Genotype-phenotype Correlation Studies Research Articles

Advances in Genetic Characterization and Genotype-Phenotype Correlation of Duchenne and Becker Muscular Dystrophy in the Personalized Medicine Era.

Currently, Duchenne muscular dystrophy (DMD) and the related condition Becker muscular dystrophy (BMD) can be usually diagnosed using physical examination and genetic testing. While BMD features partially functional dystrophin protein due to in-frame mutations, DMD largely features no dystrophin production because of out-of-frame mutations. However, BMD can feature a range of phenotypes from mild to borderline DMD, indicating a complex genotype–phenotype relationship. Despite two mutational hot spots in dystrophin, mutations can arise across the gene. The use of multiplex ligation amplification (MLPA) can easily assess the copy number of all exons, while next-generation sequencing (NGS) can uncover novel or confirm hard-to-detect mutations. Exon-skipping therapy, which targets specific regions of the dystrophin gene based on a patient’s mutation, is an especially prominent example of personalized medicine for DMD. To maximize the benefit of exon-skipping therapies, accurate genetic diagnosis and characterization including genotype–phenotype correlation studies are becoming increasingly important. In this article, we present the recent progress in the collection of mutational data and optimization of exon-skipping therapy for DMD/BMD.

Genotype–Phenotype Correlations in a Spanish Cohort of 506 Families With Biallelic ABCA4 Pathogenic Variants

To define genotype-phenotype correlations in the largest cohort study worldwide of patients with biallelic ABCA4 variants, including 434 patients with Stargardt disease (STGD1) and 72 with cone-rod dystrophy (CRD). Cohort study. We characterized 506 patients with ABCA4 variants using conventional genetic tools and next-generation sequencing technologies. Medical history and ophthalmologic data were obtained from 372 patients. Genotype-phenotype correlation studies were carried out for the following variables: variant type, age at symptom onset (AO), and clinical phenotype. A total of 228 different pathogenic variants were identified in 506 ABCA4 patients, 50 of which were novel. Genotype-phenotype correlations showed that most of the patients with biallelic truncating variants presented with CRD and that these cases had a significantly earlier AO than patients with STGD1. Three missense variants are associated with CRD for the first time (c.1804C>T; p.[Arg602Trp], c.3056C>T; p.[Thr1019Met], and c.6320G>C; p.[Arg2107Pro]). Analysis of the most prevalent ABCA4 variant in Spain, c.3386G>T; p.(Arg1129Leu), revealed that is correlated to STGD1, later AO, and foveal sparing. Our study, conducted in the largest ABCA4-associated disease cohort reported to date, updates the genotype-phenotype model established for ABCA4 variants and broadens the mutational spectrum of the gene. According to our observations, patients with ABCA4 presenting with 2 truncating variants may first present features of STGD1 but eventually develop rod dysfunction, and specific missense variants may be associated with a different phenotype, underscoring the importance of an accurate genetic diagnosis. Also, it is a prerequisite for enrollment in clinical trials, and to date, no other treatment has been approved for STGD1.

Genetic Study in Korean Pediatric Patients with Steroid-Resistant Nephrotic Syndrome or Focal Segmental Glomerulosclerosis

Steroid-resistant nephrotic syndrome (SRNS) is one of the major causes of end-stage renal disease (ESRD) in childhood and is mostly associated with focal segmental glomerulosclerosis (FSGS). More than 50 monogenic causes of SRNS or FSGS have been identified. Recently, the mutation detection rate in pediatric patients with SRNS has been reported to be approximately 30%. In this study, genotype-phenotype correlations in a cohort of 291 Korean pediatric patients with SRNS/FSGS were analyzed. The overall mutation detection rate was 43.6% (127 of 291 patients). WT1 was the most common causative gene (23.6%), followed by COQ6 (8.7%), NPHS1 (8.7%), NUP107 (7.1%), and COQ8B (6.3%). Mutations in COQ6, NUP107, and COQ8B were more frequently detected, and mutations in NPHS2 were less commonly detected in this cohort than in study cohorts from Western countries. The mutation detection rate was higher in patients with congenital onset, those who presented with proteinuria or chronic kidney disease/ESRD, and those who did not receive steroid treatment. Genetic diagnosis in patients with SRNS provides not only definitive diagnosis but also valuable information for decisions on treatment policy and prediction of prognosis. Therefore, further genotype-phenotype correlation studies are required.

RYR2 p.R169L mutation and left ventricular hypertrophy in a child with emotion-triggered sudden death.

