrocytic anemia, and neutropenia. Interestingly one of the patients was diagnosed at 3 months of age and developed ataxic gait related to cerebellar atrophy at the age of 14 months. His elder affected sibling was diagnosed at 5 months of age was completely normal. Two sibs, diagnosed at 2 months of age and immediately after birth, had autism spectrum disorder. Molecular investigations showed 2 novel mutations in TCN2 gene. Patients were treated and stayed stable on weekly injection of Cbl. In conclusion, TC deficiency has a wide heterogeneity in clinical phenotype, genotype, laboratory, and radiologic findings. Early detection of the disease and early initiation of aggressive parenteral treatment is probably associated with better prognosis and disease control.

Two novel LHX3 mutations in patients with combined pituitary hormone deficiency including cervical rigidity and sensorineural hearing loss.

BACKGROUND: Congenital combined pituitary hormone deficiency (CPHD) is a rare heterogeneous group of conditions. CPHD-type 3 (CPHD3; MIM# 221750) is caused by recessive mutations in LHX3, a LIM-homeodomain transcription factor gene. The isoforms of LHX3 are critical for pituitary gland formation and specification of the anterior pituitary hormone-secreting cell types. They also play distinct roles in the development of neuroendocrine and auditory systems. CASE PRESENTATION: Here, we summarize the clinical, endocrinological, radiological and molecular features of three patients from two unrelated families. Clinical evaluation revealed severe CPHD coupled with cervical vertebral malformations (rigid neck, scoliosis), mild developmental delay and moderate sensorineural hearing loss (SNHL). The patients were diagnosed with CPHD3 based on the array of hormone deficiencies and other associated syndromic symptoms, suggestive of targeted LHX3 gene sequencing. A novel missense mutation c.437G > T (p. Cys146Phe) and a novel nonsense mutation c.466C > T (p. Arg156Ter), both in homozygous forms, were found. The altered Cys146 resides in the LIM2 domain of the encoded protein and is a phylogenetically conserved residue, which mediates LHX3 transcription factor binding with a zinc cation. The p. Arg156Ter is predicted to result in a severely truncated protein, lacking the DNA binding homeodomain. CONCLUSIONS: Considering genotype/phenotype correlation, we suggest that the presence of SNHL and limited neck rotation should be considered in the differential diagnosis of CPHD3 to facilitate molecular diagnosis. This report describes the first LHX3 mutations from Saudi patients and highlights the importance of combining molecular diagnosis with the clinical findings. In addition, it also expands the knowledge of LHX3-related CPHD3 phenotype and the allelic spectrum for this gene.

EPG5-related Vici syndrome: a paradigm of neurodevelopmental disorders with defective autophagy.

Vici syndrome is a progressive neurodevelopmental multisystem disorder due to recessive mutations in the key autophagy gene EPG5. We report genetic, clinical, neuroradiological, and neuropathological features of 50 children from 30 families, as well as the neuronal phenotype of EPG5 knock-down in Drosophila melanogaster. We identified 39 different EPG5 mutations, most of them truncating and predicted to result in reduced EPG5 protein. Most mutations were private, but three recurrent mutations (p.Met2242Cysfs*5, p.Arg417*, and p.Gln336Arg) indicated possible founder effects. Presentation was mainly neonatal, with marked hypotonia and feeding difficulties. In addition to the five principal features (callosal agenesis, cataracts, hypopigmentation, cardiomyopathy, and immune dysfunction), we identified three equally consistent features (profound developmental delay, progressive microcephaly, and failure to thrive). The manifestation of all eight of these features has a specificity of 97%, and a sensitivity of 89% for the presence of an EPG5 mutation and will allow informed decisions about genetic testing. Clinical progression was relentless and many children died in infancy. Survival analysis demonstrated a median survival time of 24 months (95% confidence interval 0-49 months), with only a 10th of patients surviving to 5 years of age. Survival outcomes were significantly better in patients with compound heterozygous mutations (P = 0.046), as well as in patients with the recurrent p.Gln336Arg mutation. Acquired microcephaly and regression of skills in long-term survivors suggests a neurodegenerative component superimposed on the principal neurodevelopmental defect. Two-thirds of patients had a severe seizure disorder, placing EPG5 within the rapidly expanding group of genes associated with early-onset epileptic encephalopathies. Consistent neuroradiological features comprised structural abnormalities, in particular callosal agenesis and pontine hypoplasia, delayed myelination and, less frequently, thalamic signal intensity changes evolving over time. Typical muscle biopsy features included fibre size variability, central/internal nuclei, abnormal glycogen storage, presence of autophagic vacuoles and secondary mitochondrial abnormalities. Nerve biopsy performed in one case revealed subtotal absence of myelinated axons. Post-mortem examinations in three patients confirmed neurodevelopmental and neurodegenerative features and multisystem involvement. Finally, downregulation of epg5 (CG14299) in Drosophila resulted in autophagic abnormalities and progressive neurodegeneration. We conclude that EPG5-related Vici syndrome defines a novel group of neurodevelopmental disorders that should be considered in patients with suggestive features in whom mitochondrial, glycogen, or lysosomal storage disorders have been excluded. Neurological progression over time indicates an intriguing link between neurodevelopment and neurodegeneration, also supported by neurodegenerative features in epg5-deficient Drosophila, and recent implication of other autophagy regulators in late-onset neurodegenerative disease.

