Suggestive evidence for a susceptibility gene near the vitamin D receptor locus in idiopathic calcium stone formation

Monogenic features of urolithiasis: A comprehensive review

Objective

Urolithiasis formation has been attributed to environmental and dietary factors. However, evidence is accumulating that genetic background can contribute to urolithiasis formation. Advancements in the identification of monogenic causes using high-throughput sequencing technologies have shown that urolithiasis has a strong heritable component.

Methods

This review describes monogenic factors implicated in a genetic predisposition to urolithiasis. Peer-reviewed journals were evaluated by a PubMed search until July 2023 to summarize disorders associated with monogenic traits, and discuss clinical implications of identification of patients genetically susceptible to urolithiasis formation.

Results

Given that more than 80% of urolithiases cases are associated with calcium accumulation, studies have focused mainly on monogenetic contributors to hypercalciuric urolithiases, leading to the identification of receptors, channels, and transporters involved in the regulation of calcium renal tubular reabsorption. Nevertheless, available candidate genes and linkage methods have a low resolution for evaluation of the effects of genetic components versus those of environmental, dietary, and hormonal factors, and genotypes remain undetermined in the majority of urolithiasis formers.

Conclusion

The pathophysiology underlying urolithiasis formation is complex and multifactorial, but evidence strongly suggests the existence of numerous monogenic causes of urolithiasis in humans.

Review of childhood genetic nephrolithiasis and nephrocalcinosis

Nephrolithiasis (NL) is a common condition worldwide. The incidence of NL and nephrocalcinosis (NC) has been increasing, along with their associated morbidity and economic burden. The etiology of NL and NC is multifactorial and includes both environmental components and genetic components, with multiple studies showing high heritability. Causative gene variants have been detected in up to 32% of children with NL and NC. Children with NL and NC are genotypically heterogenous, but often phenotypically relatively homogenous, and there are subsequently little data on the predictors of genetic childhood NL and NC. Most genetic diseases associated with NL and NC are secondary to hypercalciuria, including those secondary to hypercalcemia, renal phosphate wasting, renal magnesium wasting, distal renal tubular acidosis (RTA), proximal tubulopathies, mixed or variable tubulopathies, Bartter syndrome, hyperaldosteronism and pseudohyperaldosteronism, and hyperparathyroidism and hypoparathyroidism. The remaining minority of genetic diseases associated with NL and NC are secondary to hyperoxaluria, cystinuria, hyperuricosuria, xanthinuria, other metabolic disorders, and multifactorial etiologies. Genome-wide association studies (GWAS) in adults have identified multiple polygenic traits associated with NL and NC, often involving genes that are involved in calcium, phosphorus, magnesium, and vitamin D homeostasis. Compared to adults, there is a relative paucity of studies in children with NL and NC. This review aims to focus on the genetic component of NL and NC in children.

Molecular mechanisms altering tubular calcium reabsorption

The majority of calcium filtered by the glomerulus is reabsorbed along the nephron. Most is reabsorbed from the proximal tubule (> 60%) via a paracellular pathway composed of the tight junction proteins claudins-2 and -12, a process driven by sodium and consequently water reabsorption. The thick ascending limb reabsorbs the next greatest amount of calcium (20-25%), also by a paracellular pathway composed of claudins-16 and -19. This pathway is regulated by the CaSR, whose activity increases the expression of claudin-14, a protein that blocks paracellular calcium reabsorption. The fine tuning of urinary calcium excretion occurs in the distal convoluted and connecting tubule by a transcellular pathway composed of the apical calcium channel TRPV5, the calcium shuttling protein calbindin-D_28K and the basolateral proteins PMCA1b and the sodium calcium exchanger, NCX. Not surprisingly, mutations in a subset of these genes cause monogenic disorders with hypercalciuria as a part of the phenotype. More commonly, "idiopathic" hypercalciuria is encountered clinically with genetic variations in CLDN14, the CASR and TRPV5 associating with kidney stones and increased urinary calcium excretion. An understanding of the molecular pathways conferring kidney tubular calcium reabsorption is employed in this review to help explain how dietary and medical interventions for this disorder lower urinary calcium excretion.