Catecholaminergic polymorphic ventricular tachycardia is a rare (prevalence: 1/10,000) channelopathy characterised by exercise-induced or emotion-triggered ventricular arrhythmias. There is an overall paucity of genotype-phenotype correlation studies in patients with catecholaminergic polymorphic ventricular tachycardia, and in vitro and in vivo effects of individual mutations have not been well characterised. We report an 8-year-old child who carried a mutation in the coding exon 8 of RYR2 (p.R169L) and presented with emotion-triggered sudden cardiac death. He was also found to have left ventricular hypertrophy, a combination which has not been reported before. We discuss the association between genetic variation in RYR2, particularly mutations causing replacement of arginine at position 169 of RYR2 and structural cardiac abnormalities.

Hypertrophic Cardiomyopathy: The Molecular Genetics

Hypertrophic Cardiomyopathy (HCM) is the common monogenic form familial pathological cardiac hypertrophy. HCM is an important cause of sudden cardiac death in the young adult and a major cause of morbidity in the elderly. We discuss here the molecular genetics and recent advances in the molecular genetics of HCM. HCM became the first cardiac disease for which a molecular genetic mechanism was identified. More than 100 mutations in nine genes, that encoding sarcomeric proteins have been identified in patients with HCM, which had led to the belief that HCM is a disease of contractile sarcomeric proteins of the cardiac muscle. Approximately two-thirds of all HCM cases are caused by the mutation of the myosin heavy chain (MyHC), cardiac troponin T (cTnT) and myosin binding protein-C (MyBP-C). Genotype-phenotype correlation studies suggest that mutations in the MyHC gene are associated with more extensive hypertrophy and a higher risk of SCD as compared to mutations in genes coding for other sarcomeric proteins, such as MyBP-C and cTnT. However, there is a noteworthy variability and factors, such as modifier genes and probably the environmental factors affect the phenotypic expression of HCM. The results of different functional studies suggest that in spite of the variety of the mutations, the initial defects in HCM is abnormal cardiac myocyte function. In this era of genetics and upcoming future of precision medicine, good knowledge of its molecular basis of any disease is crucial for patient management, and HCM is not different.
 Faridpur Med. Coll. J. Jan 2019;14(1): 44-49

CFTR gene variants, epidemiology and molecular pathology

Pathogenic variants of the CFTR gene are responsible for a broad phenotypic spectrum characterized by malfunction of some exocrine tissues, with an autosomal recessive mode of inheritance. More than 2,000 variants, distributed throughout the CFTR gene, have been identified, with different effects on the gene and protein expression and function. Genotype-phenotype correlation studies have associated severe variants with a typical multi-organ form of cystic fibrosis, while mild variants are involved in monosymptomatic or adult-onset diseases, called CFTR-related disorders. However, the interpretation of rare variants remains challenging. This review presents an overview of the epidemiology of CFTR variants worldwide and in France and describes the functional classification. Finally, some frequent cystic fibrosis-causing and mild CFTR variants are used as example to depict the molecular pathology of the CFTR locus. Finally, we give the recommendations concerning nomenclature and classification that are useful for appropriate genetic counseling. © 2020 French Society of Pediatrics. Published by Elsevier Masson SAS. All rights reserved.

P.237Functional analyses and phenotype-genotype correlation studies in patients suspected of titinopathy

P.237Functional analyses and phenotype-genotype correlation studies in patients suspected of titinopathy

ACG Clinical Guideline: Hereditary Hemochromatosis.

Hereditary hemochromatosis (HH) is one of the most common genetic disorders among persons of northern European descent. There have been recent advances in the diagnosis, management, and treatment of HH. The availability of molecular diagnostic testing for HH has made possible confirmation of the diagnosis for most patients. Several genotype-phenotype correlation studies have clarified the differences in clinical features between patients with the C282Y homozygous genotypes and other HFE mutation patterns. The increasing use of noninvasive tests such as MRI T2* has made quantification of hepatic iron deposition easier and eliminated the need for liver biopsy in most patients. Serum ferritin of <1,000 ng/mL at diagnosis remains an important diagnostic test to identify patients with a low risk of advanced hepatic fibrosis and should be used routinely as part of the initial diagnostic evaluation. Genetic testing for other types of HH is available but is expensive and generally not useful in most clinical settings. Serum ferritin may be elevated among patients with nonalcoholic fatty liver disease and in those with alcoholic liver disease. These diagnoses are more common than HH among patients with elevated serum ferritin who are not C282Y homozygotes or C282Y/H63D compound heterozygotes. A secondary cause for liver disease should be excluded among patients with suspected iron overload who are not C282Y homozygotes. Phlebotomy remains the mainstay of therapy, but emerging novel therapies such as new chelating agents may have a role for selected patients.