ADAT3-related intellectual disability: Further delineation of the phenotype.

ADAT3-related intellectual disability has been recently described in 24 individuals from eight Saudi families who had cognitive impairment and strabismus. Other common features included growth failure, microcephaly, tone abnormalities, epilepsy, and nonspecific brain abnormalities. A single homozygous founder mutation (c.382G>A:p.(V128M)) in the ADAT3 gene, which encodes a protein that functions in tRNA editing, was identified in all affected individuals. In this report, we present additional 15 individuals from 11 families (10 Saudis and 1 Emirati) who are homozygous for the same founder mutation. In addition to the universal findings of intellectual disability and strabismus, the majority exhibited microcephaly and growth failure. Additional features not reported in the original cohort include dysmorphic facial features (prominent forehead, up-slanted palpebral fissures, epicanthus, and depressed nasal bridge), behavioral problems (hyperactivity and aggressiveness), recurrent otitis media, and growth hormone deficiency. ADAT3-related intellectual disability is an important recognizable cause of intellectual disability in Arabia.

Spectrum of Mutations in 60 Saudi Patients with Mut Methylmalonic Acidemia.

Defects in the human gene encoding methylmalonyl-CoA mutase enzyme (MCM) give rise to a rare autosomal recessive inherited disorder of propionate metabolism termed mut methylmalonic acidemia (MMA). Patients with mut MMA have been divided into two subgroups: mut0 with complete loss of MCM activity and mut- with residual activity in the presence of adenosylcobalamin (AdoCbl). The disease typically presents in the first weeks or months of life and is clinically characterized by recurrent vomiting, metabolic acidosis, hyperammonemia, lethargy, poor feeding, failure to thrive and neurological deficit. To better elucidate the spectrum of mutations causing mut MMA in Saudi patients, we screened a cohort of 60 Saudi patients affected by either forms of the disease for mutations in the MUT gene. A total of 13 different mutations, including seven previously reported missense changes and six novel mutations, were detected in a homozygous state except for two compound heterozygous cases. The six novel mutations identified herein consist of three nonsense, two missense and one frameshift, distributed throughout the whole protein. This study describes for the first time the clinical and mutational spectrum of mut MMA in Saudi Arabian patients.

Clinical, neuroimaging, and genetic features of L-2-hydroxyglutaric aciduria in Arab kindreds.

