Two novel types of contiguous gene deletion of the AVPR2 and ARHGAP4 genes in unrelated Japanese kindreds with nephrogenic diabetes insipidus

Targeted long-read sequencing identified a causal structural variant in X-linked nephrogenic diabetes insipidus

BACKGROUND

X-linked nephrogenic diabetes insipidus (NDI) is a rare genetic renal disease caused by pathogenic variants in the AVPR2 gene. Single nucleotide variants and small insertions/deletions in AVPR2 are reliably detected by routine clinical sequencing. Nevertheless, structural variants involving AVPR2 are challenging to identify accurately by conventional genetic testing. Here, we report a novel deletion of AVPR2 in a Czech family identified for the first time by targeted long-read sequencing (T-LRS).

METHODS

A male proband with X-linked NDI underwent clinical sequencing of the AVPR2 gene that failed and thus indicated possible whole-gene deletion. Therefore, PCR mapping and subsequent targeted long-read sequencing (T-LRS) using a Pacific Biosciences sequencer were applied to search for the suspected deletion. To validate the deletion breakpoints and prove variant segregation in the family with X-linked NDI, Sanger sequencing of the deletion junction was performed. Quantitative real-time PCR was further carried out to confirm the carrier status of heterozygous females.

RESULTS

By T-LRS, a novel 7.5 kb deletion of AVPR2 causing X-linked NDI in the proband was precisely identified. Sanger sequencing of the deletion junction confirmed the variant breakpoints and detected the deletion in the probands´ mother, maternal aunt, and maternal cousin with X-linked NDI. The carrier status in heterozygous females was further validated by quantitative real-time PCR.

CONCLUSIONS

Identifying the 7.5 kb deletion gave a precise molecular diagnosis for the proband, enabled genetic counselling and genetic testing for the family, and further expanded the spectrum of structural variants causing X-linked NDI. Our results also show that T-LRS has significant potential for accurately identifying putative structural variants.

Identification of a novel X-linked arginine-vasopressin receptor 2 mutation in nephrogenic diabetes insipidus: Case report and pedigree analysis

INTRODUCTION

The clinical and genetic characteristics of nephrogenic diabetes insipidus (NDI) were described via assessing 2 cases of NDI patients from a Chinese family.

PATIENT CONCERNS

Two patients who manifest polyuria and polydipsia were admitted to hospital for definite diagnosis.

DIAGNOSIS

Water deprivation-vasopressin tests showed that the patients may possess renal-origin diabetes insipidus. All the levels of thyroid-stimulating hormone, luteinizing hormone, follicle stimulation hormone, adrenocorticotropic hormone, prolactin, and growth hormone in both patients were normal. These results were certified that both patients possess a nephropathy-type diabetes insipidus. B-mode ultrasonography and urinalysis test demonstrated that the patient's diabetes insipidus is unlikely to originate from renal organic disease. Remarkably, by nucleotide sequencing, we found a novel mutation c.414_418del in arginine-vasopressin receptor 2 (AVPR2) was related to the disease of NDI.

INTERVENTIONS

Two patients were treated with oral hydrochlorothiazide and indomethacin. In addition, low salt diet and potassium supplementation throughout the patients' treatment.

OUTCOMES

The clinical symptoms of 2 patients were significantly reduced after targeted therapy.

CONCLUSION

A mutation in AVPR2 was discovered to be associated with NID. It provides a new target for molecular diagnosis of NDI, enabling families to undergo genetic counseling and obtain prenatal diagnoses.

Contiguous gene deletion in a Chinese family with X-linked nephrogenic diabetes insipidus: challenges in early diagnosis and implications for affected families

Nephrogenic diabetes insipidus (NDI) is a rare disorder of the renal collecting tubules, characterized by an inability to concentrate urine due to an impaired response to arginine vasopressin (AVP), resulting in dilute urine and polyuria. Causes of NDI are heterogeneous and diagnosing congenital NDI (cNDI) in young infants is clinically challenging, as typical symptoms are often unappreciated or inconspicuous. Instead, young infants may present with non-specific signs such as vomiting, poor feeding, failure to thrive, unexplained fevers, irritability, constipation or diarrhea. We report a 37-day-old infant who presented with polyuria and severe hypernatremic dehydration that was unresponsive to vasopressin. The patient was treated with amiloride, indomethacin and hydrochlorothiazide. Genetic analysis revealed a novel contiguous deletion involving the entire AVPR2 gene and the last exon of the adjacent ARHGAP4 gene. A study of the family confirmed the carrier status in the mother. This case illustrates the importance of molecular testing in confirming the diagnosis in the index patient, as well as in identifying asymptomatic at-risk female carriers so that appropriate genetic counselling can be given for reproductive planning. All pediatric patients with suspected cNDI should undergo genetic analysis for a definitive diagnosis.

