Genetic basis of hereditary hypophosphataemic rickets and phenotype presentation in children and adults

Hypophosphataemic rickets (HR) is a genetic disorder causing defects in the renal handling of phosphorus, resulting in rickets. HR can be classified into two groups. First — those with excess fibroblast growth factor 23 (FGF23) levels, which are due to gene mutations in extrarenal factors and include X-linked dominant hypophosphataemic rickets (XLHR), autosomal dominant hypophosphataemic rickets (ADHR), autosomal recessive hypophosphataemic rickets (ARHR), and hypophosphataemic rickets with hyperparathyroidism. Second — those with normal or low FGF23, which are caused by gene mutations in renal tubular phosphate transporters and include hereditary hypophosphataemic rickets with hypercalciuria (HHRH) and X-linked recessive hypophosphataemic rickets. The radiographical changes and clinical features of rickets in various types of HR are similar but not identical. Short stature, bone deformities mainly in the lower limbs, and dental problems are typical characteristics of HR. Although the initial diagnosis of HR is usually based on physical, radiological, and biochemical features, molecular genetic analysis is important to confirm the diagnosis and differentiate the type of HR. In this review, we describe clinical and biochemical features as well as genetic causes of different types of HR. The clinical and biochemical characteristics presented in this review can help in the diagnosis of different types of HR and, therefore, direct genetic analysis to look for the specific gene mutation

Predicting the impact of PHEX, FGF23 and DMP1 gene variants found in Malaysian Malay patients with Hypophosphataemic Rickets through in silico analysis of protein function and mRNA secondary structure

Hypophosphataemic Rickets (HR) is a rare bone disorder characterised by chronic hypophosphataemia caused by defective phosphate reabsorption in the renal tubules. Variants in phosphate-regulating endopeptidase homolog, X-linked (PHEX), fibroblast growth factor-23 (FGF23) and dentin matrix protein-1 (DMP1) genes contribute to X-linked dominant, autosomal dominant and autosomal recessive forms of HR, respectively. In this study, four Malaysian patients’ DNA samples were subjected to polymerase chain reaction and Sanger sequencing to identify the types and locations of the variants. Then, in silico study was conducted based on the variants found to predict the effects of amino acid substitution on protein functions using SIFT and PolyPhen-2 software and RNAfold was used to construct the mRNA secondary structure. Mutational analyses had revealed two variants in PHEX; c.10G>C (E4Q), c.1970A>G (Y657C), one mutation in FGF23; c.716C>T (T239M) and three variants on DMP1; c.309A>T (S69C), c.1322C>T (S406S), c.1334G>A (E410E). The variants in these Malay patients were previously reported in different ethnic HR patients. Protein prediction programs suggested that the PHEX Y657C and DMP1 S69C variants may affect protein function. All variants were predicted to alter the secondary mRNA structure. These findings suggest that these missense and silent variants may lead to changes in protein function and mRNA secondary structure that are associated with the manifestation of HR phenotype

Case report: Incomplete penetrance of autosomal dominant myotonia congenita caused by a rare CLCN1 variant c.1667T>A (p.I556N) in a Malaysian family

Myotonia congenita (MC) is a rare neuromuscular disease caused by mutations within the CLCN1 gene encoding skeletal muscle chloride channels. MC is characterized by delayed muscle relaxation during contraction, resulting in muscle stiffness. There is a lack of MC case reports and data on the prevalence among Malaysians. We report a clinical case of a 50-year-old woman presents with muscle stiffness and cramp episodes that started in early childhood. She had difficulty initiating muscle movement and presented with transient muscle weakness after rest, which usually improved after repeated contraction (warm-up phenomenon). She was diagnosed with MC after myotonic discharge on electromyography (EMG). Her brother had similar symptoms; however, no additional family members showed MC symptoms. Serum creatine kinase levels were elevated in both the proband and her brother with 447 U/L and 228 U/L recorded, respectively. Genetic analysis by whole-exome sequencing (WES) revealed a previously reported pathogenic CLCN1 gene variant c.1667T>A (p.I556N). Genetic screening of all family members revealed that the same variant was observed in the children of both the proband and her brother; however, the children did not present with either clinical or electrophysiological MC symptoms. The multiplex ligation-dependent probe amplification (MLPA) analysis conducted identified neither exon deletion nor duplication in CLCN1. In conclusion, this report describes the first case of MC in Malaysia in which incomplete penetrance observed in this family is caused by a known pathogenic CLCN1 variant

