Meckel syndrome: Clinical and mutation profile in six fetuses

Novel homozygous mutations in TXNDC15 causing Meckel syndrome

BACKGROUND

Meckel syndrome (MKS) is the most severe form of an autosomal recessive ciliopathy and is clinically characterized by occipital encephalocele, severely polycystic kidneys, and postaxial polydactyly (toes). The association of TXNDC15-related MKS has been reported. We report the case of a homozygous mutation in the TXNDC15 gene, causing MKS14 in the Chinese population.

METHODS

The fetal skin tissue and parental peripheral blood were retained for whole-exome sequencing and Sanger sequencing, which investigated the potential pathogenic variants associated with MKS.

RESULTS

The fetus was homozygous for a mutation in the TXNDC15 gene (NM_024715.3), specifically c.560delA (p.Asn187llefsTer4), and both parents were heterozygous for this mutation.

CONCLUSION

Our study identified a new mutation that adds to the mutational landscape of MKS, which provide a basis for genetic counseling and the selection of reproductive options.

First preimplantation genetic testing case of Meckel syndrome with a novel homozygous TXNDC15 variant in a non-consanguineous Chinese family

BACKGROUND

Meckel-Gruber syndrome (MKS) is a perinatally lethal, genetically heterogeneous, autosomal recessive condition caused by defective primary cilium formation. So far, the association of TXNDC15-related MKS has been reported in only five independent families from diverse ethnic origins, including Saudi, Pakistani, Estonian, and Indian. Here, we report a fetus diagnosed with MKS at 12 weeks, exhibiting typical ultrasound findings.

METHODS

Low-coverage whole-genome sequencing was used to identify chromosomal abnormalities. Trio-base whole exome sequencing (trio-WES) was performed to investigate the potential pathogenic variants associated with MKS. Preimplantation genetic testing for monogenic disorders (PGT-M) was applied to prevent the transmission of the pathogenic variant.

RESULTS

A novel homozygous pathogenic variant in the TXNDC15 gene was identified through trio-WES. The application of PGT-M successfully prevented the transmission of the pathogenic variant and resulted in an ongoing pregnancy.

CONCLUSION

This is the first report of a TXNDC15 variant in the Chinese population and the first PGT case of TXNDC15-related MKS worldwide. The successful application of PGT-M in this family provides a potential approach for other monogenic diseases. Our case expands the variant spectrum of TXNDC15 and contributes to the molecular diagnosis and genetic counseling for MKS. This case underscores the importance of appropriate genetic testing methods and accurate genetic counseling in the diagnosis of rare monogenic diseases.

Novel homozygous nonsense mutation associated with Bardet-Biedl syndrome in fetuses with congenital renal malformation

BACKGROUND

The Bardet-Biedl syndrome (BBS) is a rare autosomal recessive disorder, characterized by clinical and genetic heterogeneity. BBS is more commonly reported in adults and children than in fetuses. Here, a retrospective study on 210 fetuses with congenital renal malformation was conducted.

METHODS

The fetuses were diagnosed using invasive prenatal tests, including chromosome karyotype analysis, whole exome sequencing (WES), and single-nucleotide polymorphism array. We found the intrauterine phenotype of a fetus presenting enlarged kidneys, enhanced echo, and oligohydramnios; therefore, the fetus was characterized to have BBS.

RESULTS

Chromosome karyotype analysis presented normal results. Analysis using an Affymetrix CytoScan 750K array revealed 2 homozygous regions. However, WES revealed a homozygous mutation of c.1177C>T (p.Arg393*) on exon 12 of BBS1 and a heterozygous variation of c.2704G>A (p.Asp902Asn) on exon 22 of CC2D2A. The American College of Medical Genetics and Genomics guidelines identified c.1177C>T and c.2704G>A as a pathogenic mutation and of uncertain significance, respectively. Sanger sequencing identified heterozygous mutation, that is, c.1177C>T and heterozygous variation, that is, c.2704G>A in the parents of the fetus.

CONCLUSIONS

WES identified a novel homozygous nonsense mutation c.1177C>T in BBS1 of a Chinese fetus with congenital renal malformation. This finding provides insight into the BBS1 mutations in Asian populations in general and shows the necessity of genetic counseling.