Contiguous 22.1-kb deletion embracing AVPR2 and ARHGAP4 genes at novel breakpoints leads to nephrogenic diabetes insipidus in a Chinese pedigree

Background

It has been reported that mutations in arginine vasopressin type 2 receptor (AVPR2) cause congenital X-linked nephrogenic diabetes insipidus (NDI). However, only a few cases of AVPR2 deletion have been documented in China.

Methods

An NDI pedigree was included in this study, including the proband and his mother. All NDI patients had polyuria, polydipsia, and growth retardation. PCR mapping, long range PCR and sanger sequencing were used to identify genetic causes of NDI.

Results

A novel 22,110 bp deletion comprising AVPR2 and ARH4GAP4 genes was identified by PCR mapping, long range PCR and sanger sequencing. The deletion happened perhaps due to the 4-bp homologous sequence (TTTT) at the junctions of both 5′ and 3′ breakpoints. The gross deletion co-segregates with NDI. After analyzing available data of putative clinical signs of AVPR2 and ARH4GAP4 deletion, we reconsider the potential role of AVPR2 deletion in short stature.

Conclusions

We identified a novel 22.1-kb deletion leading to X-linked NDI in a Chinese pedigree, which would increase the current knowledge in AVPR2 mutation.

ARHGAP4 mutated in a Chinese intellectually challenged family

OBJECTIVE

Mental retardation is characterized by lower intelligence compared to the average intelligence of persons the same age. These patients have low adaptive capacity acquired by society. The genetic factors of causing MR include monogenic disease, chromosome structural aberration, and chromosome number aberration and so on. We explored the cause of a Chinese family suffering from mental retardation.

METHODS

We used karyotyping technology to determine the karyotype of the proband, and we used FISH to verify the result of the karyotyping. We used whole-exome sequencing to identify the disease-causing gene and used Sanger sequencing to verify the result of whole-exome sequencing to assess the family's gene expression.

RESULTS

The G-banding of the karyotype revealed that the patient's karyotype is 46, XY. FISH revealed that the patient does not have a trisomy syndrome. The karyotype of the proband is normal. Using whole-exome sequencing, we identified 108,767 variants in the exome gene of the patient, including 101,787 SNPs and 6980 InDels. Combining clinical information and bioinformatics analysis, including databases filtering and SIFT analysis, we found ARHGAP4 in X chromosome was candidate MR disease-causing gene. PCR and Sanger sequencing results were consistent with whole-exome sequencing. ARHGAP4 (T491M) mutation was present in the genome of the proband and his mother is a carrier, while his father, sister, and brother do not carry this mutation.

CONCLUSION

According to clinical information, whole-exome sequencing results and Sanger verification results, ARHGAP4 (T491M) mutation may be disease-causing gene of the MR patient. The relation between ARHGAP4 mutation and MR clinical characteristic is needed to be illuminated with participation of more MR patients.

[Vasopressin V2 receptor-related pathologies: congenital nephrogenic diabetes insipidus and nephrogenic syndrome of inappropiate antidiuresis]

Congenital nephrogenic diabetes insipidus is a rare hereditary disease with mainly an X-linked inheritance (90% of the cases) but there are also autosomal recessive and dominant forms. Congenital nephrogenic diabetes insipidus is characterized by a resistance of the renal collecting duct to the action of the arginine vasopressin hormone responsible for the inability of the kidney to concentrate urine. The X-linked form is due to inactivating mutations of the vasopressin 2 receptor gene leading to a loss of function of the mutated receptors. Affected males are often symptomatic in the neonatal period with a lack of weight gain, dehydration and hypernatremia but mild phenotypes may also occur. Females carrying the mutation may be asymptomatic but, sometimes, severe polyuria is found due to the random X chromosome inactivation. The autosomal recessive and dominant forms, occurring in both genders, are linked to mutations in the aquaporin-2 gene. The treatment remains difficult, especially in infants, and is based on a low osmotic diet with increased water intake and the use of thiazides and indomethacin. The main goal is to avoid hypernatremic episodes and maintain a good hydration state. Potentially, specific treatment, in some cases of X-linked congenital nephrogenic diabetes insipidus, with pharmacological chaperones such as non-peptide vasopressin-2 receptor antagonists will be available in the future. Conversely, the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is linked to a constitutive activation of the V(2)-receptor due to activating mutations with clinical and biological features of inappropriate antidiuresis but with low or undetectable plasma arginine vasopressin hormone levels.