BACKGROUND AND OBJECTIVES: L-2-hydroxyglutaric aciduria is a neurometabolic disorder with autosomal recessive mode of inheritance in which patients exhibit elevated L-2-hydroxyglutaric acid in body fluids, central nervous system manifestations, and increased risk of brain tumor formation. Mutations in L2HGDH gene have been described in L-2-hydroxyglutaric aciduria patients of different ethnicities. The present study was conducted to perform a detailed clinical, imaging and genetic analysis. DESIGN AND SETTINGS: A cross-sectional clinical genetic study of 16 L-2-hydroxyglutaric aciduria patients from 4 Arab consanguineous families examined at the metabolic clinic of the hospital. PATIENTS AND METHODS: Genomic DNA was isolated from the blood of 12 patients and 10 unaffected family members, and the L2HGDH gene was sequenced. DNA sequences were compared to the L2HGDH reference sequence from GenBank. RESULTS: All patients exhibit characteristic clinical, biochemical, and imaging features of L-2-hydroxyglutaric aciduria, and 4 patients exhibited increased incidence of brain tumors. The sequencing of the L2HGDH gene revealed the c.1015delA, c.1319C > A, and c.169G > A mutations in these patients. These mutations encode for the p.Arg339AspfsX351, p.Ser440Tyr, and p.Gly57Arg changes in the L2HGDH protein, respectively. The c.169G > A mutation, which was shown to have a common origin in Italian and Portuguese patients, was also discovered in Arab patients. Finding of the homozygous c.159T SNP associated with the c.169G > A mutation in Arab patients points to an independent origin of this mutation in Arab population. CONCLUSION: The detailed description of clinical manifestations and L2HGDH mutation in this study is useful for diagnosis of L-2-hydroxyglutaric aciduria in Arab patients. While reoccurrence of an L2HGDH mutation in L-2-hydroxyglutaric aciduria patients of different ethnicity is extremely rare, the c.169G mutation has an independent origin in Arab patients. It is likely that this mutation may also be present in patients of other ethnicities.

Novel ANO5 homozygous microdeletion causing myalgia and unprovoked rhabdomyolysis in an Arabic man.

INTRODUCTION: Recessive mutations in the anoctamin-5 gene (ANO5) cause a spectrum of clinical phenotypes, including limb-girdle muscular dystrophy (LGMD 2L), distal myopathy, and asymptomatic hyperCKemia. METHODS: In this report we describe our clinical, electrophysiological, pathological, and molecular findings in a subject with anoctaminopathy-5. RESULTS: A 49-year-old Arabic man from a consanguineous family presented with a 5-year history of myalgias, hyperCKemia and an episode of unprovoked rhabdomyolysis. Muscle biopsy showed mild myopathic changes and interstitial amyloid deposition. ANO5 analysis detected a novel homozygous deletion of approximately 11.9 kb encompassing exons 13-17, predicted to be pathogenic. CONCLUSIONS: Anoctaminopathy-5 can manifest with a phenotype reminiscent of metabolic myopathy and should be considered as a potential cause of myalgia and myoglobinuria. Amyloid deposition in the muscle biopsy is helpful for the diagnosis. A novel homozygous ANO5 deletion was identified, suggesting that screening for common mutations may have low yield in non-European subjects.

COL4A4-related nephropathy caused by a novel mutation in a large consanguineous Saudi family.

INTRODUCTION: Collagen type IV related nephropathies are due to the defects in collagen IV genes COL4A3, COL4A4, or COL4A5 and comprise a spectrum of phenotypes ranging from Alport Syndrome (AS) to its mild variants, termed as familial haematuria or thin basement membrane nephropathy. Classical AS is a progressive renal disease presenting with a triad of progressive hematuric nephritis and typical extra-renal complications, such as sensorineural hearing loss (SNHL) and variable ocular anomalies. The mode of inheritance in AS is X-linked in 85%, autosomal recessive in 15%, and autosomal dominant in rare cases. OBJECTIVES: This study aims to identify underlying mutation in multiple individuals from a large consanguineous Saudi family with inherited nephropathy, including our index patient who manifested all the features of classical AS. PATIENTS AND METHODS: Patients were diagnosed by nephrologists and clinical geneticists. All the individuals underwent clinical, audiological and ophthalmological evaluation. Blood samples were collected after written informed consent. DNA extraction, homozygosity mapping and PCR amplification followed standard methodologies. RESULTS: The disease locus was mapped to 2q36.3, where both COL4A3 and COL4A4 reside. Sanger sequencing of COL4A3 and COL4A4 revealed an underlying novel homozygous disease-causing COL4A4 mutation (c.2420delG; p.G807fsX60) in the affected proband. Considerable phenotypic variability segregating with this COL4A4 mutation in our study family is documented. The homozygous mutants were manifesting end-stage renal disease (ESRD) in their adolescence, while the heterozygous carrier members were presenting with considerable phenotypic heterogeneity ranging from intermittent hematuria to late onset ESRD. In addition, there is a relatively severe involvement of the ear (SNHL) and eye in the homozygotes than the heterozygotes. Fertility problems were also noted in both of the homozygous females. CONCLUSION: Identification of the causative mutation is an efficient strategy for conclusive molecular diagnosis in the patients and to establish genotype/phenotype correlation. It is important to study and evaluate asymptomatic carriers, to predict prognosis of the disease and to obviate the need for another renal biopsy in at-risk related family members. While an accurate genetic diagnosis of AS provides valuable information for genetic counseling in the extended family members, it can also facilitate future prenatal diagnosis and planning for pre-implantation genetic diagnosis.

