Male patients affected by mosaic PCDH19 mutations: five new cases.

Pathogenic variants in the PCDH19 gene are associated with epilepsy, intellectual disability (ID) and behavioural disturbances. Only heterozygous females and mosaic males are affected, likely due to a disease mechanism named cellular interference. Until now, only four affected mosaic male patients have been described in literature. Here, we report five additional male patients, of which four are older than the oldest patient reported so far. All reported patients were selected for genetic testing because of developmental delay and/or epilepsy. Custom-targeted next generation sequencing gene panels for epilepsy genes were used. Clinical data were collected from medical records. All patients were mosaic in blood for likely pathogenic variants in the PCDH19 gene. In most, clinical features were very similar to the female phenotype, with normal development before seizure onset, which occurred between 5 and 10 months of age, clustering of seizures and sensitivity to fever. Four out of five patients had mild to severe ID and behavioural problems. We reaffirm the similarity between male and female PCDH19-related phenotypes, now also in a later phase of the disorder (ages 10-14 years).

Mutations in the vasopressin type 2 receptor gene (AVPR2) associated with nephrogenic diabetes insipidus.

Nephrogenic diabetes insipidus (DIR) is an X-linked disorder characterized by insensitivity of the distal nephron for the pituitary hormone, vasopressin. The genetic map location of the DIR gene on chromosome Xq28 coincides with the physical map location of the functional vasopressin renal V2-type receptor. Recently, the human and rat cDNAs for the vasopressin V2 receptor (AVPR2) have been identified. We show here that the structural AVPR2 gene is localized between DXS52 and G6PD, which is within the genetic map location of DIR. We also tested eight X-linked DIR probands and their families for mutations in one of the most conserved extracellular regions of AVPR2: in three of them, we have identified point mutations resulting in non-conservative amino acid substitutions which cosegregated with DIR in all families.

Dominant isolated renal magnesium loss is caused by misrouting of the Na(+),K(+)-ATPase gamma-subunit.

Primary hypomagnesaemia is composed of a heterogeneous group of disorders characterized by renal or intestinal Mg(2+) wasting, often associated with disturbances in Ca(2+) excretion. We identified a putative dominant-negative mutation in the gene encoding the Na(+), K(+)-ATPase gamma-subunit (FXYD2), leading to defective routing of the protein in a family with dominant renal hypomagnesaemia.

Mutation of BSND causes Bartter syndrome with sensorineural deafness and kidney failure.

Antenatal Bartter syndrome (aBS) comprises a heterogeneous group of autosomal recessive salt-losing nephropathies. Identification of three genes that code for renal transporters and channels as responsible for aBS has resulted in new insights into renal salt handling, diuretic action and blood-pressure regulation. A gene locus of a fourth variant of aBS called BSND, which in contrast to the other forms is associated with sensorineural deafness (SND) and renal failure, has been mapped to chromosome 1p. We report here the identification by positional cloning, in a region not covered by the human genome sequencing projects, of a new gene, BSND, as the cause of BSND. We examined ten families with BSND and detected seven different mutations in BSND that probably result in loss of function. In accordance with the phenotype, BSND is expressed in the thin limb and the thick ascending limb of the loop of Henle in the kidney and in the dark cells of the inner ear. The gene encodes a hitherto unknown protein with two putative transmembrane alpha-helices and thus might function as a regulator for ion-transport proteins involved in aBS, or else as a new transporter or channel itself.

Autosomal recessive cerebellar ataxias: the current state of affairs.

Among the hereditary ataxias, autosomal recessive cerebellar ataxias (ARCAs) encompass a diverse group of rare neurodegenerative disorders in which a cerebellar syndrome is the key clinical feature. The clinical overlap between the different cerebellar ataxias, the occasional atypical phenotypes, and the genetic heterogeneity often complicate the clinical management of such patients. Despite the steady increase in newly discovered ARCA genes, many patients with a putative ARCA cannot be genotyped yet, proving that more genes must be involved. This review presents an updated overview of the various ARCAs. The clinical and genetic characteristics of those forms with a known molecular genetic defect are discussed, along with the emerging insights in the underlying pathophysiological mechanisms.

From karyotyping to array-CGH in prenatal diagnosis.