Mechanisms Underlying Calcium Nephrolithiasis

Nephrolithiasis is a worldwide problem with increasing prevalence, enormous costs, and significant morbidity. Calcium-containing kidney stones are by far the most common kidney stones encountered in clinical practice. Consequently, hypercalciuria is the greatest risk factor for kidney stone formation. Hypercalciuria can result from enhanced intestinal absorption, increased bone resorption, or altered renal tubular transport. Kidney stone formation is complex and driven by high concentrations of calcium-oxalate or calcium-phosphate in the urine. After discussing the mechanism mediating renal calcium salt precipitation, we review recent discoveries in renal tubular calcium transport from the proximal tubule, thick ascending limb, and distal convolution. Furthermore, we address how calcium is absorbed from the intestine and mobilized from bone. The effect of acidosis on bone calcium resorption and urinary calcium excretion is also considered. Although recent discoveries provide insight into these processes, much remains to be understood in order to provide improved therapies for hypercalciuria and prevent kidney stone formation. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Genetics of kidney stone disease-Polygenic meets monogenic

Kidney stone disease comprising nephrolithiasis and nephrocalcinosis is a clinical syndrome of increasing prevalence with remarkable heterogeneity. Stone composition, age of manifestation, rate of recurrence, and impairment of kidney function varies with underlying etiologies. While calcium-based kidney stones account for the vast majority their etiology is still poorly understood. Recent studies underline the notion that genetic susceptibility together with dietary habits constitutes the major driver of kidney stone formation. In addition to single gene (Mendelian) disorders, which are most likely underestimated in the adult population, common risk alleles explain part of the observed heritability. Interestingly, identified GWAS loci often match those of Mendelian disease genes and vice versa (CASR, SLC34A1, CYP24A1). These findings provide mechanistic links related to renal calcium homeostasis, vitamin D metabolism, and CaSR-signaling regulated by the CaSR-CLDN14-CLDN16/19 axis (paracellular Ca²⁺ reabsorption) and TRPV5 (transcellular Ca²⁺ reabsorption). Recent identification of new single gene disorders of calcium-oxalate-nephrolithiasis (SLC26A1, CLDN2) and distal renal tubular acidosis with nephrocalcinosis (FOXI1, WDR72, ATP6V1C2) enabled additional insights into the kidney-gut axis and molecular prerequisites of proper urinary acidification. Implementation of centralized patient registries on hereditary kidney stone diseases are necessary to build up well characterized cohorts for urgently needed clinical studies.

A polymorphism in the 3'-untranslated region of the matrix metallopeptidase 9 gene is associated with susceptibility to idiopathic calcium nephrolithiasis in the Chinese population

Objective

To investigate whether single nucleotide polymorphisms (SNPs) in the 3′ untranslated region (UTR) of the matrix metallopeptidase 9 gene (MMP9) are associated with susceptibility to calcium oxalate stones.

Methods

A total of 428 patients with kidney stone disease (KSD) and 450 control individuals were enrolled. Three MMP9 SNPs (rs20544, rs9509, and rs1056628) were genotyped, and MMP9 mRNA and protein expression was determined in patients and controls. The dual luciferase reporter gene assay was conducted by transfecting HEK293 cells with miR-491-5p mimics and plasmids containing MMP9 with rs1056628 AA/CC genotypes.

Results

The rs1056628 CC genotype was significantly increased in KSD patients compared with controls (CC vs AA: odds ratio [OR] = 2.279, 95% confidence interval [CI] = 1.048–4.956). The rs1056628 C allele frequency was higher in KSD patients than controls. The increased KSD risks associated with rs1056628 were more evident in individuals aged <30 years (OR = 3.504, 95% CI = 1.102–11.139) and men (OR = 2.522, 95% CI = 1.004–6.334). mRNA and protein levels of MMP9 were significantly higher in KSD patients with the CC genotype than in those with the AA genotype.

Conclusion

This study demonstrates that MMP9 SNP rs1056628 is associated with a significant KSD risk in Chinese Han individuals.