Introducing Thioredoxin-Related Transmembrane Proteins: Emerging Roles of Human TMX and Clinical Implications

Significance: The presence of a large number of thioredoxin superfamily members suggests a complex mechanism of redox-based regulation in mammalian cells. However, whether these members are functionally redundant or play separate and distinct roles in each cellular compartment remains to be elucidated. Recent Advances: In the mammalian endoplasmic reticulum (ER), ∼20 thioredoxin-like proteins have been identified. Most ER oxidoreductases are soluble proteins located in the luminal compartment, whereas a small family of five thioredoxin-related transmembrane proteins (TMX) also reside in the ER membrane and play crucial roles with specialized functions. Critical Issues: In addition to the predicted function of ER protein quality control, several independent studies have suggested the diverse roles of TMX family proteins in the regulation of cellular processes, including calcium homeostasis, bioenergetics, and thiol-disulfide exchange in the extracellular space. Moreover, recent studies have provided evidence of their involvement in the pathogenesis of various diseases. Future Directions: Extensive research is required to unravel the physiological roles of TMX family proteins. Given that membrane-associated proteins are prime targets for drug discovery in a variety of human diseases, expanding our knowledge on the mechanistic details of TMX action on the cell membrane will provide the molecular basis for developing novel diagnostic and therapeutic approaches as a potent molecular target in a clinical setting. Antioxid. Redox Signal. 36, 984-1000.

Evaluation of novel compound variants of CEP290 in prenatally suspected case of Meckel syndrome through whole exome sequencing

Background

Meckel syndrome (MKS) is a fatal disease characterized by multisystem fibrosis during the prenatal or perinatal period. It has an autosomal recessive genetic pattern and is characterized by meningo occipital encephalocele, polycystic kidney dysplasia, polydactyly, and hepatobiliary ductal plate malformation. Germline variations in CEP290 have been shown to cause MKS4.

Methods

In this study, a 23‐year‐old Chinese woman who was 18 weeks pregnant was examined. The pregnancy was terminated due to occipital meningocele and enlarged cystic dysplastic kidney revealed by ultrasonography. In addition, the patient had a history of adverse pregnancy whereby the fetus presented with double kidney enlargement. Karyotype analysis and chromosomal microarray examination (CMA) were carried out using amniotic fluid samples. Whole exome sequencing (WES) was performed using tissue specimens of the aborted fetus.

Results

Karyotype and CMA analyses showed normal results. However, compound heterozygous mutations of CEP290 c.3175dup and CEP290 c.1201dup were detected through WES. CEP290 c.1201dup is a novel heterozygous mutation of CEP290 that has not been reported previously.

Conclusions

The findings of this study provide information on the correlation between MKS phenotype and genotype in CEP290. In addition, these findings indicate that WES is an effective method for detecting genetic causes of multiple structural defects especially those showing normal karyotype and CMA results.

The neonatal liver: Normal development and response to injury and disease

The liver emerges from the ventral foregut endoderm around 3 weeks in human and 1 week in mice after fertilization. The fetal liver works as a hematopoietic organ and then develops functions required for performing various metabolic reactions in late fetal and neonatal periods. In parallel with functional differentiation, the liver establishes three dimensional tissue structures. In particular, establishment of the bile excretion system consisting of bile canaliculi of hepatocytes and bile ducts of cholangiocytes is critical to maintain healthy tissue status. This is because hepatocytes produce bile as they functionally mature, and if allowed to remain within the liver tissue can lead to cytotoxicity. In this review, we focus on epithelial tissue morphogenesis in the perinatal period and cholestatic liver diseases caused by abnormal development of the biliary system.

Three Novel Variants of CEP290 and CC2D2DA and a Link Between ZNF77 and SHH Signaling Pathway Are Found in Two Meckel-Gruber Syndrome Fetuses

Meckel-Gruber syndrome (MKS) is a rare lethal autosomal recessive inherited disorder. Missed diagnosis might happen in clinical works due to an unclear genotype–phenotype correlation. We analyzed two families visiting our center; the parents are normal; each of the family aborted a fetus at 12WG. Following ultrasonography and pathological examination, both were diagnosed as MKS. Whole exome sequencing identified a compound heterozygous of two novel variants of CEP290 and a heterozygous of a novel variant of CC2D2A. Frameshift mutations in ZNF77 were also detected. Western blot analyzing whole-brain tissue showed that the expression of ZNF77, CC2D2A, and CEP290 was enhanced. HEK293T transfected with over-expression wildtype/mutated ZNF77 plasmid showed that SHH was increased in wildtype ZNF77 cells, while SHH and CC2D2A were increased in mutated ZNF77 cells. Our research provided two novel pathogenic variants of CEP290 and CC2D2A and suggested that ZNF77 might promote the expression of CC2D2A and regulate the amount of SHH.

