Genomic structure of the human congenital chloride diarrhea (CLD) gene

Genetic Determinants of Non-Syndromic Enlarged Vestibular Aqueduct: A Review

Hearing loss is the most common sensorial deficit in humans and one of the most common birth defects. In developed countries, at least 60% of cases of hearing loss are of genetic origin and may arise from pathogenic sequence alterations in one of more than 300 genes known to be involved in the hearing function. Hearing loss of genetic origin is frequently associated with inner ear malformations; of these, the most commonly detected is the enlarged vestibular aqueduct (EVA). EVA may be associated to other cochleovestibular malformations, such as cochlear incomplete partitions, and can be found in syndromic as well as non-syndromic forms of hearing loss. Genes that have been linked to non-syndromic EVA are SLC26A4, GJB2, FOXI1, KCNJ10, and POU3F4. SLC26A4 and FOXI1 are also involved in determining syndromic forms of hearing loss with EVA, which are Pendred syndrome and distal renal tubular acidosis with deafness, respectively. In Caucasian cohorts, approximately 50% of cases of non-syndromic EVA are linked to SLC26A4 and a large fraction of patients remain undiagnosed, thus providing a strong imperative to further explore the etiology of this condition.

Step-Up Approach for Sodium Butyrate Treatment in Children With Congenital Chloride Diarrhea

OBJECTIVES

Oral salt substitutive therapy is pivotal for the survival of patients with congenital chloride diarrhea (CLD), however this therapy is unable to influence the symptoms severity. Butyrate has been proposed to limit diarrhea severity in CLD. Unfortunately, the optimal dose schedule is still largely undefined. In addition, butyrate seems not to be well-tolerated by all patients, with some subjects reporting diarrhea worsening. We investigated the efficacy of a step-up therapeutic approach with sodium butyrate in patients who experienced a diarrhea worsening or an absent improvement after the direct administration of 100 mg/kg/day of sodium butyrate.

METHODS

The efficacy of a step-up therapeutic approach starting from 50 mg/Kg/day with a subsequent 25 mg/kg/day weekly increase up to 100 mg/kg/day of oral sodium butyrate was investigated in previously three unresponsive CLD children.

RESULTS

The step-up therapeutic approach resulted effective in limiting diarrhea severity in all our three previously unresponsive CLD patients.

CONCLUSIONS

Our results suggest the efficacy of the step-up therapeutic approach in CLD children.

Congenital chloride diarrhea and Pendred syndrome: case report of siblings with two rare recessive disorders of SLC26 family genes

BACKGROUND

Congenital chloride diarrhea (CLD; OMIM 214700) is a rare autosomal recessive disorder caused by pathogenic variations in the solute carrier family 26 member A3 (SLC26A3) gene. Without salt substitution, this chronic diarrheal disorder causes severe dehydration and electrolyte disturbances. Homozygous variants in the nearby gene SLC26A4 disrupt anion exchange in the inner ear and the thyroid, causing Pendred syndrome (PDS; OMIM 274600), which is the most frequent form of syndromic deafness.

CASE PRESENTATION

We report an unusual co-occurrence of two rare homozygous mutations in both the SLC26A3 and SLC26A4 genes, causing a rare combination of both CLD and PDS in two siblings. Although the clinical pictures were typical, the combined loss of these anion transporters might modulate the risk of renal injury associated with CLD.

CONCLUSIONS

Familial presentation of two rare autosomal recessive disorders with loss of function of different SLC26 anion transporters is described. Independent homozygous variants in the SLC26A3 and SLC26A4 genes cause CLD and PDS in siblings, shedding light on co-occurrence of rare recessive traits in the progeny of consanguineous couples.

Clinical Features, Molecular Genetics, and Long-Term Outcome in Congenital Chloride Diarrhea: A Nationwide Study in Japan

OBJECTIVE

To clarify clinical and genetic features of Japanese children with congenital chloride diarrhea (CCD).

STUDY DESIGN

This was a multi-institutional, retrospective survey of 616 pediatric centers in Japan with identified patients with CCD between 2014 and 2018. Mutations involving SLC26A3 were detected by Sanger sequencing.