Genetics of kidney stone disease

Kidney stone disease (nephrolithiasis) is a common problem that can be associated with alterations in urinary solute composition including hypercalciuria. Studies suggest that the prevalence of monogenic kidney stone disorders, including renal tubular acidosis with deafness, Bartter syndrome, primary hyperoxaluria and cystinuria, in patients attending kidney stone clinics is ∼15%. However, for the majority of individuals, nephrolithiasis has a multifactorial aetiology involving genetic and environmental factors. Nonetheless, the genetic influence on stone formation in these idiopathic stone formers remains considerable and twin studies estimate a heritability of >45% for nephrolithiasis and >50% for hypercalciuria. The contribution of polygenic influences from multiple loci have been investigated by genome-wide association and candidate gene studies, which indicate that a number of genes and molecular pathways contribute to the risk of stone formation. Genetic approaches, studying both monogenic and polygenic factors in nephrolithiasis, have revealed that the following have important roles in the aetiology of kidney stones: transporters and channels; ions, protons and amino acids; the calcium-sensing receptor (a G protein-coupled receptor) signalling pathway; and the metabolic pathways for vitamin D, oxalate, cysteine, purines and uric acid. These advances, which have increased our understanding of the pathogenesis of nephrolithiasis, will hopefully facilitate the future development of targeted therapies for precision medicine approaches in patients with nephrolithiasis.

Association of vitamin D receptor gene polymorphisms and risk of urolithiasis: results of a genetic epidemiology study and comprehensive meta-analysis

Polymorphisms of vitamin D receptor (VDR) gene have been associated with risk of urolithiasis, but, with inconsistent results and lack data from Pakistani population. Therefore, after including our indigenous study data, a comprehensive meta-analysis was performed to provide an evidence-based estimate of any association between VDR polymorphisms and urolithiasis risk. A total of 483 Pakistani subjects, comprising 235 urolithiasis patients and 248 healthy controls, were genotyped for 6 VDR polymorphisms. Additionally, a systematic literature search with subsequent meta-analysis was conducted and pooled odds ratios (ORs) were used to determine the strength of any existent associations. Trial sequential analysis (TSA) was also performed. Results revealed no significant association of any VDR polymorphism and urolithiasis risk in indigenous Pakistani patients. However, meta-analysis of 29 relevant studies indicated that VDR FokI polymorphism significantly increased the risk of urolithiasis in allelic (f vs. F: OR = 1.13; 95% CI = 1.05-1.22; p ≤ 0.01) and recessive (ff vs. FF + Ff: OR = 1.20; 95% CI = 1.05-1.38; p = 0.01) models with no significant heterogeneity. No associations were evident for VDR ApaI, BsmI and TaqI polymorphic variants and urolithiasis risk after correction for multiple testing. Subgroup analysis by ethnicity suggested significant association for FokI variant among Asians. The TSA results demonstrated that the evidence reflecting association of FokI polymorphism and urolithiasis risk was sufficient and conclusive. In conclusion, this meta-analysis suggests that VDR FokI polymorphism is significantly associated with urolithiasis risk, especially in Asians, whereas ApaI, BsmI and TaqI polymorphisms are not associated.

Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR): clinical heterogeneity and long-term efficacious management of eight patients from four unrelated Arab families with a loss of function VDR mutation

BACKGROUND

Vitamin D regulates the concentrations of calcium and phosphate in blood and promotes the growth and remodeling of bones. The circulating active form of vitamin D, 1,25-dihydroxyvitamin D, binds to the vitamin D receptor (VDR), which heterodimerizes with the retinoid X receptor to regulate the expression of target genes. Inactivating mutations in the VDR gene cause hereditary vitamin D-resistant rickets (HVDRR), a rare disorder characterized by an early onset of rickets, growth retardation, skeletal deformities, hypocalcemia, hypophosphatemia and secondary hyperparathyroidism, and in some cases alopecia.

METHODS

We describe eight new HVDRR patients from four unrelated consanguineous families. The VDR gene was sequenced to identify mutations. The management of patients over a period of up to 11 years following the initial diagnosis is assessed.

RESULTS

Although all patients exhibit main features of HVDRR and carry the same c.885C>A (p.Y295*) loss of function mutation in the VDR gene, there was heterogeneity of the manifestations of HVDRR-associated phenotypes and developmental milestones. These eight patients were successfully treated over a period of 11 years. All clinical symptoms were improved except alopecia.

CONCLUSIONS

The study concludes that VDR sequencing and laboratory tests are essential to confirm HVDRR and to assess the effectiveness of the treatment.