Pseudoachondroplasia: Phenotype and genotype in 11 Indian patients

Pseudoachondroplasia (PSACH) is an autosomal dominant disorder characterized by rhizomelic short-limbed skeletal dysplasia. The primary clinical and radiographic features include disproportionate dwarfism, joint laxity and hyperextensibility, exaggerated lumbar lordosis, and late ossification of the epiphyses. Identification of disease-causing variants in heterozygous state in COMP establishes the molecular diagnosis of PSACH. We examined 11 families with clinical features suggestive of PSACH. In nine families, we used Sanger sequencing of exons 8-19 of COMP (NM_000095.2) and in two families exome sequencing was used for confirming the diagnosis. We identified 10 de novo variants, including five known variants (c.925G>A, c.976G>A, c.1201G>T, c.1417_1419del, and c.1511G>A) and five variants (c.874T>C, c.1201G>C, c.1309G>A, c.1416_1421delCGACAA, and c.1445A>T) which are not reported outside Indian ethnicity. We hereby report the largest series of individuals with molecular diagnosis of PSACH from India and reiterate the well-known genotype-phenotype corelation in PSACH.

Molecular Diagnosis and Prenatal Phenotype Analysis of Eight Fetuses With Ciliopathies

Human ciliopathies are hereditary conditions caused by variants in ciliary-associated genes. Ciliopathies are often characterized by multiple system defects. However, it is not easy to make a definite diagnosis in the prenatal period only based on the imageology. In this report, eight new prenatal cases from five unrelated families diagnosed with ciliopathies were systematically examined. The clinical manifestations of these fetuses showed such prenatal diagnostic features as occipital encephalocele, and polydactyly and polycystic kidneys. Situs inversus caused by CPLANE1 variant was first reported. In Family 1 and Family 3, homozygous variants of CPLANE1 and NPHP4 caused by consanguineous marriage and uniparental disomy were detected by whole-exome sequencing, respectively. In Family 2, Family 4 and Family 5, compound heterozygotes of TMEM67 and DYNC2H1 including two novel missense variants and one novel nonsense variant were identified. The distribution of pathogenic missense variants along TMEM67 gene mainly clustered in the extracellular cysteine rich region, extracellular area with unknown structure, and the transmembrane regions. Genotype-phenotype relationship between CPLANE1 and TMEM67 genes was concluded. This report describes new clinical manifestations and novel variants in CPLANE1, TMEM67, NPHP4, and DYNC2H1.

On the Wrong Track: Alterations of Ciliary Transport in Inherited Retinal Dystrophies

Ciliopathies are a group of heterogeneous inherited disorders associated with dysfunction of the cilium, a ubiquitous microtubule-based organelle involved in a broad range of cellular functions. Most ciliopathies are syndromic, since several organs whose cells produce a cilium, such as the retina, cochlea or kidney, are affected by mutations in ciliary-related genes. In the retina, photoreceptor cells present a highly specialized neurosensory cilium, the outer segment, stacked with membranous disks where photoreception and phototransduction occurs. The daily renewal of the more distal disks is a unique characteristic of photoreceptor outer segments, resulting in an elevated protein demand. All components necessary for outer segment formation, maintenance and function have to be transported from the photoreceptor inner segment, where synthesis occurs, to the cilium. Therefore, efficient transport of selected proteins is critical for photoreceptor ciliogenesis and function, and any alteration in either cargo delivery to the cilium or intraciliary trafficking compromises photoreceptor survival and leads to retinal degeneration. To date, mutations in more than 100 ciliary genes have been associated with retinal dystrophies, accounting for almost 25% of these inherited rare diseases. Interestingly, not all mutations in ciliary genes that cause retinal degeneration are also involved in pleiotropic pathologies in other ciliated organs. Depending on the mutation, the same gene can cause syndromic or non-syndromic retinopathies, thus emphasizing the highly refined specialization of the photoreceptor neurosensory cilia, and raising the possibility of photoreceptor-specific molecular mechanisms underlying common ciliary functions such as ciliary transport. In this review, we will focus on ciliary transport in photoreceptor cells and discuss the molecular complexity underpinning retinal ciliopathies, with a special emphasis on ciliary genes that, when mutated, cause either syndromic or non-syndromic retinal ciliopathies.