RESULTS

Thirteen patients met all entry criteria including mutations in SLC26A3, and 14 patients satisfied clinical diagnostic criteria. Homozygous or compound heterozygous mutations in SLC26A3, including 6 novel mutations, were identified in 13 of these 14 patients (93%). The most common (detected in 7 of 13) was c.2063-1g>t. Median age at diagnosis was 1 day. Nine of the patients meeting all criteria were diagnosed as neonates (69%). Median follow-up duration was 10 years. When studied, 8 patients had <5 stools daily (62%), and all had fewer than in infancy. Only 1 patient had nephrocalcinosis, and 3 (23%) had mild chronic kidney disease. Neurodevelopment was generally good; only 1 patient required special education. Five patients (38%) received long-term sodium, potassium, and chloride supplementation.

CONCLUSIONS

Early fetal ultrasound diagnosis and prompt long-term sodium, potassium, and chloride supplementation were common management features. Genetic analysis of SLC26A3 provided definitive diagnosis of CCD. In contrast with previously reported localities, c.2063-1g>t might be a founder mutation in East Asia.

A missense mutation in SLC26A3 is associated with human male subfertility and impaired activation of CFTR

Chloride absorption and bicarbonate excretion through exchange by the solute carrier family 26 member 3 (SLC26A3) and cystic fibrosis transmembrane conductance regulator (CFTR) are crucial for many tissues including sperm and epithelia of the male reproductive tract. Homozygous SLC26A3 mutations cause congenital chloride diarrhea with male subfertility, while homozygous CFTR mutations cause cystic fibrosis with male infertility. Some homozygous or heterozygous CFTR mutations only manifest as male infertility. Accordingly, we studied the influence of SLC26A3 on idiopathic infertility by sequencing exons of SLC26A3 in 283 infertile and 211 control men. A heterozygous mutation c.2062 G > C (p.Asp688His) appeared in nine (3.2%) infertile men, and additionally, in two (0.9%) control men, whose samples revealed a sperm motility defect. The p.Asp688His mutation is localized in the CFTR-interacting STAS domain of SLC26A3 and enriched in Finland, showing a significant association with male infertility in comparison with 6,572 Finnish (P < 0.05) and over 120,000 global alleles (P < 0.0001) (ExAC database). Functional studies showed that while SLC26A3 is a strong activator of CFTR-dependent anion transport, SLC26A3-p.Asp688His mutant retains normal Cl⁻/HCO₃⁻ exchange activity but suppresses CFTR, despite unaffected domain binding and expression. These results suggest a novel mechanism for human male infertility─impaired anion transport by the coupled SLC26A3 and CFTR.

Twelve Novel Mutations in the SLC26A3 Gene in 17 Sporadic Cases of Congenital Chloride Diarrhea

OBJECTIVES

We aimed to improve the knowledge of pathogenic mutations in sporadic cases of congenital chloride diarrhea (CCD) and emphasize the importance of functional studies to define the effect of novel mutations.

METHODS

All member 3 of solute carrier family 26 (SLC26A3) coding regions were sequenced in 17 sporadic patients with CCD. Moreover, the minigene system was used to analyze the effect of 2 novel splicing mutations.

RESULTS

We defined the SLC26A3 genotype of all 17 patients with CCD and identified 12 novel mutations. Using the minigene system, we confirmed the in silico prediction of a complete disruption of splicing pattern caused by 2 of these novel mutations: the c.971+3_971+4delAA and c.735+4_c.735+7delAGTA. Moreover, several prediction tools and a structure-function prediction defined the pathogenic role of 6 novel missense mutations.

CONCLUSIONS

We confirm the molecular heterogeneity of sporadic CCD adding 12 novel mutations to the list of known pathogenic mutations. Moreover, we underline the importance, for laboratories that offer molecular diagnosis and genetic counseling, to perform fast functional analysis of novel mutations.

Genotype-dependency of butyrate efficacy in children with congenital chloride diarrhea

Background

Congenital chloride diarrhea (CLD) is an autosomal recessive disorder characterized by life-long, severe diarrhea with intestinal Cl^- malabsorption. It results from a reduced activity of the down regulated in adenoma exchanger (DRA), due to mutations in the solute carrier family 26, member 3 (SLC26A3) gene. Currently available therapies are not able to limit the severity of diarrhea in CLD. Conflicting results have been reported on the therapeutic efficacy of oral butyrate.

Methods

We investigated the effect of oral butyrate (100 mg/kg/day) in seven CLD children with different SLC26A3 genotypes. Nasal epithelial cells were obtained to assess the effect of butyrate on the expression of the two main Cl^- transporters: DRA and putative anion transporter-1 (PAT-1).