Hereditary Hemorrhagic Telangiectasia: Breakpoint Characterization of a Novel Large Deletion in ACVRL1 Suggests the Causing Mechanism

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant vascular dysplasia. Mutations in either ENG or ACVRL1 account for around 85% of cases, and 10% are large deletions and duplications. Here we present a large novel deletion in ACVRL1 gene and its molecular characterization in a 3 generation Italian family. We employed short tandem repeats (STRs) analysis, direct sequencing, multiplex ligation-dependant probe amplification (MLPA) analysis, and 'deletion-specific' PCR methods. STRs Analysis at ENG and ACVRL1 loci suggested a positive linkage for ACVRL1. Direct sequencing of this gene did not identify any mutations, while MLPA identified a large deletion. These results were confirmed and exactly characterized with a 'deletion-specific' PCR: the deletion size is 4,594 bp and breakpoints in exon 3 and intron 8 show the presence of short direct repeats of 7 bp [GCCCCAC]. We hypothesize, as causative molecular mechanism, the replication slippage model. Understanding the fine mechanisms associated with genomic rearrangements may indicate the nonrandomness of these events, highlighting hot spots regions. The complete concordance among MLPA, STRs analysis and 'deletion-specific PCR' supports the usefulness of MLPA in HHT molecular analysis.

A novel contiguous gene deletion of AVPR2 and ARHGAP4 genes in male dizygotic twins with nephrogenic diabetes insipidus and intellectual disability

The clinical features of loss of ARHGAP4 function remain unclear despite several reports of different patterns of deletions inactivating different functional regions of the protein. The protein encoded by ARHGAP4 is thought to function as a Rho GTPase activating protein. Characterization of the genetic defect causing X-linked nephrogenic diabetes insipidus (NDI) and intellectual disability in two dizygotic twin brothers revealed a novel contiguous deletion of 17,905 bp encompassing the entire AVPR2 gene and extending into intron 7 of the ARHGAP4 gene. Examination of their mother showed that she was a carrier of this deletion. An attempt was made to distinguish the putative clinical signs of an ARHGAP4 deletion from the well-defined phenotype of X-linked NDI caused by an AVPR2 gene deletion. By reviewing all characterized deletions encompassing ARHGAP4, we reconsider the potential role of ARHGAP4 in cognition.

Two new large deletions of the AVPR2 gene causing nephrogenic diabetes insipidus and a review of previously published deletions

BACKGROUND

In this paper, we report two new original deletions and present an extended review of the previously characterized AVPR2 gene deletions to better understand the underlying deletion mechanisms.

METHODS

The two novel deletions were defined using polymerase chain reaction mapping and junction fragment sequencing. Bioinformatic analysis was performed on both the previously mapped deletions and the novel ones through several web tools.

RESULTS

In our two patients with nephrogenic diabetes insipidus, we found a 23 755 bp deletion and a 9264 bp deletion both comprising the entire AVPR2 gene and part of the ARHGAP4 gene. Through bioinformatic studies, the smallest overlapping region as well as several motifs and repeats that are known to promote rearrangements were confirmed.

CONCLUSIONS

Through this study, it was determined that the deletion mechanisms in the AVPR2 region do not follow the rules of non-allelic homologous recombination. Two of the 13 deletions can be attributed to the fork stalling and template switching (FoSTeS) mechanism, whereas the remaining 11 deletions could be caused either by non-homologous end joining or by the FoSTeS mechanism. Although no recurrence was found, several groupings of deletion breakpoints were identified.

Small G proteins and their regulators in cellular signalling

Small molecular weight GTPases (small G proteins) are essential in the transduction of signals from different plasma membrane receptors. Due to their endogenous GTP-hydrolyzing activity, these proteins function as time-dependent biological switches controlling diverse cellular functions including cell shape and migration, cell proliferation, gene transcription, vesicular transport and membrane-trafficking. This review focuses on endocrine diseases linked to small G proteins. We provide examples for the regulation of the activity of small G proteins by various mechanisms such as posttranslational modifications, guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs) or guanine nucleotide dissociation inhibitors (GDIs). Finally we summarize endocrine diseases where small G proteins or their regulatory proteins have been revealed as the cause.

Ectopic production of parathyroid hormone in a patient with sporadic medullary thyroid cancer

Elevation of serum parathyroid hormone (PTH) in patients with medullary thyroid cancer (MTC) is usually found in multiple endocrine neoplasia type 2A (MEN2A). However, ectopic production of PTH is rare and its molecular etiology remains largely uninvestigated. We report a case of ectopic production of PTH by a sporadic MTC. The etiology of ectopic PTH gene expression was examined, focusing on GCM2 which has a crucial role in developing parathyroid glands. We observed ectopic expression of the PTH and GCM2 genes in tissues from the tumor and metastatic lymph nodes. However, GCM2 gene expression was also detected in adjacent thyroid tissue and lymphoblasts, in which PTH gene expression was absent. Hypomethylation of the PTH promoter, which is reportedly associated with ectopic production of PTH, was not seen in either the tumor tissue or metastatic lymph nodes. Meanwhile, DNA hypomethylation was seen in a CpG island identified in the GCM2 promoter region, regardless of whether or not the GCM2 gene was expressed. We showed that transcriptional activity of the CpG island sequences cloned into a reporter plasmid was dependent upon DNA methylation. Finally, we present the first report of a PTH-producing MTC. There was no apparent association between ectopic PTH and GCM2 gene expression, despite co-expression of the two genes. Neither genomic rearrangement nor DNA hypomethylation in the PTH gene appeared responsible for ectopic production of PTH. Although DNA hypomethylation may be necessary for the GCM2 gene expression, ectopic expression of GCM2 won't be possible by DNA hypomethylation alone.