Symmetry of folds in FEVR: A genotype-phenotype correlation study

Familial exudative vitreoretinopathy (FEVR) is a hereditary retinal vascular disorder. Among the various clinical phenotypes of this disease, retinal folds are the primary and typical feature of FEVR. However, little is known about the clinical characteristics, genetic spectrum, or potential phenotype–genotype correlation of retinal folds. Herein, we describe and analyze the clinical and genetic characteristics of retinal folds in FEVR. Eighty-nine patients with unilateral or bilateral retinal folds were included in this study. Clinical examinations showed that the retinal folds were bilateral in 37 patients (41.6%). Various retinal abnormalities were noted in the fellow eyes in the remaining 52 patients with unilateral folds. Most of the folds were located temporally (98.4%, 124/126), and were complete (97.6%, 123/126). 67.5% (60/89) probands were genetic confirmed FEVR. 25 novel pathogenic mutations (7 in FZD4, 7 in LRP5, 1 in NDP, 4 in TSPAN12, and 6 in KIF11) were identified in 25 families. Overall, 87.5% (14/16) and 73.7%(14/19) patients with LRP5 and FZD4 mutations were with unilateral folds, respectively.Nevertheless, only 25% (2/8), 36.4%(4/11) and 16.7%(1/6) patients with NDP, TSPAN12, and KIF11 mutations were with unilateral folds. Moreover, 85.7%(12/14),100% (6/6) and 100%(8/8) of the patients with mutated TSPAN12, KIF11, and NDP genes presented with symmetry in disease staging, while 55% and 64.7% of patients with FZD4 and LRP5 mutation displayed symmetry in staging. In conclusion, the majority of retinal folds extended completely and radially in the temporal peripheral retina. Patients with causative mutations in NDP, TSPAN12, or KIF11 were more likely to have bilaterally symmetrical severe retinopathy. In contrast, patients with LRP5 and FZD4 mutations displayed a relatively milder but broader spectrum of phenotypes and a higher frequency of asymmetry.

Improvement of DIAgnostic and Phenotype-genotype Correlation Studies in Patients With MYOpathy Suspected of TITinopathy

Improvement of DIAgnostic and Phenotype-genotype Correlation Studies in Patients With MYOpathy Suspected of TITinopathy

Investigation of Chromosomal Abnormalities and Microdeletion/ Microduplication(s) in Fifty Iranian Patients with Multiple Congenital Anomalies.

Major birth defects are inborn structural or functional anomalies with long-term disability and adverse impacts on individuals, families, health-care systems, and societies. Approximately 20% of birth defects are due to chromosomal and genetic conditions. Inspired by the fact that neonatal deaths are caused by birth defects in about 20 and 10% of cases in Iran and worldwide respectively, we conducted the present study to unravel the role of chromosome abnormalities, including microdeletion/microduplication(s), in multiple congenital abnormalities in a number of Iranian patients. In this descriptive cross-sectional study, 50 sporadic patients with Multiple Congenital Anomalies (MCA) were selected. The techniques employed included conventional karyotyping, fluorescence in situ hybridization (FISH), multiplex ligation-dependent probe amplification (MLPA), and array comparative genomic hybridisation (array-CGH), according to the clinical diagnosis for each patient. Chromosomal abnormalities and microdeletion/microduplication(s) were observed in eight out of fifty patients (16%). The abnormalities proved to result from the imbalances in chromosomes 1, 3, 12, and 18 in four of the patients. However, the other four patients were diagnosed to suffer from the known microdeletions of 22q11.21, 16p13.3, 5q35.3, and 7q11.23. In the present study, we report a patient with 46,XY, der(18)[12]/46,XY, der(18), +mar[8] dn presented with MCA associated with hypogammaglobulinemia. Given the patient's seemingly rare and highly complex chromosomal abnormality and the lack of any concise mechanism presented in the literature to justify the case, we hereby propose a novel mechanism for the formation of both derivative and ring chromosome 18. In addition, we introduce a new 12q abnormality and a novel association of an Xp22.33 duplication with 1q43q44 deletion syndrome. The phenotype analysis of the patients with chromosome abnormality would be beneficial for further phenotype-genotype correlation studies.