Results

A variable clinical response to butyrate was observed regarding the stool pattern and fecal ion loss. The best response was observed in subjects with missense and deletion mutations. Variable response to butyrate was also observed on SLC26A3 (DRA) and SLC26A6 (PAT1) gene expression in nasal epithelial cells of CLD patients.

Conclusions

We demonstrate a genotype-dependency for butyrate therapeutic efficacy in CLD. The effect of butyrate is related in part on a different modulation of the expression of the two main apical membrane Cl^- exchangers of epithelial cells, members of the SLC26 anion family.

Trial registration

Australian New Zealand Clinical trial Registry ACTRN12613000450718.

Congenital chloride diarrhea in Korean children: novel mutations and genetic characteristics

Congenital chloride diarrhea (CLD, OMIM#214700) is an autosomal recessive disorder caused by mutations in the solute carrier family 26 member 3 (SLC26A3) gene, which encodes an intestinal chloride/bicarbonate exchanger. While more than 50 mutations have been identified throughout the world, there have been no data on the genetic characteristics of the patients of East Asian ethnic origin. In this study, we performed genetic analysis by direct sequencing of the 20 exons and parts of exon-intron boundaries of the SLC26A3 gene in eight patients of Korean origin with non-consanguineous parents. We identified three novel mutations, including two splice-site mutations (c.2063-1G>T in intron 18, c.1047+3 A>C in intron 12) and one missense mutation (p.Ser134Asn in exon 5). One previously identified mutation was also found (p.Pro131Leu in exon 5). The most common mutation was c.2063-1G>T, which was found in at least one allele of all patients.

CONCLUSION

This is the first report to demonstrate the genetic background of CLD in a single ethnic group of East Asian descent. The c.2063-1G>T mutation could be suggested as a founder mutation in Korean population so that the targeting sequencing for the mutation would be a cost-efficient screening method to confirm a diagnosis of CLD in patients of Korean descent.

Transcriptional regulation of the pendrin gene

Pendrin (SLC26A4), a Cl(-)/anion exchanger encoded by the gene PDS, is highly expressed in the kidney, thyroid and inner ear epithelia and is essential for bicarbonate secretion/chloride reabsorption, iodide accumulation and endolymph ion balance, respectively. The molecular mechanisms controlling pendrin activity in renal, thyroid and inner ear epithelia have been the subject of recent studies. The effects of ambient pH, the hormone aldosterone and the peptide uroguanylin (UGN; the "intestinal natriuretic hormone"), known modulators of electrolyte balance, on transcription of the pendrin gene, have been investigated. Luciferase reporter plasmids containing different length fragments of the human PDS (hPDS) promoter were transfected into renal HEK293, thyroid LA2, and inner ear VOT36 epithelial cells. Acidic pH decreased and alkaline pH increased hPDS promoter activity in transfected HEK293 and VOT36, but not in LA2 cells. Aldosterone reduced hPDS promoter activity in HEK293 but had no effect in LA2 and VOT36 cells. These pH and aldosterone-induced effects on the hPDS promoter occurred within 96-bp and 89-bp regions, respectively, which likely contain distinct response elements to these modulators. Injection of UGN into mice resulted in decreased pendrin mRNA and protein expression in the kidney. Exposure of transfected HEK293 to UGN decreased hPDS promoter activity. The findings provided evidence for the presence of a UGN response element within the 96-bp region overlapping with the pH response element on the hPDS promoter. Pendrin is also expressed in airway epithelium. The cytokins interleukin 4 (IL-4) and interleukin-13 (IL-13), known regulators of airway surface function, have been shown to increase hPDS promoter activity by a STAT6-dependent mechanism. In conclusion, systemic pH, the hormone aldosterone, and the peptide UGN influence renal tubular pendrin gene expression and, perhaps, pendrin-mediated Cl(-)/HCO(3)(-) exchange at the transcriptional level. Pendrin-driven anion transport in the endolymph and at the airway surface may be regulated transcriptionally by systemic pH and IL-3/IL-4, respectively. The distinct response elements and the corresponding transcription factors mediating the effect of these modulators on the PDS promoter remain to be identified and characterized.