AVPR2 variants and mutations in nephrogenic diabetes insipidus: review and missense mutation significance

Almost 90% of nephrogenic diabetes insipidus (NDI) is due to mutations in the arginine-vasopressin receptor 2 gene (AVPR2). We retrospectively examined all the published mutations/variants in AVPR2. We planned to perform a comprehensive review of all the AVPR2 mutations/variants and to test whether any amino acid change causing a missense mutation is significantly more or less common than others. We performed a Medline search and collected detailed information regarding all AVPR2 mutations and variants. We performed a frequency comparison between mutated and wild-type amino acids and codons. We predicted the mutation effect or reported it based on published in vitro studies. We also reported the ethnicity of each mutation/variant carrier. In summary, we identified 211 AVPR2 mutations which cause NDI in 326 families and 21 variants which do not cause NDI in 71 NDI families. We described 15 different types of mutations including missense, frameshift, inframe deletion, deletion, insertion, nonsense, duplication, splicing and combined mutations. The missense mutations represent the 55.83% of all the NDI published families. Arginine and tyrosine are significantly (P = 4.07E-08 and P = 3.27E-04, respectively) the AVPR2 most commonly mutated amino acids. Alanine and glutamate are significantly (P = 0.009 and P = 0.019, respectively) the least mutated AVPR2 amino acids. The spectrum of mutations varies from rare gene variants or polymorphisms not causing NDI to rare mutations causing NDI, among which arginine and tyrosine are the most common missense. The AVPR2 mutations are spread world-wide. Our study may serve as an updated review, comprehensive of all AVPR2 variants and specific gene locations. J. Cell. Physiol. 217: 605-617, 2008. (c) 2008 Wiley-Liss, Inc.

Nephrogenic diabetes insipidus in a patient with L1 syndrome: a new report of a contiguous gene deletion syndrome including L1CAM and AVPR2

We report on an infant boy with congenital hydrocephalus due to L1 syndrome and polyuria due to diabetes insipidus. We initially believed his excessive urine loss was from central diabetes insipidus and that the cerebral malformation caused a secondary insufficient pituitary vasopressin release. However, he failed to respond to treatment with a vasopressin analogue, which pointed to nephrogenic diabetes insipidus (NDI). L1 syndrome and X-linked NDI are distinct clinical disorders caused by mutations in the L1CAM and AVPR2 genes, respectively, located in adjacent positions in Xq28. In this boy we found a deletion of 61,577 basepairs encompassing the entire L1CAM and AVPR2 genes and extending into intron 7 of the ARHGAP4 gene. To our knowledge this is the first description of a patient with a deletion of these three genes. He is the second patient to be described with L1 syndrome and NDI. During follow-up he manifested complications from the hydrocephalus and NDI including global developmental delay and growth failure with low IGF-1 and hypothyroidism.

Immunological profile in a family with nephrogenic diabetes insipidus with a novel 11 kb deletion in AVPR2 and ARHGAP4 genes

Background

Congenital nephrogenic diabetes insipidus (NDI) is characterised by an inability to concentrate urine despite normal or elevated plasma levels of the antidiuretic hormone arginine vasopressin. We report a Japanese extended family with NDI caused by an 11.2-kb deletion that includes the entire AVPR2 locus and approximately half of the Rho GTPase-activating protein 4 (ARHGAP4) locus. ARHGAP4 belongs to the RhoGAP family, Rho GTPases are critical regulators of many cellular activities, such as motility and proliferation which enhances intrinsic GTPase activity.

ARHGAP4 is expressed at high levels in hematopoietic cells, and it has been reported that an NDI patient lacking AVPR2 and all of ARHGAP4 showed immunodeficiency characterised by a marked reduction in the number of circulating CD3+ cells and almost complete absence of CD8+ cells.

Methods

PCR and sequencing were performed to identify the deleted region in the Japanese NDI patients. Immunological profiles of the NDI patients were analysed by flow cytometry. We also investigated the gene expression profiles of peripheral blood mononuclear cells (PBMC) from NDI patients and healthy controls in microarray technique.

Results

We evaluated subjects (one child and two adults) with 11.2-kb deletion that includes the entire AVPR2 locus and approximately half of the ARHGAP4. Hematologic tests showed a reduction of CD4+ cells in one adult patient, a reduction in CD8+ cells in the paediatric patient, and a slight reduction in the serum IgG levels in the adult patients, but none of them showed susceptibility to infection. Gene expression profiling of PBMC lacking ARHGAP4 revealed that expression of RhoGAP family genes was not influenced greatly by the lack of ARHGAP4.