Genotype-phenotype correlation study and mutational and hormonal analysis in a Chinese cohort with 21-hydroxylase deficiency.

BackgroundSteroid 21‐hydroxylase deficiency (21OHD) is the most common enzymatic defect, but the genotype–phenotype associations have not been well established in Chinese patients. Here, a Chinese 21OHD cohort was enrolled to investigate the clinical, biochemical, and genetic characteristics of this disorder.MethodsMutation analysis of CYP21A2 gene, 21‐hydroxylase activity assays and in silico predictions of protein structure were performed. Genotype–phenotype associations were analyzed in both the cohort and 487 Chinese CAH patients ever reported.ResultsAmong the total cohort (72 patients), 47 patients (65.3%) were diagnosed as salt‐wasting (SW) phenotype, 11 (15.3%) were simple virilizing (SV) type, and 14 (19.4%) were nonclassic (NC) type. The value of FSH and LH for prediction of the SW phenotype was up to 0.862 and 0.669, respectively. Overall, the detection rate of CYP21A2 mutation was 97.9%, which revealed 25 mutations and 36 genotypes. Four novel mutations (p.L199X, p.E321del, p.H393Q, and p.L459‐P464del) were detected and induced a significantly reduced 21‐hydroxylase activity. Generally, disease severity can be predicted with the genotypes. The most common genotypes in Chinese population were I2G/I2G (12.5%), I2G/Large lesion (12.1%), I173N/I2G (10.3%), and I173N/Large lesion (9.2%). The SW form of CAH is prominent in deletion or intronic splice mutations, namely I2G/I2G (18.6%), I2G/Large lesion (17.2%) and Large lesion/Large lesion (8.6%).ConclusionFour novel mutations were identified and a high consistency of genotype–phenotype association was found in SW CAH. Moreover, FSH and LH levels were proved to be a promising marker for predicting the severity of the disease.

Up-to-date classification and multidisciplinary symptoms of Ehlers-Danlos syndromes

In this review article, the authors summarize the clinical aspects of the novel classification of Ehlers-Danlos syndrome, which is a group of rare, hereditary connective tissue disorders. The leading symptom of the Ehlers-Danlos syndrome group is joint hypermobility, skin hyperextensibility and generalized tissue fragility. Ehlers-Danlos syndrome displays a high clinical and genetic heterogeneity and harbors many multidisciplinary properties. Certain subtypes only affect the quality of life, while other forms may lead to severe, even fatal vascular or intestinal complications. Last year, based on the data of various international genotype-phenotype correlation studies of large populations, a new classification of the syndrome's clinical subtypes was introduced. The novel international nosology of Ehlers-Danlos syndromes published in 2017 delineates 13 clinical subtypes, describes their genetic background and defines major and minor diagnostic criteria for each subtype. We gathered the complex, multidisciplinary symptoms of Ehlers-Danlos syndromes in a table to assist the diagnosis from a differential diagnostic point of view. In the clinical practice, the proper diagnosis of patients affected by the Ehlers-Danlos syndrome group is essential to give optimal clinical care and to prevent the development of severe complications. Orv Hetil. 2019; 160(16): 603-612.

Analysis of the Phenotypes in the Rett Networked Database.

Rett spectrum disorder is a progressive neurological disease and the most common genetic cause of intellectual disability in females. MECP2 is the major causative gene. In addition, CDKL5 and FOXG1 mutations have been reported in Rett patients, especially with the atypical presentation. Each gene and different mutations within each gene contribute to variability in clinical presentation, and several groups worldwide performed genotype-phenotype correlation studies using cohorts of patients with classic and atypical forms of Rett spectrum disorder. The Rett Networked Database is a unified registry of clinical and molecular data of Rett patients, and it is currently one of the largest Rett registries worldwide with several hundred records provided by Rett expert clinicians from 13 countries. Collected data revealed that the majority of MECP2-mutated patients present with the classic form, the majority of CDKL5-mutated patients with the early-onset seizure variant, and the majority of FOXG1-mutated patients with the congenital form. A computation of severity scores further revealed significant differences between groups of patients and correlation with mutation types. The highly detailed phenotypic information contained in the Rett Networked Database allows the grouping of patients presenting specific clinical and genetic characteristics for studies by the Rett community and beyond. These data will also serve for the development of clinical trials involving homogeneous groups of patients.

A systematic review of hearing and vestibular function in carriers of the Pro51Ser mutation in the COCH gene.