Hereditary hypophosphatemic rickets with hypercalciuria: pathophysiology, clinical presentation, diagnosis and therapy

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH; OMIM: 241530) is a rare autosomal recessive disorder with an estimated prevalence of 1:250,000 that was originally described by Tieder et al. Individuals with HHRH carry compound-heterozygous or homozygous (comp/hom) loss-of-function mutations in the sodium-phosphate co-transporter NPT2c. These mutations result in the development of urinary phosphate (Pi) wasting and hypophosphatemic rickets, bowing, and short stature, as well as appropriately elevated 1,25(OH)₂D levels, which sets this fibroblast growth factor 23 (FGF23)-independent disorder apart from the more common X-linked hypophosphatemia. The elevated 1,25(OH)₂D levels in turn result in hypercalciuria due to enhanced intestinal calcium absorption and reduced parathyroid hormone (PTH)-dependent calcium-reabsorption in the distal renal tubules, leading to the development of kidney stones and/or nephrocalcinosis in approximately half of the individuals with HHRH. Even heterozygous NPT2c mutations are frequently associated with isolated hypercalciuria (IH), which increases the risk of kidney stones or nephrocalcinosis threefold in affected individuals compared with the general population. Bone disease is generally absent in individuals with IH, in contrast to those with HHRH. Treatment of HHRH and IH consists of monotherapy with oral Pi supplements, while active vitamin D analogs are contraindicated, mainly because the endogenous 1,25(OH)₂D levels are already elevated but also to prevent further worsening of the hypercalciuria. Long-term studies to determine whether oral Pi supplementation alone is sufficient to prevent renal calcifications and bone loss, however, are lacking. It is also unknown how therapy should be monitored, whether secondary hyperparathyroidism can develop, and whether Pi requirements decrease with age, as observed in some FGF23-dependent hypophosphatemic disorders, or whether this can lead to osteoporosis.

Polymorphisms of the VDR gene in patients with nephrolithiasis in a Han Chinese population

An association study between VDR gene polymorphisms and nephrolithiasis was conducted in different populations, but it is not yet known whether the association exists in the Han Chinese population. Here, we genotyped three SNPs (rs731236, rs7975232 and rs10735810) in the promoter region of the VDR gene by iMLDR genotyping assays in a large case-control cohort. The results demonstrated that there was no association found between the three SNPs (rs731236, rs7975232 and rs10735810) in the VDR gene and nephrolithiasis, whether in allele or genotype distribution. However, SNP rs10735810 was correlated with the level of serum calcium in control groups, but not in patient groups. In conclusion, considering the large sample size, we believe that the SNP rs10735810 allele A in the VDR gene promoter region may influence the level of serum calcium, but not influence the formation of nephrolithiasis in a Han Chinese population.

Vitamin D, Hypercalciuria and Kidney Stones

The estimated lifetime risk of nephrolithiasis is growing nowadays, and the formation of kidney stones is frequently promoted by hypercalciuria. Vitamin D, and especially its active metabolite calcitriol, increase digestive calcium absorption—as urinary calcium excretion is directly correlated with digestive calcium absorption, vitamin D metabolites could theoretically increase calciuria and promote urinary stone formation. Nevertheless, there was, until recently, low evidence that 25-hydroxyvitamin D serum levels would be correlated with kidney stone formation, even if high calcitriol concentrations are frequently observed in hypercalciuric stone formers. Low 25-hydroxyvitamin D serum levels have been associated with a broad spectrum of diseases, leading to a huge increase in vitamin D prescription in the general population. In parallel, an increased frequency of kidney stone episodes has been observed in prospective studies evaluating vitamin D alone or in association with calcium supplements, and epidemiological studies have identified an association between high 25-hydroxyvitamin D serum levels and kidney stone formation in some groups of patients. Moreover, urinary calcium excretion has been shown to increase in response to vitamin D supplements, at least in some groups of kidney stone formers. It seems likely that predisposed individuals may develop hypercalciuria and kidney stones in response to vitamin D supplements.

Update on Hereditary Kidney Stone Disease and Introduction of a New Clinical Patient Registry in Germany

Kidney stone disease is an increasingly prevalent condition with remarkable clinical heterogeneity, with regards to stone composition, age of manifestation, rate of recurrence, and impairment of kidney function. Calcium-based kidney stones account for the vast majority of cases, but their etiology is poorly understood, notably their genetic drivers. As recent studies indicate, hereditary conditions are most likely underestimated in prevalence, and new disease genes are constantly being identified. As a consequence, there is an urgent need of a more efficient documentation and collection of cases with underlying hereditary conditions, to better understand shared phenotypic presentation and common molecular mechanisms. By implementation of a centralized patient registry on hereditary kidney stone disease in Germany, we aim to help closing the vast knowledge gap on genetics of kidney stone disease. In this context, clinical registries are indispensable for several reasons: first, delineating better phenotype-genotype associations will allow more precise patient stratification in future clinical research studies. Second, identifying new disease genes and new mechanisms will further reduce the rate of unknown nephrolithiasis/nephrocalcinosis etiology; and third, deciphering new molecular targets will pave the way to develop drugs for recurrence prevention in severely affected families.