Conclusion

These results suggest that loss of ARHGAP4 expression is not compensated for by other family members. ARHGAP4 may play some role in lymphocyte differentiation but partial loss of ARHGAP4 does not result in clinical immunodeficiency.

Affective and psychiatric disorders in celiac disease

Several extraintestinal clinical manifestations have been reported in celiac disease (CD). Among them, growing evidence suggests the association between CD and affective and psychiatric disorders. In this review the most frequent affective and psychiatric disorders associated with CD and the possible mechanisms involved in these associations were analyzed. The available data suggest that screening for CD in patients with affective and/or psychiatric symptoms may be useful since these disorders could be the expression of an organic disease rather than primary psychiatric illnesses.

Genotyping of single-nucleotide polymorphisms by primer extension reaction in a dry-reagent dipstick format

The primer extension (PEXT) reaction is the most widely used approach to genotyping of single-nucleotide polymorphisms (SNPs). Current methods for analysis of PEXT reaction products are based on electrophoresis, fluorescence resonance energy transfer, fluorescence polarization, pyrosequencing, mass spectrometry, microarrays, and spectrally encoded microspheres. We report the first dry-reagent dipstick method that enables rapid visual detection of PEXT products without instrumentation. The method is applied to the analysis of six SNPs in the mannose-binding lectin gene (MBL2). The genomic region that spans each SNP of interest is amplified by PCR. Two primer extension reactions are performed with allele-specific primers (for one or the other variant nucleotide), which contain an oligo(dA) segment at the 5'-end. Biotin-dUTP is incorporated in the extended strand. The product is applied to the strip followed by immersion in the appropriate buffer. As the DNA moves along the strip by capillary action, it hybridizes with oligo(dT)-functionalized gold nanoparticles, such that only extended products are captured by immobilized streptavidin at the test zone, generating a red line. A second red line is formed at the control zone of the strip by hybridization of the nanoparticles with immobilized oligo(dA). The dipstick test is complete within 10 min. We analyzed six SNPs of the mannose-binding lectin gene (MBL2) using genomic DNA from 27 patients, representing a total of 74 variant nucleotide positions. Patient genotypes showed 100% concordance with direct DNA sequencing data. The described PEXT-dipstick assay is rapid and highly accurate; it does not require specialized instrumentation or highly trained technical personnel. It is appropriate for a diagnostic laboratory where a few selected SNP markers are examined per patient with a low cost per assay.

Contiguous gene deletion involvingL1CAM andAVPR2 causes X-linked hydrocephalus with nephrogenic diabetes insipidus

X-linked hydrocephalus with aqueductal stenosis (HSAS) is caused by mutation or deletion of the L1 cell adhesion molecule gene (L1CAM) at Xq28. Central diabetes insipidus (CDI) can arise as a consequence of resultant hypothalamic dysfunction from hydrocephalus and must be distinguished from nephrogenic diabetes insipidus (NDI) by exogenous vasopressin response. Causes of NDI are heterogeneous and include mutation or deletion of the arginine vasopressin receptor 2 gene (AVPR2), which is located approximately 29 kb telomeric to L1CAM. We identified a patient with both HSAS and NDI where DNA sequencing failure suggested the possibility of a contiguous gene deletion. A 32.7 kb deletion mapping from L1CAM intron1 to AVPR2 exon2 was confirmed. A 90 bp junctional insertion fragment sharing short direct repeat homology with flanking sequences was identified. To our knowledge this is the first reported case of an Xq28 microdeletion involving both L1CAM and AVPR2, defining a new contiguous gene syndrome comprised of HSAS and NDI. Contiguous gene deletion should be considered as a mechanism for all patients presenting with hydrocephalus and NDI.

Gene expression profiles of erythroid precursors characterise several mechanisms of the action of hydroxycarbamide in sickle cell anaemia

Hydroxycarbamide (HC) (or hydroxyurea) has been reported to increase fetal haemoglobin levels and improve clinical symptoms in sickle cell anaemia (SCA) patients. However, the complete pathway by which HC acts remains unclear. To study the mechanisms involved in the action of HC, global gene expression profiles were obtained from the bone marrow cells of a SCA patient before and after HC treatment using serial analysis of gene expression. In the comparison of both profiles, 147 differentially expressed transcripts were identified. The functional classification of these transcripts revealed a group of gene categories associated with transcriptional and translational regulation, e.g. EGR-1, CENTB1, ARHGAP4 and RIN3, suggesting a possible role for these pathways in the improvement of clinical symptoms of SCA patients. The genes involved in these mechanisms may represent potential tools for the identification of new targets for SCA therapy.

Deletion of the V2 vasopressin receptor gene in two Chinese patients with nephrogenic diabetes insipidus

Background

Congenital nephrogenic diabetes insipidus (NDI) is a rare X-linked inherited disorder characterized by the excretion of large volumes of diluted urine and caused by mutations in arginine vasopressin receptor 2 (AVPR2) gene. To investigate the mutation of AVPR2 gene in a Chinese family with congenital NDI, we screened AVPR2 gene in two NDI patients and eight family members by PCR amplification and direct sequencing.