The Pro51Ser (P51S) COCH mutation is characterized by a late-onset bilateral sensorineural hearing loss (SNHL) and progressive vestibular deterioration. The aim of this study was to carry out a systematic review of all reported hearing and vestibular function data in P51S COCH mutation carriers and its correlation with age. Scientific databases including Medline, Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, ISI Web of Knowledge, and Web of Science were searched to accumulate information about hearing outcome and vestibular function. Eleven genotype-phenotype correlation studies of the P51S COCH variant were identified and analyzed. The SNHL starts at the age of 32.8years. The Annual Threshold Deterioration is 3 decibel hearing loss (dB HL) per year (1-24 dB HL/year). Profound SNHL was observed at 76years on average (60-84years). 136 individual vestibular measurements were collected from 86 carriers. The onset of the vestibular dysfunction was estimated around 34years (34-40years), and vestibular deterioration rates were higher than those of the SNHL, with complete bilateral loss observed between 49 and 60years. Both audiometric and vestibular data were processed with much different methodologies and pre-symptomatic P51S carriers were systematically underrepresented. Further delineation of this correlation would benefit cross-sectional and longitudinal study involving all (pre-symptomatic and symptomatic) P51S carriers.

Genetic analysis of 63 Chinese patients with mucopolysaccharidosis type II: Functional characterization of seven novel IDS variants

Mucopolysaccharidosis type II (MPS II) is an X-linked recessive lysosomal storage disorder resulting from the deficiency of the enzyme iduronate-2-sulfatase (IDS).This study described the molecular characteristics of 63 Chinese children with MPS II and investigated functional characterization of seven novel IDS variants. We analyzed mutations in the IDS gene of 63 children with MPS II. Seven novel mutations were further characterized by transient expression studies. 49 different mutations were identified in the IDS gene including 33 previously reported and 16 novel mutations. The mutation p.R443X and c.1122C > T(p.G374G) may be link to attenuated type. The novel missense mutations were predicted damaging in silico. The bioinformatic structural analysis of the novel missense mutations showed that these amino acid replacements would cause a severe impairment of protein structure and function. In vitro functional analysis of the seven novel mutants, showing a very low IDS activity, clearly demonstrated their pathogenic nature. In western blotting analysis of the IDS protein, the examined mutations showed a similar or slightly lower molecular mass of precursor without mature forms being detected. Our study expands the spectrum of genotype of MPS II, provides new insights into the molecular mechanism of MPS II and helps to the future studies of genotype-phenotype correlations to estimate prognosis and develop new therapeutic approach.

Investigation of Chromosomal Abnormalities and Microdeletion/ Microduplication(s) in Fifty Iranian Patients with Multiple Congenital Anomalies

Objective Major birth defects are inborn structural or functional anomalies with long-term disability and adverse impacts on individuals, families, health-care systems, and societies. Approximately 20% of birth defects are due to chromosomal and genetic conditions. Inspired by the fact that neonatal deaths are caused by birth defects in about 20 and 10% of cases in Iran and worldwide respectively, we conducted the present study to unravel the role of chromosome abnormalities, including microdeletion/microduplication(s), in multiple congenital abnormalities in a number of Iranian patients. Materials and Methods In this descriptive cross-sectional study, 50 sporadic patients with Multiple Congenital Anomalies (MCA) were selected. The techniques employed included conventional karyotyping, fluorescence in situ hybridization (FISH), multiplex ligation-dependent probe amplification (MLPA), and array comparative genomic hybridisation (array-CGH), according to the clinical diagnosis for each patient. Results Chromosomal abnormalities and microdeletion/microduplication(s) were observed in eight out of fifty patients (16%). The abnormalities proved to result from the imbalances in chromosomes 1, 3, 12, and 18 in four of the patients. However, the other four patients were diagnosed to suffer from the known microdeletions of 22q11.21, 16p13.3, 5q35.3, and 7q11.23. ConclusionIn the present study, we report a patient with 46,XY, der(18)[12]/46,XY, der(18), +mar[8] dn presented with MCA associated with hypogammaglobulinemia. Given the patient’s seemingly rare and highly complex chromosomal abnormality and the lack of any concise mechanism presented in the literature to justify the case, we hereby propose a novel mechanism for the formation of both derivative and ring chromosome 18. In addition, we introduce a new 12q abnormality and a novel association of an Xp22.33 duplication with 1q43q44 deletion syndrome. The phenotype analysis of the patients with chromosome abnormality would be beneficial for further phenotype-genotype correlation studies.