Conventional karyotyping detects chromosomal anomalies in up to 35% of pregnancies with fetal ultrasound anomalies, depending on the number and type of these anomalies. Extensive experience gained in the past decades has shown that prenatal karyotyping is a robust technique which can detect the majority of germline chromosomal anomalies. For most of these anomalies the phenotype is known. In postnatal diagnosis of patients with congenital anomalies and intellectual disability, array-CGH/SNP array has become the first-tier investigation. The higher abnormality detection yield and its amenability to automation renders array-CGH also suitable for prenatal diagnosis. As both findings of unclear significance and unexpected findings may be detected, studies on the outcome of array-CGH in prenatal diagnosis were initially performed retrospectively. Recently, prospective application of array-CGH in pregnancies with ultrasound anomalies, and to a lesser extent in pregnancies referred for other reasons, was studied. Array-CGH showed an increased diagnostic yield compared to karyotyping, varying from 1-5%, depending on the reason for referral. Knowledge of the spectrum of array-CGH anomalies detected in the prenatal setting will increase rapidly in the years to come, thus facilitating pre- and posttest counseling. Meanwhile, new techniques like non-invasive prenatal diagnosis are emerging and will claim their place. In this review, we summarize the outcome of studies on prenatal array-CGH, the clinical relevance of differences in detection rate and range as compared to standard karyotyping, and reflect on the future integration of new molecular techniques in the workflow of prenatal diagnosis.

Hereditary spastic paraplegia due to SPAST mutations in 151 Dutch patients: new clinical aspects and 27 novel mutations.

BACKGROUND: In the clinically and genetically heterogeneous group of the hereditary spastic paraplegias (HSPs), mutations in the SPAST gene are most frequently found and cause a pure autosomal dominant form. OBJECTIVE: To provide the clinical and genetic characteristics of Dutch patients with HSP due to a SPAST mutation (SPG4). METHODS: SPAST mutation carriers were identified through a comprehensive national database search. Available medical records were reviewed. RESULTS: 151 mutation carriers carried 60 different changes in the SPAST gene, of which one was a known polymorphism, and 27 were novel. Missense mutations were most frequently found (39%). Clinical information was available from 72 mutation carriers. Age at onset ranged from 1 to 63 years with a bimodal peak distribution in the first decade and above age 30. The predominantly pure spastic paraplegia was accompanied by deep sensory disturbances and sphincter problems in almost 50%. An additional hand tremor was found in 10%. Patients with missense mutations and exon deletions did not reveal a distinctive phenotype. CONCLUSIONS: Dutch SPAST mutation carriers show a broad mutation spectrum, with 27 novel mutations in the present series. A bimodal peak distribution in age at onset was found and an accompanying tremor as peculiar feature of SPG4. The pathogenicity of S44L, the first exon 4 mutation, and a possible autosomal recessive mode of inheritance are discussed.

CNTNAP2 gene dosage variation is associated with schizophrenia and epilepsy.

A homozygous mutation of the CNTNAP2 gene has been associated with a syndrome of focal epilepsy, mental retardation, language regression and other neuropsychiatric problems in children of the Old Order Amish community. Here we report genomic rearrangements resulting in haploinsufficiency of the CNTNAP2 gene in association with epilepsy and schizophrenia. Genomic deletions of varying sizes affecting the CNTNAP2 gene were identified in three non-related Caucasian patients. In contrast, we did not observe any dosage variation for this gene in 512 healthy controls. Moreover, this genomic region has not been identified as showing large-scale copy number variation. Our data thus confirm an association of CNTNAP2 to epilepsy outside the Old Order Amish population and suggest that dosage alteration of this gene may lead to a complex phenotype of schizophrenia, epilepsy and cognitive impairment.

Requirement of human renal water channel aquaporin-2 for vasopressin-dependent concentration of urine.

Concentration of urine in mammals is regulated by the antidiuretic hormone vasopressin. Binding of vasopressin to its V2 receptor leads to the insertion of water channels in apical membranes of principal cells in collecting ducts. In nephrogenic diabetes insipidus (NDI), the kidney fails to concentrate urine in response to vasopressin. A male patient with an autosomal recessive form of NDI was found to be a compound heterozygote for two mutations in the gene encoding aquaporin-2, a water channel. Functional expression studies in Xenopus oocytes revealed that each mutation resulted in nonfunctional water channel proteins. Thus, aquaporin-2 is essential for vasopressin-dependent concentration of urine.

Decreased bone density and treatment in patients with autosomal recessive cutis laxa.