Autism spectrum disorders and inborn errors of metabolism: an update.

BACKGROUND: Autism spectrum disorder is characterized by social communicative deficits with restricted interests occurring in about 1% of the population. Although its exact cause is not known, several factors have been implicated in its etiology, including inborn errors of metabolism. Although relatively uncommon, these disorders frequently occur in countries with high rates of consanguinity and are often associated with behavioral problems, such as hyperactivity and aggression. The aim of this review is to examine the association of autism with these conditions. METHOD: A computer-assisted search was performed to identify the most common inborn errors of metabolism associated with autism. RESULTS: The following disorders were identified: phenylketonuria, glucose-6-phosphatase deficiency, propionic acidemia, adenosine deaminase deficiency, Smith-Lemli-Opitz syndrome and mitochondrial disorders, and the recently described branched chain ketoacid dehydrogenase kinase deficiency. CONCLUSION: The risk of autistic features is increased in children with inborn errors of metabolism, especially in the presence of cognitive and behavioral deficits. We propose that affected children should be screened for autism.

Recessive mutations in EPG5 cause Vici syndrome, a multisystem disorder with defective autophagy.

Vici syndrome is a recessively inherited multisystem disorder characterized by callosal agenesis, cataracts, cardiomyopathy, combined immunodeficiency and hypopigmentation. To investigate the molecular basis of Vici syndrome, we carried out exome and Sanger sequence analysis in a cohort of 18 affected individuals. We identified recessive mutations in EPG5 (previously KIAA1632), indicating a causative role in Vici syndrome. EPG5 is the human homolog of the metazoan-specific autophagy gene epg-5, encoding a key autophagy regulator (ectopic P-granules autophagy protein 5) implicated in the formation of autolysosomes. Further studies showed a severe block in autophagosomal clearance in muscle and fibroblasts from individuals with mutant EPG5, resulting in the accumulation of autophagic cargo in autophagosomes. These findings position Vici syndrome as a paradigm of human multisystem disorders associated with defective autophagy and suggest a fundamental role of the autophagy pathway in the immune system and the anatomical and functional formation of organs such as the brain and heart.

Genomic analysis of pediatric cataract in Saudi Arabia reveals novel candidate disease genes.

BACKGROUND: Pediatric cataract is an important preventable blinding disease. Previous studies have estimated 10-25% of cases to be genetic in etiology. METHODS: In an effort to characterize the genetics of cataract in our population, we have conducted a comprehensive clinical and genomic analysis (including autozygome and exome analysis) on a series of 38 index patients. RESULTS: Pediatric cataract is genetic in at least 79% of the study families. Although crystallins accounted for most of the mutant alleles, mutations in other genes were encountered, including recessive mutations in genes that usually cause the disease in a dominant manner. In addition, several novel candidate genes (MFSD6L, AKR1E2, RNLS, and CYP51A1) were identified. CONCLUSION: Pediatric cataract is typically a genetic disease, usually autosomal recessive, in Saudi Arabia. Although defining a specific cataract phenotype can sometimes predict the genetic cause, genomic analysis is often required to unravel the causative mutation given the marked genetic heterogeneity. The identified novel candidate genes require independent confirmation in future studies.