ApaL1 urokinase and Taq1 vitamin D receptor gene polymorphisms in first-stone formers, recurrent stone formers, and controls in a Caucasian population

The purpose of this study was to determine differences in genotype distribution and allele frequency of urokinase and vitamin D receptor (VDR) single nucleotide polymorphisms (SNPs) between first-stone formers, recurrent stone formers, and controls in a Caucasian population. A total of 86 first-stone formers, 78 recurrent stone formers, and 167 controls were included. Urokinase and VDR SNPs were tested by gene amplification followed by ApaL1 and Taq1 endonuclease digestion, respectively. Baseline variables, genotype, and allele frequencies were compared between the three groups, using descriptive statistics. Adjusted odds ratios were calculated to estimate the risk for recurrent urolithiasis associated with genotypes. We found that differences in the distribution of ApaL1 SNP and Taq1 SNP genotypes were statistically different between recurrent stone formers and first-stone formers, and between recurrent stone formers and controls. Allele frequency analysis showed that the T allele for ApaL1 SNP and the C allele for Taq1 SNP were significantly associated with recurrent urolithiasis. For Taq1 SNP, logistic regression analysis showed that the C/C genotype was associated with a more than threefold higher risk for recurrent urolithiasis. We conclude that ApaL1 and Taq1 SNPs of the urokinase and VDR genes are associated with recurrent urolithiasis in a Caucasian population.

Prevalence of Monogenic Causes in Pediatric Patients with Nephrolithiasis or Nephrocalcinosis

BACKGROUND AND OBJECTIVES

Nephrolithiasis is a prevalent condition that affects 10%-15% of adults in their lifetime. It is associated with high morbidity due to colicky pain, the necessity for surgical intervention, and sometimes progression to CKD. In recent years, multiple monogenic causes of nephrolithiasis and nephrocalcinosis have been identified. However, the prevalence of each monogenic gene in a pediatric renal stone cohort has not yet been extensively studied.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

To determine the percentage of cases that can be explained molecularly by mutations in one of 30 known nephrolithiasis/nephrocalcinosis genes, we conducted a high-throughput exon sequencing analysis in an international cohort of 143 individuals <18 years of age, with nephrolithiasis (n=123) or isolated nephrocalcinosis (n=20). Over 7 months, all eligible individuals at three renal stone clinics in the United States and Europe were approached for study participation.

RESULTS

We detected likely causative mutations in 14 of 30 analyzed genes, leading to a molecular diagnosis in 16.8% (24 of 143) of affected individuals; 12 of the 27 detected mutations were not previously described as disease causing (44.4%). We observed that in our cohort all individuals with infantile manifestation of nephrolithiasis or nephrocalcinosis had causative mutations in recessive rather than dominant monogenic genes. In individuals who manifested later in life, causative mutations in dominant genes were more frequent.

CONCLUSIONS

We present the first exclusively pediatric cohort examined for monogenic causes of nephrolithiasis/nephrocalcinosis, and suggest that important therapeutic and preventative measures may result from mutational analysis in individuals with early manifestation of nephrolithiasis or nephrocalcinosis.

Exploring the genetic basis of early-onset chronic kidney disease

The primary causes of chronic kidney disease (CKD) in children differ from those of CKD in adults. In the USA the most common diagnostic groups of renal disease that manifest before the age of 25 years are congenital anomalies of the kidneys and urinary tract, steroid-resistant nephrotic syndrome, chronic glomerulonephritis and renal cystic ciliopathies, which together encompass >70% of early-onset CKD diagnoses. Findings from the past decade suggest that early-onset CKD is caused by mutations in any one of over 200 different monogenic genes. Developments in high-throughput sequencing in the past few years has rendered identification of causative mutations in this high number of genes feasible. Use of genetic analyses in patients with early onset-CKD will provide patients and their families with a molecular genetic diagnosis, generate new insights into disease mechanisms, facilitate aetiology-based classifications of patient cohorts for clinical studies, and might have consequences for personalized approaches to the prevention and treatment of CKD. In this Review, we discuss the implications of next-generation sequencing in clinical genetic diagnostics and the discovery of novel genes in early-onset CKD. We also delineate the resulting opportunities for deciphering disease mechanisms and the therapeutic implications of these findings.