AIM: Due to the occasional association pathological fractures and osteoporosis we evaluated four patients with cutis laxa syndrome for skeletal anomalies. PATIENT/METHODS: We prospectively evaluated four patients, a male and a female child and a brother-sister sib pair, with dysmorphic features, growth delay, joint anomalies, psychomotor retardation and congenital cutis laxa. The clinical features and the family history were suggestive for autosomal recessive cutis laxa syndrome type II, partially overlapping with geroderma osteodysplastica. Skeletal survey, sequential bone density measurements, endocrine and metabolic investigations were performed including N- and O-linked glycosylation analysis. ATP6V0A2 and FBLN5 mutations were ruled out in all patients. RESULTS: All children were diagnosed with significantly decreased bone density, especially in the lumbar spine, including spontaneous vertebral and rib fractures in three children. Following 24 months of bisphosphonate treatment a total restitution of bone density was observed in three cases and no relapse was detected in the 2-year follow-up period. A spontaneous improvement was found in one female during puberty. CONCLUSION: Bone disease might occur early in the course in autosomal recessive cutis laxa syndrome. We report on a significant clinical improvement and stabilization in our patients following bisphosphonate therapy. We suggest early, systemic evaluation and follow up of bone density in all children presenting with inherited cutis laxa.

Rescue of vasopressin V2 receptor mutants by chemical chaperones: specificity and mechanism.

Because missense mutations in genetic diseases of membrane proteins often result in endoplasmic reticulum (ER) retention of functional proteins, drug-induced rescue of their cell surface expression and understanding the underlying mechanism are of clinical value. To study this, we tested chemical chaperones and sarco(endo)plasmic reticulum Ca2+ ATPase pump inhibitors on Madin-Darby canine kidney cells expressing nine ER-retained vasopressin type-2 receptor (V2R) mutants involved in nephrogenic diabetes insipidus. Of these nine, only V2R-V206D showed improved maturation and plasma membrane rescue with glycerol, dimethyl sulfoxide (DMSO), thapsigargin/curcumin, and ionomycin but not with other osmolytes or growth at 27 degrees C. This revealed that rescue is mutant specific and that this mutant is prone to rescue by multiple compounds. Rescue did not involve changed expression of molecular chaperones calnexin, heat-shock protein (HSP) 70, or HSP90. V2R antagonist SR121463B treatment revealed that V2R-V206D and V2R-S167T were rescued and matured to a greater extent, suggesting that the rescuing activity of a pharmacological versus chemical chaperone is broader and stronger. Calcium measurements showed that rescue of V2R-V206D by thapsigargin, curcumin, and ionomycin was because of increased cytosolic calcium level, rather than decreased endoplasmic reticulum calcium level. The molecular mechanism underlying rescue by DMSO, glycerol, and SR121463B is different, because with these compounds intracellular calcium levels were unaffected.

Regulation of the vasopressin V2 receptor by vasopressin in polarized renal collecting duct cells.

Binding of arginine-vasopressin (AVP) to its V2 receptor (V2R) in the basolateral membrane of principal cells induces Aquaporin-2-mediated water reabsorption in the kidney. To study the regulation of the V2R by dDAVP in a proper model, a polarized renal cell line stably-expressing V2R-GFP was generated. Labeled AVP-binding studies revealed an equal basolateral vs. apical membrane distribution for V2R-GFP and endogenous V2R. In these cells, GFP-V2R was expressed in its mature form and localized for 75% in the basolateral membrane and for 25% to late endosomes/lysosomes. dDAVP caused a dose- and time-dependent internalization of V2R-GFP, which was completed within 1 h with 100 nM dDAVP, was prevented by coincubation with a V2R antagonist, and which reduced its half-life from 11.5 to 2.8 h. Semiquantification of the V2R-GFP colocalization with E-cadherin (basolateral membrane), early endosomal antigen-1 (EEA-1) and lysosome-associated membrane protein-2 (LAMP-2) in time revealed that most dDAVP-bound V2R was internalized via early endosomes to late endosomes/lysosomes, where it was degraded. The dDAVP-internalized V2R did not recycle to the basolateral membrane. In conclusion, we established the itinerary of the V2R in a polarized cell model that likely resembles the in vivo V2R localization and regulation by AVP to a great extent.

[From gene to disease; Dent's disease caused by abnormalities in the CLCN5 and OCRL1 genes]. / Van gen naar ziekte; de ziekte van Dent veroorzaakt door afwijkingen in CLCN5- en OCRL1-genen.

Dent's disease is an X-linked disorder, characterized by generalized proximal tubular dysfunction, nephrolithiasis, nephrocalcinosis and the development ofend-stage renal disease, generally occurring after the age of thirty. In the majority of cases, the disease is caused by mutations in the CLCN5-gene. The pathogenesis of the disease has not yet been clarified. Defective recycling of multi-ligand proximal tubular receptors megalin and cubilin is considered responsible for the defective reabsorption of low molecular weight proteins, albumin, hormones and vitamins. Treatment with thiazide diuretics to diminish the hypercalciuria in combination with citrate supplements might prevent renal stone formation and deterioration of renal function. In the laboratory ofDNA diagnostics in the Radboud University Nijmegen Medical Centre, the molecular analysis of the CLCN5-gene in patients suspected with this disease is performed.