Molecular cloning and promoter analysis of downregulated in adenoma (DRA)

Downregulated in adenoma (DRA), also referred to as SLC26A3, is an intestinal anion transporter essential for intestinal chloride absorption. Mutations in DRA result in congenital chloride diarrhea. DRA expression has been shown to be induced by differentiation and to be modulated by cytokines. However, mechanisms of DRA gene transcription and its tissue-specific targeting have not yet been investigated. In this study, we cloned a 3,765-bp promoter fragment of human DRA gene and characterized its activity in human colonic LS174T and Caco-2 human colon cell lines. Primer extension identified a single transcriptional initiation site that was identical in both colon cancer cell lines and normal colon. Although hepatic nuclear factor HNF-4 is involved in the basal activity of DRA promoter, sodium butyrate induces its activity in LS174T cells via the binding of Yin Yang 1 (YY1) and GATA transcription factors to their respective cis-elements in promoter region. We also demonstrated a reduction in DRA promoter activity in Caco-2 cells by IFN-gamma, suggesting that regulation of DRA promoter by IFN-gamma may contribute to the pathophysiology of intestinal inflammation. Furthermore, we showed that the DRA promoter fragment is sufficient to drive human growth hormone transgene expression specifically in villus epithelial cells of the small intestine and in differentiated upper crypt and surface epithelial cells of the colon. Our studies provide evidence for the involvement of HNF-4, YY1, and GATA transcription factors in DRA expression in intestinal differentiated epithelial cells.

Characterization of the human sulfate anion transporter (hsat-1) protein and gene (SAT1; SLC26A1)

Sulfate plays an essential role during growth, development, bone/cartilage formation, and cellular metabolism. In this study, we have isolated the human sulfate anion transporter cDNA (hsat-1; SCL26A1) and gene (SAT1), determined its protein function in Xenopus oocytes and characterized SAT1 promoter activity in mammalian renal cell lines. hsat-1 encodes a protein of 75 kDa, with 12 putative transmembrane domains, that induces sulfate, chloride, and oxalate transport in Xenopus oocytes. hsat-1 mRNA is expressed most abundantly in the kidney and liver, with lower levels in the pancreas, testis, brain, small intestine, colon, and lung. The SAT1 gene is comprised of four exons stretching 6 kb in length, with an alternative splice site formed from an optional exon. SAT1 5' flanking region led to promoter activity in renal OK and LLC-PK1 cells. Using SAT1 5' flanking region truncations, the first 135 bp was shown to be sufficient for basal promoter activity. Mutation of the activator protein-1 (AP-1) site at position -52 in the SAT1 promoter led to loss of transcriptional activity, suggesting its requirement for SAT1 basal expression. This study represents the first functional characterization of the human SAT1 gene and protein encoded by the anion transporter hsat-1.

The mouse sulfate anion transporter gene Sat1 (Slc26a1): cloning, tissue distribution, gene structure, functional characterization, and transcriptional regulation thyroid hormone

Sulfate (SO(4)(2-)) is required for bone/cartilage formation and cellular metabolism. sat-1 is a SO(4)(2-) anion transporter expressed on basolateral membranes of renal proximal tubules, and is suggested to play an important role in maintaining SO(4)(2-) homeostasis. As a first step towards studying its tissue-specific expression, hormonal regulation, and in preparation for the generation of knockout mice, we have cloned and characterized the mouse sat-1 cDNA (msat-1), gene (sat1; Slc26a1) and promoter region. msat-1 encodes a 704 amino acid protein (75.4 kDa) with 12 putative transmembrane domains that induce SO(4)(2-) (also oxalate and chloride) transport in Xenopus oocytes. msat-1 mRNA was expressed in kidney, liver, cecum, calvaria, brain, heart, and skeletal muscle. Two distinct transcripts were expressed in kidney and liver due to alternative utilization of the first intron, corresponding to an internal portion of the 5'-untranslated region. The Sat1 gene (~6 kb) consists of 4 exons. Its promoter is ~52% G + C rich and contains a number of well-characterized cis-acting elements, including sequences resembling hormone responsive elements T(3)REs and VDREs. We demonstrate that Sat1 promoter driven basal transcription in OK cells was stimulated by tri-iodothyronine. Site-directed mutagenesis identified an imperfect T(3)RE at -454-bp in the Sat1 promoter to be responsible for this activity. This study represents the first characterization of the structure and regulation of the Sat1 gene encoding a SO(4)(2-)/chloride/oxalate anion transporter.