Altered Calcium and Vitamin D Homeostasis in First-Time Calcium Kidney Stone-Formers

Background

Elevated serum 1,25-dihydroxyvitamin D (1,25(OH)₂D) concentrations have been reported among cohorts of recurrent calcium (Ca) kidney stone-formers and implicated in the pathogenesis of hypercalciuria. Variations in Ca and vitamin D metabolism, and excretion of urinary solutes among first-time male and female Ca stone-formers in the community, however, have not been defined.

Methods

In a 4-year community-based study we measured serum Ca, phosphorus (P), 25-hydroxyvitamin D (25(OH)D), 1,25(OH)₂D, 24,25-dihydroxyvitamin D (24,25(OH)₂D), parathyroid hormone (PTH), and fibroblast growth factor-23 (FGF-23) concentrations in first-time Ca stone-formers and age- and gender frequency-matched controls.

Results

Serum Ca and 1,25(OH)₂D were increased in Ca stone-formers compared to controls (P = 0.01 and P = 0.001). Stone-formers had a lower serum 24,25(OH)₂D/25(OH)D ratio compared to controls (P = 0.008). Serum PTH and FGF-23 concentrations were similar in the groups. Urine Ca excretion was similar in the two groups (P = 0.82). In controls, positive associations between serum 25(OH)D and 24,25(OH)₂D, FGF-23 and fractional phosphate excretion, and negative associations between serum Ca and PTH, and FGF-23 and 1,25(OH)₂D were observed. In SF associations between FGF-23 and fractional phosphate excretion, and FGF-23 and 1,25(OH)₂D, were not observed. 1,25(OH)₂D concentrations associated more weakly with FGF-23 in SF compared with C (P <0.05).

Conclusions

Quantitative differences in serum Ca and 1,25(OH)₂D and reductions in 24-hydroxylation of vitamin D metabolites are present in first-time SF and might contribute to first-time stone risk.

Role of 1,25-dihydroxy vitamin D3 and parathyroid hormone in urinary calcium excretion in calcium stone formers

PURPOSE

To find out the possible role of 1,25(OH)₂ vitamin D₃ [1,25(OH)₂D₃] and parathyroid hormone (PTH) as intrinsic factors in urinary calcium stone formers (SFs), we investigated their relationship with serum and urinary biochemical parameters.

MATERIALS AND METHODS

A total of 326 calcium SFs (male: 204, female: 122) were enrolled and underwent outpatient metabolic evaluations including 1,25(OH)₂D₃ and PTH as well as serum and 24-hour urinary biochemical parameters. As control, 163 age- and sex-matched (2:1) individuals (non-SFs) who have never urinary stone episode were included.

RESULTS

1,25(OH)₂D₃ level was positively correlated with urinary calcium excretion (r=0.347, p<0.001). The hypercalciuric group and recurrent SFs had higher serum 1,25(OH)₂D₃ levels than the normocalciuric group (p<0.001) and first SFs (p=0.050). In the adjusted multiple linear regression analysis, serum 1,25(OH)₂D₃ level (β=0.259, p<0.001) and serum PTH level (β=-0.160, p<0.001) were significantly correlated with urinary calcium excretion. The patients in highest tertile of 1,25(OH)₂D₃ had a more than 3.1 fold risk of hypercalciuria than those in the lowest tertile (odds ratio=3.14, 95% confidence interval: 1.431-6.888, p=0.004). No correlation was observed between PTH and 1,25(OH)₂D₃ (R=0.005, p=0.929) in calcium SFs, while a negative correlation was found in controls (R=-0.269, p=0.001).

CONCLUSION

1,25(OH)₂D₃ was closely correlated with urinary calcium excretion, and high 1,25(OH)₂D₃ levels were detected in the hypercalciuric group and in recurrent SFs. However, 1,25(OH)₂D₃ was not correlated with PTH in calcium SFs. These findings suggest that 1,25(OH)₂D₃ might be important intrinsic factor for altered calcium regulation in SFs.