[From gene to disease; 'apparent mineralocorticoid excess' syndrome, a syndrome with an apparent excess of mineral corticoids]. / Van gen naar ziekte; 'apparent mineralocorticoid excess'-syndroom, een syndroom met ogenschijnlijke overmaat aan mineralocorticoïden.

Apparent mineralocorticoid excess (AME) is an autosomal recessive disease caused by deficiency of the enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2). 11beta-HSD2 converts cortisol into inactive cortisone and prevents the stimulation of the mineralocorticoid receptor by cortisol. In patients with AME, an enhanced stimulation of mineralocorticoid receptors by cortisol in the distal nephron causes an elevated sodium reabsorption and increased potassium excretion. Sodium retention leads to severe low renin hypertension. The diagnosis of AME is based on the detection of an increased concentration of cortisol metabolites and a low or undetectable concentration of cortisone metabolites in urine. Molecular analysis of the HSD11B2 gene confirms the diagnosis. AME is successfully treated by potassium-sparing diuretics, sometimes in combination with loop diuretics (furosemide). Mild forms of AME might occur more frequently than is currently known and should be suspected in patients with hypertension, hypokalemia and decreased plasma renin concentration. Since liquorice can induce the clinical symptoms of AME due to reversible inhibition of the 11beta-HSD2 enzyme by glycyrrhetinic acid, the active ingredient of liquorice, patients suspected of having AME should not consume liquorice.

Molecular characterization of nephrogenic diabetes insipidus.

Nephrogenic diabetes insipidus (NDI) is characterized by insensitivity of the distal renal nephron to the antidiuretic effect of the neurohypophyseal hormone arginine vasopressin. In the last 2 years, two different genetic defects causing the NDI phenotype have been identified. The genes involved encode proteins that reside at both ends of the cellular vasopressin signaling cascade, namely the vasopressin V(2) receptor and the aquaporin-2 water channel. Analysis of naturally occurring mutations in the V(2) receptor and the aquaporin-2 water channel will facilitate the study of structure-function correlates of both proteins, which will lead to substantial progress in elucidating the cellular mechanisms involved in the antidiuretic effect of vasopressin.

Screening for subtelomeric rearrangements in 210 patients with unexplained mental retardation using multiplex ligation dependent probe amplification (MLPA).

BACKGROUND: Subtelomeric rearrangements contribute to idiopathic mental retardation and human malformations, sometimes as distinct mental retardation syndromes. However, for most subtelomeric defects a characteristic clinical phenotype remains to be elucidated. OBJECTIVE: To screen for submicroscopic subtelomeric aberrations using multiplex ligation dependent probe amplification (MLPA). METHODS: 210 individuals with unexplained mental retardation were studied. A new set of subtelomeric probes, the SALSA P036 human telomere test kit, was used. RESULTS: A subtelomeric aberration was identified in 14 patients (6.7%) (10 deletions and four duplications). Five deletions were de novo; four were inherited from phenotypically normal parents, suggesting that these were polymorphisms. For one deletion, DNA samples of the parents were not available. Two de novo submicroscopic duplications were detected (dup 5qter, dup 12pter), while the other duplications (dup 18qter and dup 22qter) were inherited from phenotypically similarly affected parents. All clinically relevant aberrations (de novo or inherited from similarly affected parents) occurred in patients with a clinical score of >or=3 using an established checklist for subtelomeric rearrangements. Testing of patients with a clinical score of >or=3 increased the diagnostic yield twofold to 12.4%. Abnormalities with clinical relevance occurred in 6.3%, 5.1%, and 1.7% of mildly, moderately, and severely retarded patients, respectively, indicating that testing for subtelomeric aberrations among mildly retarded individuals is necessary. CONCLUSIONS: The value of MLPA is confirmed. Subtelomeric screening can be offered to all mentally retarded patients, although clinical preselection increases the percentage of chromosomal aberrations detected. Duplications may be a more common cause of mental retardation than has been appreciated.

[From gene to disease; mutations in the SLC12A3 gene as the cause of Gitelman's syndrome]. / Van gen naar ziekte; mutaties in het SLC12A3-gen als oorzaak van het syndroom van Gitelman.