SLC26A3 mutations in congenital chloride diarrhea

Congenital chloride diarrhea (CLD) is an autosomal recessive disorder of intestinal electrolyte absorption. It is characterized by persistent secretory diarrhea resulting in polyhydramnios and prematurity prenatally, and dehydration, hypoelectrolytemia, hyperbilirubinemia, abdominal distention, and failure to thrive immediately after birth. CLD is caused by mutations in the solute carrier family 26, member 3 gene (SLC26A3, alias CLD or DRA), which encodes a Na+-independent Cl-/HCO3- (or OH-) exchanger. SLC26A3 is a member of the SLC26 sulfate permease/anion transporter family and it is expressed mainly in the apical brush border of intestinal epithelium. The only extraintestinal tissues showing SLC26A3 expression are eccrine sweat glands and seminal vesicles. A wide variety of different mutations in the SLC26A3 gene have been associated with CLD with no apparent evidence of phenotype-genotype correlation. The clinical course of CLD, however, is variable and may rather depend on environmental factors and compensatory mechanisms than mutations. In this report, we present a summary of all published and two novel SLC26A3 mutations and polymorphisms, and review them in the context of their functional consequences and clinical implications.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Identification of seven novel mutations including the first two genomic rearrangements in SLC26A3 mutated in congenital chloride diarrhea

Congenital chloride diarrhea (CLD) is an autosomal recessive disorder characterized by defective intestinal electrolyte absorption, resulting in voluminous osmotic diarrhea with high chloride content. A variety of mutations in the solute carrier family 26, member 3 gene (SLC26A3, previously known as CLD or DRA) are responsible for the disease. Since the identification of the SLC26A3 gene and the determination of its genomic structure, altogether three founder and 17 private mutations have been characterized within miscellaneous ethnic groups. We screened for mutations in seven unrelated families with CLD. The diagnoses were confirmed by fecal chloride measurements. The combined PCR-SSCP and sequencing analyses revealed altogether seven novel mutations including two missense mutations (S206P, D468V), two splicing defects (IVS12-1G>C, IVS13-2delA), one nonsense mutation (Q436X), one insertion/deletion mutation (2104-2105delGGins29-bp), and an intragenic deletion of SLC26A3 exons 7 and 8. Two previously identified mutations were also found. This is the first report of rearrangement mutations in SLC26A3. Molecular features predisposing SLC26A3 for the two rearrangements may include repetitive elements and palindromic-like sequences. The increasingly wide diversity of SLC26A3 mutations suggests that mutations in the SLC26A3 gene may not be rare events.

Distinct outcomes of chloride diarrhoea in two siblings with identical genetic background of the disease: implications for early diagnosis and treatment

BACKGROUND

Congenital chloride diarrhoea (CLD, OMIM 214700) is a serious inherited defect of intestinal electrolyte absorption transmitted in an autosomal recessive fashion. The major clinical manifestation is diarrhoea with high chloride content which can be balanced by substitution. The molecular pathology involves an epithelial Cl(-)/HCO(3)(-) exchanger protein, encoded by the solute carrier family 26, member 3 gene (SLC26A3), previously known as CLD or DRA (downregulated in adenomas). To date, almost 30 different mutations in the SLC26A3 gene have been identified throughout the world. No clear genotype-phenotype correlation has been established.

PATIENTS/METHODS

Two siblings presenting with CLD were studied for disease history, supplementation, or other treatments, and for mutations in the SLC26A3 gene.

RESULTS

Mutation analysis revealed a homozygous I544N mutation in both patients. However, despite the uniform genetic background of CLD in this family, the clinical picture and outcome of the disease were remarkably different between siblings. The older sibling had a late diagnosis and chronic course of the disease whereas the younger one, who was diagnosed soon after birth and immediately received supplementation therapy, grows and develops normally.

CONCLUSION

Time of diagnosis, substitution therapy, compliance, and compensatory mechanisms are more important modulators of the clinical picture of CLD than the type of mutation in the SLC26A3 gene.

The human renal sodium sulfate cotransporter (SLC13A1; hNaSi-1) cDNA and gene: organization, chromosomal localization, and functional characterization

Sulfate plays an essential role during growth, development, bone/cartilage formation, and cellular metabolism. In this study, we have determined the structure of the human Na+-sulfate cotransporter (hNaSi-1) cDNA (Human Genome Nomenclature Committee-approved symbol SLC13A1) and gene (NAS1). hNaSi-1 encodes a protein of 595 amino acids with 13 putative transmembrane domains. hNaSi-1 mRNA expression was exclusive to the human kidney. Expression of hNaSi-1 protein in Xenopus oocytes demonstrated a high-affinity Na+-sulfate cotransporter that was inhibited by selenate, thiosulfate, molybdate, tungstate, citrate, and succinate. Antisense inhibition experiments suggest hNaSi-1 to represent the major Na+-sulfate cotransporter in the human kidney. NAS1 was localized on human chromosome 7, mapped to 7q31-q32, near the sulfate transporter genes, DRA and SUT-1. The NAS1 gene contains 15 exons, spanning over 83 kb in length. Knowledge of the structure, function, and chromosomal localization of hNaSi-1 will permit the screening of NAS1 mutations in humans with disorders in renal sulfate reabsorption and homeostasis.