Results

Five specific fragments, covering entire coding sequence and their flanking intronic sequences of AVPR2 gene, were not observed in both patients, while those fragments were all detected in the control subjects. Several different fragments around the AVPR2 locus were amplified step by step. It was revealed that a genomic fragment of 5,995-bp, which contained the entire AVPR2 gene and the last exon (exon 22) of the C1 gene, was deleted and a 3-bp (GAG) was inserted. Examination of the other family members showed that the mothers and the grandmother were carriers for this deletion.

Conclusion

Our findings suggest that the two patients in a Chinese family suffering from congenital NDI had a 5,995-bp deletion and 3-bp (GAG) insertion at Xq28. The deletion contained the entire AVPR2 gene and exon 22 of the C1 gene.

Molecular basis and clinical features of nephrogenic diabetes insipidus

Maintenance of water and electrolyte homeostasis is central to mammalian survival and, therefore, under stringent hormonal control. Water homeostasis is achieved by balancing fluid intake with water excretion, governed by the antidiuretic action of arginine vasopressin. Arginine vasopressin stimulation of renal V₂ vasopressin receptors in the basolateral membrane of principal cells induces aquaporin-2-mediated water reabsorption in the kidney. The importance of this system is apparent when mutations inactivate V₂ vasopressin receptors and aquaporin-2 and cause the clinical phenotype of nephrogenic diabetes insipidus. To date, over 190 mutations in the V₂ vasopressin receptors gene (AVPR2) and approximately 38 mutations in the aquaporin-2 gene have been identified in patients with inherited nephrogenic diabetes insipidus. Extensive in vitro expression and mutagenesis studies of V₂ vasopressin receptors and aquaporin-2 have provided detailed insights into the molecular mechanisms of G-protein-coupled receptor and water channel dysfunction per se. Targeted deletions of AVPR2 and AQP2 in mice have extended the knowledge of nephrogenic diabetes insipidus pathophysiology and have stimulated testing of old and new ideas to therapeutically restore normal kidney function in animal models and patients with this disease. In this review, we summarize the current knowledge relevant to understand the molecular basis of inherited nephrogenic diabetes insipidus forms and the rationales for the current pharmacological treatment of patients with this illness.

Novel mutations underlying nephrogenic diabetes insipidus in Arab families

PURPOSE

Nephrogenic Diabetes Insipidus (NDI) is genetically heterogeneous and may be inherited in an X-linked or autosomal recessive manner. We aimed to investigate the molecular basis of NDI among Arab families.

METHODS

Direct sequencing of coding regions for AQP2 and AVPR2 was used to identify underlying mutations. One large deletion required Southern blot analysis and a PCR-based strategy to identify deletion junctions.

RESULTS

We identified two novel missense mutations (AQP2:p.Gly100Arg and p.Gly180Ser) in AQP2 and one novel missense mutation (AVPR2:p.Gly122Asp), one previously reported missense mutation (AVPR2:p.Arg137His) and one novel contiguous deletion (AVPR2:c.25 + 273_ARHGAP4o:2650-420del) affecting AVPR2. We also describe evidence of lyonization associated with the novel deletion.

CONCLUSIONS

Two novel mutations were identified in each of AVPR2 and AQP2 underlying CNDI in Arab families. Identification of these mutations will facilitate early diagnosis of CNDI, counseling of families and provide opportunities for early intervention aimed at reducing morbidity. The presence of affected females and consanguinity, as is often observed in Arab communities should not be used to rule out AVPR2 as a candidate when considering diagnostic testing. Careful observation of phenotypic heterogeneity should be used in referring such families for both AQP2 and AVPR2 molecular genetic testing.

Severe combined immunodeficiency associated with nephrogenic diabetes insipidus and a deletion in the Xq28 region

We evaluated a baby boy with severe combined immunodeficiency (SCID) and X-linked nephrogenic diabetes insipidus (NDI). This patient had less than 10% CD3+ T cells, almost all of which were positive for CD4 and CD45RO. Genetic studies demonstrated a 34.4 kb deletion at Xq28 which included AVPR2, the gene responsible for NDI; ARHGAP4, a hematopoietic specific gene encoding a GTPase-activating protein; and a highly conserved segment of DNA between ARHGAP4 and ARD1A, a gene involved in the response to hypoxia. Other patients with NDI, but without immunodeficiency, have had deletions that remove all ARHGAP4 except exon 1; however, no other patients have had deletions of the highly conserved intragenic region between ARHGAP4 and ARD1A. X chromosome inactivation studies, done on sorted cells from the mother and grandmother of the patient, carriers of the deletion, demonstrated exclusive use of the non-mutant X chromosome as the active X in CD4 and CD8 T cells. Surprisingly, NK cells, monocytes and neutrophils from these women demonstrated preferential use of the mutant X chromosome as the active X. These results are consistent with an X-linked form of SCID, due to the loss of regulatory elements that control the response to hypoxia in hematopoietic cells.