Gitelman's syndrome is characterised by persistent hypokalaemia, hypomagnesaemia and hypocalciuria (OMIM 263800). This rare autosomal recessive disorder is caused by renal Na+, Cl-, K+ and Mg2+ wasting. Other typical features include hypocalciuria and an intact renal concentrating ability. Gitelman's syndrome is caused by mutations in the SLC12A3 gene, encoding the thiazide-sensitive sodium-chloride co-transporter (NCC). NCC is located in the distal convoluted tubule of the kidney, a segment known to play an important role in active magnesium reabsorption in the nephron. The exact mechanisms underlying hypomagnesaemia and hypocalciuria in Gitelman's syndrome are still poorly understood, but point to enhanced proximal Na+ and Ca2+ reabsorption and apoptosis of distal convoluted tubule cells.

Autosomal recessive nephrogenic diabetes insipidus caused by an aquaporin-2 mutation.

Vasopressin V2 receptors, expressed from an x-chromosomal gene, are involved in antidiuresis, but also in release of coagulation factor VIII and von Willebrand factor (vWF). The present study describes autosomal recessive nephrogenic diabetes insipidus (NDI) in a large cluster of patients in Israel's Lower-Galilee. Evidence for an intact V2 receptor was concluded by their normal increase in factor VIII and vWF after desmopressin infusion. Thus, in these patients a defect in the pathway beyond the V2 receptor was suspected. The recent cloning of the human Aquaporin-2 gene enabled us to test this gene as a candidate for such a postreceptor defect. Direct sequencing of the Aquaporin-2 gene revealed a G298T substitution causing a Gly100Stop nonsense mutation in the third transmembrane region. Because this putative disease-causing mutation was identified in index patients of different families, we suggest that all patients are descendants of a common ancestor. Thus, this new entity is characterized by an autosomal recessive NDI. The differential response of clotting factors and urine osmolality to desmopressin may provide a simple tool for clinical diagnosis of a V2-postreceptor defect. The early stop-codon of Aquaporin-2 results in complete resistance to vasopressin antidiuretic effect.

Molecular cytogenetic detection of 9q34 breakpoints associated with nail patella syndrome.

The nail patella syndrome (NPS1) is an autosomal dominant disorder characterised by dysplasia of the finger nails and skeletal abnormalities. NPS1 has been mapped to 9q34, to a 1 cM interval between D9S315 and the adenylate kinase gene (AK1). We have mapped the breakpoints within the candidate NPS1 region in two unrelated patients with balanced translocations. One patient [46,XY,t(1;9)(q32.1;q34)] was detected during a systematic survey of old cytogenetic files in Denmark and southern Sweden. The other patient [46,XY,t(9;17)(q34.1;q25)] was reported previously. D9S315 and AK1 were used to isolate YACs, from which endclones were used to isolate PACs. Two overlapping PAC clones span the 9q34 breakpoints in both patients, suggesting that NPS1 is caused by haploinsufficiency due to truncation or otherwise inactivation of a gene at or in the vicinity of the breakpoints.

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis maps to chromosome 3q27 and is associated with mutations in the PCLN-1 gene.

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC, MIM 248250) is a complex renal tubular disorder characterised by hypomagnesaemia, hypercalciuria, advanced nephrocalcinosis, hyposthenuria and progressive renal failure. The mode of inheritance is autosomal recessive. A primary defect in the reabsorption of magnesium in the medullary thick ascending limb of the loop of Henle (mTAL) has been proposed to be essential in FHHNC pathophysiology. To identify the underlying genetic defect we performed linkage analysis in eight families, including three with consanguineous marriages. We found linkage to microsatellite markers on chromosome 3q27 with a maximum two-point lod score (Zmax) of 5.208 for D3S3530 without evidence for genetic heterogeneity. Haplotype analysis revealed crucial recombination events reducing the critical interval to 6.6cM. Recently, mutations in the gene PCLN-1, mapping to 3q27 and coding for paracellin-1, were identified by Simon et al (1999) as the underlying genetic defect in FHHNC. Paracellin-1 represents a renal tight junction protein predominantly expressed in the TAL. Mutational analysis in our patient cohort revealed eight different mutations in the PCLN-1 gene, within six novel mutations. In seven of 13 mutant alleles we detected a Leu151 substitution without evidence for a founder effect. Leu151 is a residue of the first extracellular loop of paracellin-1, the part of the protein expected to bridge the intercellular space and to be important for paracellular conductance. This study confirms the implication of paracellin-1 defects in FHHNC and points to a predominant role of this protein in the paracellular reabsorption of divalent cations in the TAL.