Pierquin Syndrome: Report of a New Case

Pierquin syndrome is a rare genetic entity characterized by the association of Dandy–Walker malformation and postaxial polydactyly. The incidence is uncertain with only six cases previously reported in the literature. In this study, we reported a new case of Pierquin syndrome born from consanguineous parents, characterized by Dandy–Walker malformation, postaxial polydactyly, and congenital heart disease. The case reinforces an autosomal recessive modality of inheritance and expands the phenotypic spectrum of this rare malformation syndrome.

Whole exome sequencing identified a novel missense alteration in CC2D2A causing Joubert syndrome 9 in a Pakhtun family

BACKGROUND

Joubert syndrome (JBTS) is a heterogenous disorder characterized by intellectual disability, developmental delays, molar tooth sign in brain imaging, hypotonia, ocular motor apraxia and overlapping features of ciliopathies. There are 36 clinical subtypes of JBTS, with an equal number of genes known so far for this phenotype.

METHODS

Whole exome sequencing (WES) and Sanger sequencing were performed for the molecular diagnosis of a Pakhtun family affected with Joubert syndrome type 9 (JBTS9).

RESULTS

A novel homozygous missense variant (c.4417C>G; Pro1473Ala) in exon 34 was identified in coiled-coil and C2 domains-containing the protein 2A (CC2D2A; NM_001080522) gene. The variant co-segregated in autosomal recessive fashion within the family and was not found in 200 ethnically matched unaffected individuals. In silico analyses supported the pathogenic effect of the altered CC2D2A protein.

CONCLUSIONS

To the best of our knowledge, this is the first report of CC2D2A alteration co-segragating with a JBTS9 phenotype in a Pakhtun family from Pakistan. Our findings broaden the pathogenic spectrum of JBTS9, adding a novel variant to CC2D2A variation pool. WES analysis is a successful molecular diagnostic tool for rare genetic disorders, especially in those populations where the marriage of cousins is more frequent. Efficient and accurate genetic testing and counselling of the affected families are helpful for patient management and for reducing the disease burden in future generations.

Thioredoxin-Related Transmembrane Proteins: TMX1 and Little Brothers TMX2, TMX3, TMX4 and TMX5

The endoplasmic reticulum (ER) is site of synthesis and maturation of membrane and secretory proteins in eukaryotic cells. The ER contains more than 20 members of the Protein Disulfide Isomerase (PDI) family. These enzymes regulate formation, isomerization and disassembly of covalent bonds between cysteine residues. As such, PDIs ensure protein folding, which is required to attain functional and transport-competent structure, and protein unfolding, which facilitates dislocation of defective gene products across the ER membrane for ER-associated degradation (ERAD). The PDI family includes over a dozen of soluble members and few membrane-bound ones. Among these latter, there are five PDIs grouped in the thioredoxin-related transmembrane (TMX) protein family. In this review, we summarize the current knowledge on TMX1, TMX2, TMX3, TMX4 and TMX5, their structural features, regulation and roles in biogenesis and control of the mammalian cell’s proteome.

Two novel TCTN2 mutations cause Meckel-Gruber syndrome

Meckel-Gruber syndrome (MKS) is a clinically and genetically heterogeneous ciliopathy characterized by a triad of occipital encephalocele, polycystic kidneys, and postaxial polydactyly. Pathogenesis of MKS is related to dysfunction of primary cilia. However, reports on MKS caused by Tectonic2 (TCTN2) mutations are scanty whilst. There is no direct evidence of ciliogenesis in such MKS patients. Here, we identified two novel nonsense variants of TCTN2 (c.343G > T, p.E115*; c.1540C > T, p.Q514*) in a Chinese MKS fetus. Compared to reported TCTN2-causing MKS patients, our case represented an endocardial pad defect, which was not reported previously. We also found primary cilia protruded normally from the surface of epithelial cells in the affected fetal kidney tubules compared to controls, indicating TCTN2 is not necessary for ciliogenesis in the kidney. To our knowledge, this is the first case of MKS fetus caused by TCTN2 mutations from China.