First characterization of a large deletion of the PDHA 1 gene

Pyruvate dehydrogenase complex (PDC) deficiency is one of the major recognized causes of congenital lactic acidosis. The most common form is due to PDHA 1 gene (Xp22.12) defects. Here, we report the case of a Polynesian girl presenting with delayed neurological development, cortical atrophy, and posterior corpus callosum agenesis. Elevated lactate and pyruvate levels in blood and cerebrospinal fluid suggested PDC deficiency. However, PDC activity was within the normal range in lymphocytes and the direct sequencing of the 11 exons and intron-exon junctions of the PDHA 1 gene did not show any changes. Long-range PCR amplification of the whole gene (16 kb) from blood DNA revealed a heterozygous deletion of approximately 4.2kb. Fine mapping of the deletion breakpoint was achieved using purified long-range PCR products for restriction enzyme analysis and direct sequencing. The deletion removed a 4,227 bp region covering part of intron 5 to part of intron 9 [g.10,145_14,371 del 4,227]. The deletion breakpoint contained a short direct repeat (GTAG), which may be derived either from the upstream or the downstream homologous sequence. The presence of a GAG triplet and inverted repeats in the vicinity of the deletion suggest replication slippage at a polymerase alpha arrest site. This is the first time that a large intragenic deletion of the PDHA 1 gene has been characterized.

[Congenital nephrogenic diabetes insipidus]

Nephrogenic diabetes insipidus is a rare hereditary disease, characterized by a resistance of the renal collecting duct to the action of the antidiuretic hormone, arginine vasopressin, responsible for the inability of the kidney to concentrate urine. More than 90% of the patients are males and have the X-linked recessive form of the disease usually presenting with polyuria and polydipsia in infancy. This mode of inheritance is related to mutations in the V(2) receptor gene, located in the Xq28 chromosomal region. Less than 10% of the patients have an autosomal-recessive or an autosomal-dominant mode of inheritance with clinical manifestations occurring in males and females, related to mutations in the aquaporin-2 gene, located in chromosome region 12q13. The aim of the treatment is to avoid chronic and acute dehydration episodes. It remains symptomatic, mainly based on an hypoosmotic diet and the use of hydrochlorothiazide and indomethacin. Recent findings showed that pharmacological chaperones, such as V(2) nonpeptide antagonists, are able to rescue some of the V(2) receptor mutants and could be useful tools for treatment in the future.

Molecular basis of evolutionary events that shaped the hardness locus in diploid and polyploid wheat species (Triticum and Aegilops)

The Hardness (Ha) locus controls grain hardness in hexaploid wheat (Triticum aestivum) and its relatives (Triticum and Aegilops species) and represents a classical example of a trait whose variation arose from gene loss after polyploidization. In this study, we investigated the molecular basis of the evolutionary events observed at this locus by comparing corresponding sequences of diploid, tertraploid, and hexaploid wheat species (Triticum and Aegilops). Genomic rearrangements, such as transposable element insertions, genomic deletions, duplications, and inversions, were shown to constitute the major differences when the same genomes (i.e., the A, B, or D genomes) were compared between species of different ploidy levels. The comparative analysis allowed us to determine the extent and sequences of the rearranged regions as well as rearrangement breakpoints and sequence motifs at their boundaries, which suggest rearrangement by illegitimate recombination. Among these genomic rearrangements, the previously reported Pina and Pinb genes loss from the Ha locus of polyploid wheat species was caused by a large genomic deletion that probably occurred independently in the A and B genomes. Moreover, the Ha locus in the D genome of hexaploid wheat (T. aestivum) is 29 kb smaller than in the D genome of its diploid progenitor Ae. tauschii, principally because of transposable element insertions and two large deletions caused by illegitimate recombination. Our data suggest that illegitimate DNA recombination, leading to various genomic rearrangements, constitutes one of the major evolutionary mechanisms in wheat species.

Mutant G-protein-coupled receptors as a cause of human diseases

G-protein-coupled receptors (GPCR) are involved in directly and indirectly controlling an extraordinary variety of physiological functions. Their key roles in cellular communication have made them the target for more than 60% of all currently prescribed drugs. Mutations in GPCR can cause acquired and inherited diseases such as retinitis pigmentosa (RP), hypo- and hyperthyroidism, nephrogenic diabetes insipidus, several fertility disorders, and even carcinomas. To date, over 600 inactivating and almost 100 activating mutations in GPCR have been identified which are responsible for more than 30 different human diseases. The number of human disorders is expected to increase given the fact that over 160 GPCR have been targeted in mice. Herein, we summarize the current knowledge relevant to understanding the molecular basis of GPCR function, with primary emphasis on the mechanisms underlying GPCR malfunction responsible for different human diseases.