A novel synonymous KMT2B variant in a patient with dystonia causes aberrant splicing

BACKGROUND: Heterozygous KMT2B variants are a common cause of dystonia. A novel synonymous KMT2B variant, c.5073C>T (p.Gly1691=) was identified in an individual with childhood-onset progressive dystonia. METHODS: The splicing impact of c.5073C>T was assessed using an in vitro exon-trapping assay. The genomic region of KMT2B exons 23-26 was cloned into the pSpliceExpress plasmid between exon 2 and 3 of the rat Ins2 gene. The c.5073C>T variant was then introduced through site-directed mutagenesis. The KMT2B wild-type and c.5073C>T plasmids were transfected separately into HeLa cells and RNA was extracted 48 hours after transfection. The RNA was reverse transcribed to produce cDNA, which was PCR amplified using primers annealing to the flanking rat Ins2 sequences. RESULTS: Sanger sequencing of the PCR products revealed that c.5073C>T caused a novel splice donor site and therefore a 5-bp deletion of KMT2B exon 23 in mature mRNA, leading to a coding frameshift and premature stop codon (p.Lys1692AsnfsTer7). CONCLUSION: To our knowledge, this is the first report of a KMT2B synonymous variant associated with dystonia. Reassessment of synonymous variants may increase diagnostic yield for inherited disorders including monogenic dystonia. This is of clinical importance, given the generally favourable response to deep brain stimulation for KMT2B-related dystonia

Advances and challenges in modeling inherited peripheral neuropathies using iPSCs

Inherited peripheral neuropathies (IPNs) are a group of diseases associated with mutations in various genes with fundamental roles in the development and function of peripheral nerves. Over the past 10 years, significant advances in identifying molecular disease mechanisms underlying axonal and myelin degeneration, acquired from cellular biology studies and transgenic fly and rodent models, have facilitated the development of promising treatment strategies. However, no clinical treatment has emerged to date. This lack of treatment highlights the urgent need for more biologically and clinically relevant models recapitulating IPNs. For both neurodevelopmental and neurodegenerative diseases, patient-specific induced pluripotent stem cells (iPSCs) are a particularly powerful platform for disease modeling and preclinical studies. In this review, we provide an update on different in vitro human cellular IPN models, including traditional two-dimensional monoculture iPSC derivatives, and recent advances in more complex human iPSC-based systems using microfluidic chips, organoids, and assembloids

Missense mutations in the copper transporter gene ATP7A cause X-Linked distal hereditary motor neuropathy

Distal hereditary motor neuropathies comprise a clinically and genetically heterogeneous group of disorders. We recently mapped an X-linked form of this condition to chromosome Xq13.1-q21 in two large unrelated families. The region of genetic linkage included ATP7A, which encodes a copper-transporting P-type ATPase mutated in patients with Menkes disease, a severe infantile-onset neurodegenerative condition. We identified two unique ATP7A missense mutations (p.P1386S and p.T994I) in males with distal motor neuropathy in two families. These molecular alterations impact highly conserved amino acids in the carboxyl half of ATP7A and do not directly involve the copper transporter's known critical functional domains. Studies of p.P1386S revealed normal ATP7A mRNA and protein levels, a defect in ATP7A trafficking, and partial rescue of a S. cerevisiae copper transport knockout. Although ATP7A mutations are typically associated with severe Menkes disease or its milder allelic variant, occipital horn syndrome, we demonstrate here that certain missense mutations at this locus can cause a syndrome restricted to progressive distal motor neuropathy without overt signs of systemic copper deficiency. This previously unrecognized genotype-phenotype correlation suggests an important role of the ATP7A copper transporter in motor-neuron maintenance and function