The mouse Na(+)-sulfate cotransporter gene Nas1. Cloning, tissue distribution, gene structure, chromosomal assignment, and transcriptional regulation by vitamin D

NaSi-1 is a Na(+)-sulfate cotransporter expressed on the apical membrane of the renal proximal tubule and plays an important role in sulfate reabsorption. To understand the molecular mechanisms that mediate the regulation of NaSi-1, we have isolated and characterized the mouse NaSi-1 cDNA (mNaSi-1), gene (Nas1), and promoter region and determined Nas1 chromosomal localization. The mNaSi-1 cDNA encodes a protein of 594 amino acids with 13 putative transmembrane segments, inducing high affinity Na(+)-dependent transport of sulfate in Xenopus oocytes. Three different mNaSi-1 transcripts derived from alternative polyadenylation and splicing were identified in kidney and intestine. The Nas1 gene is a single copy gene comprising 15 exons spread over 75 kilobase pairs that maps to mouse chromosome 6. Transcription initiation occurs from a single site, 29 base pairs downstream to a TATA box-like sequence. The promoter is AT-rich (61%), contains a number of well characterized cis-acting elements, and can drive basal transcriptional activity in opossum kidney cells but not in COS-1 or NIH3T3 cells. We demonstrated that 1,25-dihydroxyvitamin D(3) stimulated the transcriptional activity of the Nas1 promoter in transiently transfected opossum kidney cells. This study represents the first characterization of the genomic organization of a Na(+)-sulfate cotransporter gene. It also provides the basis for a detailed analysis of Nas1 gene regulation and the tools required for assessing Nas1 role in sulfate homeostasis using targeted gene manipulation in mice.

Genetic Disorders of Membrane Transport III. Congenital chloride diarrhea

Congenital chloride diarrhea (CLD) is a recessively inherited disorder of intestinal electrolyte absorption that involves, specifically, Cl-/HCO-3 exchange. CLD is caused by mutations in a chromosome 7 gene, first known as DRA (for downregulated in adenoma). The disease occurs in all parts of the world but is more common in some populations with genetic founder effects. More than 20 mutations in the gene are known to date. The CLD (or DRA) gene encodes a transmembrane protein belonging to the sulfate transporter family with three known members in humans, all associated with a distinct genetic disease. Members of the gene family can transport other anions as well that may turn out to be physiologically more important than sulfate transport. The gene family is well conserved in many prokaryotic and eukaryotic species and is expected to be much larger than presently known.

Genetic background of congenital chloride diarrhea in high-incidence populations: Finland, Poland, and Saudi Arabia and Kuwait

Congenital chloride diarrhea (CLD) is an inherited intestinal disorder caused by mutations in the down-regulated in adenoma gene. In Finland, the disease is prevalent because of a founder effect, and all but one of the CLD-associated chromosomes carry the same mutation, V317del. In Poland, another area with a high incidence of CLD, as many as seven different mutations have been detected so far. A third known cluster of CLD, around the Persian Gulf, has not been genetically studied. We studied the allelic diversity of CLD in Poland, in Saudi Arabia and Kuwait, and in three isolated families from North America and Hong Kong. Altogether, eight novel mutations were identified, making a total of 19 known CLD gene mutations. The Polish major mutation I675-676ins was found in 47% of the Polish CLD-associated chromosomes. Haplotype analysis and clustering of the I675-676ins mutation supported a founder effect and common ancestral origin. As in Finland, a major founder effect was observed in Arab patients: 94% of the CLD-associated chromosomes carried a nonsense mutation, G187X, which occurred in either a conserved ancestral haplotype or its derivative. Our data confirm that the same locus is mutated in all cases of CLD studied so far. In Poland, a relatively common founder mutation is likely to highlight a set of rare mutations that would very rarely produce homozygosity alone. This suggests that mutations in the CLD locus are not rare events. Although the disease is thought to be rare, undiagnosed patients may not be uncommon.