Treatment of Congenital Nephrogenic Diabetes insipidus with Hydrochlorothiazide and Amiloride in an Adult Patient

AIM

The effects of treatment with hydrochlorothiazide (HCTZ) combined with amiloride were elucidated and compared to HCTZ treatment alone and combined with acemetacin or triamterene in a Japanese adult patient with congenital nephrogenic diabetes insipidus.

METHODS

The study was divided into seven periods: (1) HCTZ and acemetacin; (2) control period; (3) HCTZ; (4) a second control period; (5) HCTZ and amiloride; (6) a third control period, and (7) HCTZ and triamterene. Fluid intake, urine volume, urinary Na, K, creatinine, and osmolality and serum Na, K, Cl, CO2, and osmolality were measured, and free water clearance and proximal and distal tubular Na reabsorption rates were calculated.

RESULTS

Without drug administration, the urine volume was about 8,000 ml/day. The urine volume was reduced to about 6,000 ml/day with HCTZ. A further urine volume reduction to about 5,000 ml/day was obtained with the second drug administration, and the effects were similar among the three regimens. Serum and urinary osmolality and free water clearance were also similar among the three combinations, whereas the urinary potassium excretion was the least, and the serum potassium concentration was the highest with HCTZ plus amiloride. Besides, no alkalosis was observed only with this combination.

CONCLUSION

HCTZ plus amiloride may be superior to HCTZ plus acemetacin and HCTZ plus triamterene in preventing hyperkaliuria, hypokalemia, and metabolic alkalosis.

Translocation and gross deletion breakpoints in human inherited disease and cancer I: Nucleotide composition and recombination-associated motifs

Translocations and gross deletions are important causes of both cancer and inherited disease. Such gene rearrangements are nonrandomly distributed in the human genome as a consequence of selection for growth advantage and/or the inherent potential of some DNA sequences to be frequently involved in breakage and recombination. Using the Gross Rearrangement Breakpoint Database [GRaBD; www.uwcm.ac.uk/uwcm/mg/grabd/grabd.html] (containing 397 germ-line and somatic DNA breakpoint junction sequences derived from 219 different rearrangements underlying human inherited disease and cancer), we have analyzed the sequence context of translocation and deletion breakpoints in a search for general characteristics that might have rendered these sequences prone to rearrangement. The oligonucleotide composition of breakpoint junctions and a set of reference sequences, matched for length and genomic location, were compared with respect to their nucleotide composition. Deletion breakpoints were found to be AT-rich whereas by comparison, translocation breakpoints were GC-rich. Alternating purine-pyrimidine sequences were found to be significantly over-represented in the vicinity of deletion breakpoints while polypyrimidine tracts were over-represented at translocation breakpoints. A number of recombination-associated motifs were found to be over-represented at translocation breakpoints (including DNA polymerase pause sites/frameshift hotspots, immunoglobulin heavy chain class switch sites, heptamer/nonamer V(D)J recombination signal sequences, translin binding sites, and the chi element) but, with the exception of the translin-binding site and immunoglobulin heavy chain class switch sites, none of these motifs were over-represented at deletion breakpoints. Alu sequences were found to span both breakpoints in seven cases of gross deletion that may thus be inferred to have arisen by homologous recombination. Our results are therefore consistent with a role for homologous unequal recombination in deletion mutagenesis and a role for nonhomologous recombination in the generation of translocations.

Completely skewed X-inactivation in a mentally retarded young female with pseudohypoparathyroidism type IB and juvenile renin-dependent hypertension

Genetic analysis of the GNAS gene was performed in a patient with idiopathic renin-dependent hypertension, PTH resistance, and Albright's hereditary osteodystrophy-like characteristics such as a round face, short stature, obesity, and mental retardation (IQ, 49). Mutational analysis showed no mutations in exons 1-13 or in any exon-intron boundary. However, methylation-status analysis revealed a bialleic methylation defect in GNAS exon 1A, indicating that a GNAS-imprinting defect is the cause of her PTH resistance, as commonly observed in pseudohypoparathyroidism type IB. The imprinting defect, however, could not explain her renin-dependent hypertension and Albright's hereditary osteodystrophy-like phenotype. There are many types of X-linked mental retardation. Syndromic X-linked mental retardation, such as X-linked alpha-thalassemia mental retardation syndrome and Rett syndrome, is reportedly associated with abnormal imprinting. To further investigate this unexplained phenotype, we tested whether this patient showed skewed X-inactivation (SXI) presumably as a result of postinactivation selection against cells with a mutated gene on the active X-chromosome. Completely SXI was observed in the DNA from her leukocytes, urinary sediment, and renal tissue. A mutation of the X-chromosome might be correlated with this phenotype because of a close association between completely SXI and X-chromosomal mutation.