Detection of Charcot-Marie-Tooth type 1A duplication by the polymerase chain reaction

Charcot-Marie-Tooth disease type 1A (CMT1A) is a hereditary peripheral neuropathy with a genetic locus on chromosome 17p11.2. The majority of patients carry a duplicated DNA segment that encompasses the gene PMP22, which encodes a peripheral myelin protein. PMP22 is the crucial gene involved in the pathogenesis of CMT1A. Molecular diagnosis of CMT1A requires detection of this duplicated segment. Existing methods for detection of the duplication are laborious and time consuming. We have developed a set of polymorphic (AC)(n) repeat markers (contained within the duplication) for use in the polymerase chain reaction, which give a high probability of detecting three unique alleles in affected individuals. This test detected 85% of a panel of 52 CMT1A patients in which the duplication had previously been demonstrated.4 page(s

Genetic basis of hereditary hypophosphataemic rickets and phenotype presentation in children and adults

Hypophosphataemic rickets (HR) is a genetic disorder causing defects in the renal handling of phosphorus, resulting in rickets. HR can be classified into two groups. First- those with excess fibroblast growth factor 23(FGF23) levels, which are due to gene mutations in extrarenal factors and include X-linked dominant hypophosphataemic rickets (XLHR), autosomal dominant hypophosphataemic rickets (ADHR), autosomal recessive hypophosphataemic rickets (ARHR), and hypophosphataemic rickets with hyperparathyroidism. Second- those with normal or low FGF23, which are caused by gene mutations in renal tubular phosphate transporters and include hereditary hypophosphataemic rickets with hypercalciuria (HHRH) and X-linked recessive hypophosphataemic rickets. The radiographical changes and clinical features of rickets in various types of HR are similar but not identical. Short stature, bone deformities mainly in the lower limbs, and dental problems are typical characteristics of HR. Although the initial diagnosis of HR is usually based on physical, radiological, and biochemical features, molecular genetic analysis is important to confirm the diagnosis and differentiate the type of HR. In this review, we describe clinical and biochemical features as well as genetic causes of different types of HR. The clinical and biochemical characteristics presented in this review can help in the diagnosis of different types of HR and, therefore, direct genetic analysis to look for the specific gene mutation

Single test for two hereditary neuropathies, CMT1A and HNPP

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A Novel locus for distal motor neuron degeneration maps to chromosome 7q34-q36

The motor neuron diseases (MND) are a group of related neurodegenerative diseases that cause the relative selective progressive death of motor neurons. These diseases range from slowly progressive forms including hereditary motor neuropathy (HMN), to the rapidly progressive disorder amyotrophic lateral sclerosis (ALS). There is clinical and genetic overlap among these MNDs, implicating shared pathogenic mechanisms. We recruited a large family with a MND that was previously described as juvenile ALS and distal HMN. We identified a novel MND/HMN locus on chromosome 7q34-q36 following a genome-wide scan for linkage in this family. The disease causing mutation maps to a 26.2 cM (12.3 Mb) interval flanked by D7S2513 and D7S637 on chromosome 7q34-q36. Recombinant haplotype analysis including unaffected individuals suggests that the refined candidate interval spans 14.3 cM (6.3 Mb) flanked by D7S2511 and D7S798. One gene in the candidate interval, CDK5, was selected for immediate mutation analysis based upon its known association with an ALS-like phenotype in mice however, no mutations were identified. Identification of genes causing familial MND will lead to a greater understanding of the biological basis of both familial and sporadic motor neuron degeneration including ALS.6 page(s