Genetic determinants of urolithiasis

Urolithiasis affects approximately 10% of individuals in Western societies by the seventh decade of life. The most common form, idiopathic calcium oxalate urolithiasis, results from the interaction of multiple genes and their interplay with dietary and environmental factors. To date, considerable progress has been made in identifying the metabolic risk factors that predispose to this complex trait, among which hypercalciuria predominates. The specific genetic and epigenetic factors involved in urolithiasis have remained less clear, partly owing to the candidate gene and linkage methods that have been available until now, being inherently low in their power of resolution and in assessing modest effects in complex traits. However, together with investigations of rare, Mendelian forms of urolithiasis associated with various metabolic risk factors, these methods have afforded insights into biological pathways that seem to underlie the development of stones in the urinary tract. Monogenic diseases account for a greater proportion of stone formers in children and adolescents than in adults. Early diagnosis of monogenic forms of urolithiasis is of importance owing to associated renal injury and other potentially treatable disease manifestations, but diagnosis is often delayed because of a lack of familiarity with these rare disorders. In this Review, we will discuss advances in the understanding of the genetics underlying polygenic and monogenic forms of urolithiasis.

Adenovirus-delivered microRNA targeting the vitamin D receptor reduces intracellular Ca²⁺ concentrations by regulating the expression of Ca²⁺-transport proteins in renal epithelial cells

What’s known on the subject? and What does the study add? Experimental data have shown that VDR overexpression in the duodenum and kidney cortex is a biological characteristic of genetic hypercalciuric stone-forming rats (GHS rat), and a link between idiopathic calcium stone formation and the microstatellite marker D12S339 (near the VDR locus) has been proven in humans. Our study shows that VDR can positively regulate the mRNA and protein expression of TRPV5, calbindin-D28k and PMCA1b in NRK cell lines. VDR knockdown results in a decrease in intracellular Ca²⁺ concentration in NRK cell lines. The effect of the elevated VDR level in the kidney on hypercalciuria and the underlying mechanisms need to be further addressed.

OBJECTIVE

• To determine the effects of vitamin D receptor (VDR) on hypercalciuria and the mechanisms underlying such effects.

MATERIALS AND METHODS

• The adenovirus vector-delivered microRNA targeting rat VDR was constructed. We infected the normal rat kidney epithelial cell line NRK (Cellbank, China) with the adenovirus and then collected the cells at 0, 48, 72, 96, 120 h after infection. The mRNA and protein levels of VDR and VDR-dependent epithelial Ca2+ transport proteins were detected using real-time polymerase chain reaction and Western blot assays, respectively. • Fluorescent Ca²⁺ indicator Fluo-4 NW (Fluo-4 NW calcium assay kit, Molecular Probes, Invitrogen, USA) and laser scanning confocal microscope (Olympus, FV500-IX71, Japan) were used to detect the cytosolic free Ca²⁺ concentration at different time points after infection.

RESULTS

• The mRNA and protein level of VDR, transient receptor potential vanilloid receptor subtype 5 (TRPV5), calbindin-D28k and plasma membrane Ca²⁺-ATPase (PMCA1b) in infected NRK cells was significantly lower at 72 and 96 h after infection than that in control cells. • There was no significant difference between the two groups in the mRNA and protein level of TRPV6 and the Na⁺/Ca²⁺-exchanger (NCX1). • Furthermore, VDR knockdown results in a decrease in intracellular Ca²⁺ concentration ([Ca²⁺]i) in NRK cell lines.

CONCLUSIONS

• Our study shows that VDR can positively regulate the mRNA and protein expression of TRPV5, calbindin-D28k and PMCA1b, but not of TRPV6 or NCX1, in NRK cell lines. VDR knockdown results in a decrease in [Ca²⁺]i in NRK cell lines. • The effect of the elevated VDR level in the kidney on hypercalciuria and the mechanisms underlying need to be further addressed.

Nephrocalcinosis and urolithiasis in children

The incidence of adult urolithiasis has increased significantly in industrialized countries over the past decades. Sound incidence rates are not available for children, nor are they known for nephrocalcinosis, which can appear as a single entity or together with urolithiasis. In contrast to the adult kidney stone patient, where environmental factors are the main cause, genetic and/or metabolic disorders are the main reason for childhood nephrocalcinosis and urolithiasis. While hypercalciuria is considered to be the most frequent risk factor, several other metabolic disorders such as hypocitraturia or hyperoxaluria, as well as a variety of renal tubular diseases, e.g., Dent's disease or renal tubular acidosis, have to be ruled out by urine and/or blood analysis. Associated symptoms such as growth retardation, intestinal absorption, or bone demineralization should be evaluated for diagnostic and therapeutic purposes. Preterm infants are a special risk population with a high incidence of nephrocalcinosis arising from immature kidney, medication, and hypocitraturia. In children, concise evaluation will reveal an underlying pathomechanism in >75% of patients. Early treatment reducing urinary saturation of the soluble by increasing fluid intake and by providing crystallization inhibitors, as well as disease-specific medication, are mandatory to prevent recurrent kidney stones and/or progressive nephrocalcinosis, and consequently deterioration of renal function.