Genotype-Phenotype Correlation Study and Mutational Analysis in a Chinese Cohort with 21-Hydroxylase Deficiency

Background: Steroid 21-hydroxylase deficiency (21OHD) is the most common enzymatic defect. Genotype-phenotype associations were concluded in different ethnic group. However, such correlations have not been well established in Chinese. This study aimed to analyze this correlation in a Chinese 21OHD cohort and the function of four novel mutations of CYP21A2 gene. Methods: A Chinese 21OHD cohort was enrolled to investigate the clinical, biochemical, and genetic characteristics of this disorder. Symptoms and laboratory findings of all the members were collected. Mutation analysis of CYP21A2 gene, 21-hydroxylase activity assays and in silico predictions of protein structure were performed. Genotype and phenotype associations were analyzed in both the cohort and 487 Chinese CAH patients ever reported. Findings: Among the total cohort, 47 patients (65.3%) were diagnosed as SW phenotype, 11 (15.3%) were SV type and 14 (19.4%) were NC type. Interestingly, the value of FSH and LH for the prediction of the severe SW phenotype was up to 0.862 and 0.669, respectively. Overall, the detection rate of CYP21A2 mutation was 97.9%, which revealed 25 mutations and 36 genotypes. Four novel mutations (p.L199X, p.E321del, p.H393Q and p.L459-P464del) were detected. All these four mutations were predicted to destroy 21-hydroxylase three-dimensional structure and induced a significantly reduced CYP21 activity of less than 5% compared with the wild type. Interpretation: A high consistency of genotype-phenotype association was found in SW CAH and a less matched correlation in SV and NC CAH. Moreover, FSH and LH levels were proved to be a promising marker for predicting the severity of the disease. Funding: This work was supported by grants from the National Natural Science Foundation (No. 81370891 and No. 81670720), Special Funds for Taishan Scholar Project (No. tsqn20161071). Declaration of Interests: The author reports no conflicts of interest in this work. Ethical Approval Statement: This study was approved by the Ethics Committee of the Shandong Provincial Hospital. Written informed consent was obtained from all patients or their guardians.

Multiparametric Approach to Arrhythmogenic Cardiomyopathy: Clinical, Instrumental, and Lifestyle Indications

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a genetic disorder with an autosomal dominant inheritance and incomplete penetrance. It predominantly affects the right ventricle (RV), predisposing to the origin of ventricular arrhythmias and sudden death (SD). The structural basis of the disease consists of the progressive loss of myocardium with fibro-adipose replacement. ARVC is a “desmosomes” disease involving mutations of proteins such as placoglobin, desmoplachine, placophylline, desmoglein, and desmocollin. In the “classical” form, the disease mainly implicates the RV chamber, while the left ventricle (LV) is involved in advanced stages. Genotype-phenotype correlation studies have identified some phenotypic variants characterized by an early participation of the LV, which can proceed in parallel toward the two ventricles (“biventricular” variant) or prevails over the RV (variant to “left dominance”). These data led to the evolution of the initial definition of ARVC, which is currently considered a genetic disease of both ventricles and, therefore, deserves the denomination “arrhythmogenic cardiomyopathy”. Many aspects of diagnosis, treatment, and indications for a correct lifestyle are important in sports medicine. This paper will discuss the clinical management of ARVC, with particular reference to diagnosis, risk stratification, therapy, and indications for physical activity.

ZIC2 in Holoprosencephaly.

The ZIC2 transcription factor is one of the most commonly mutated genes in Holoprosencephaly (HPE) probands. HPE is a severe congenital defect of forebrain development which occurs when the cerebral hemispheres fail to separate during the early stages of organogenesis and is typically associated with mispatterning of the embryonic midline. Recent study of genotype-phenotype correlations in HPE cases has defined distinctive features of ZIC2-associated HPE presentation and genetics, revealing that ZIC2 mutation does not produce the craniofacial abnormalities generally thought to characterise HPE but leads to a range of non-forebrain phenotypes. Furthermore, the studies confirm the extent of ZIC2 allelic heterogeneity and that pathogenic variants of ZIC2 are associated with both classic and middle interhemispheric variant (MIHV) HPE which arise from defective ventral and dorsal forebrain patterning, respectively. An allelic series of mouse mutants has helped to delineate the cellular and molecular mechanisms by which one gene leads to defects in these related but distinct embryological processes.