Epidemiology of pediatric urolithiasis

Pediatric urolithiasis has increased globally in the last few decades. There has been a change in the pattern of stone composition with an increase in the frequency of kidney stones and a decrease in bladder stones. The role of familial predisposition and environmental factors in pediatric urolithiasis is now better understood. Metabolic factors are more common in pediatric urolithiasis than in adult stone disease. This review updates on the epidemiology of pediatric urolithiasis with a focus on the changing trends in the stone disease, current spectrum of stone disease encountered in clinical practice, individual predisposition and the role of environmental factors in stone formation.

Genetics and calcium nephrolithiasis

Calcium nephrolithiasis is one of the most prevalent uronephrologic disorders in the western countries. Studies in families and twins evidenced a genetic predisposition to calcium nephrolithiasis. Family-based or case-control studies of single-candidate genes evidenced the possible involvement of calcium-sensing receptor (CASR), vitamin D receptor (VDR), and osteopontin (OPN) gene polymorphisms in stone formation. The only high-throughput genome-wide association study identified claudin 14 (CLDN14) gene as a possible major gene of nephrolithiasis. Specific phenotypes were related with these genes: CASR gene in normocitraturic patients, VDR gene in hypocitraturic patients with severe clinical course, and CLDN14 gene in hypercalciuric patients. The pathogenetic weight of these genes remains unclear, but an alteration of their expression may occur in stone formers. Technological skills, accurate clinical examination, and a detailed phenotype description are the basis to get new insight about the genetic basis of nephrolithiasis.

Genetic basis of renal cellular dysfunction and the formation of kidney stones

Nephrolithiasis is a result of formation and retention of crystals within the kidneys. The driving force behind crystal formation is urinary supersaturation with respect to the stone-forming salts, which means that crystals form when the concentrations of participating ions are higher than the thermodynamic solubility for that salt. Levels of supersaturation are kept low and under control by proper functioning of a variety of cells including those that line the renal tubules. It is our hypothesis that crystal deposition, i.e., formation and retention in the kidneys, is a result of impaired cellular function, which may be intrinsic and inherent or triggered by external stimuli and challenges. Cellular impairment or dysfunction affects the supersaturation, by influencing the excretion of participating ions such as calcium, oxalate and citrate and causing hypercalciuria, hyperoxaluria or hypocitraturia. The production and excretion of macromolecular promoters and inhibitors of crystallization is also dependent upon proper functioning of the renal epithelial cells. Insufficient or ineffective crystallization modulators such as osteopontin, Tamm-Horsfall protein, bikunin, etc. are most likely produced by the impaired cells.

Genetic causes of hypercalciuric nephrolithiasis

Renal stone disease (nephrolithiasis) affects 3-5% of the population and is often associated with hypercalciuria. Hypercalciuric nephrolithiasis is a familial disorder in over 35% of patients and may occur as a monogenic disorder that is more likely to manifest itself in childhood. Studies of these monogenic forms of hypercalciuric nephrolithiasis in humans, e.g. Bartter syndrome, Dent's disease, autosomal dominant hypocalcemic hypercalciuria (ADHH), hypercalciuric nephrolithiasis with hypophosphatemia, and familial hypomagnesemia with hypercalciuria have helped to identify a number of transporters, channels and receptors that are involved in regulating the renal tubular reabsorption of calcium. Thus, Bartter syndrome, an autosomal disease, is caused by mutations of the bumetanide-sensitive Na-K-Cl (NKCC2) co-transporter, the renal outer-medullary potassium (ROMK) channel, the voltage-gated chloride channel, CLC-Kb, the CLC-Kb beta subunit, barttin, or the calcium-sensing receptor (CaSR). Dent's disease, an X-linked disorder characterized by low molecular weight proteinuria, hypercalciuria and nephrolithiasis, is due to mutations of the chloride/proton antiporter 5, CLC-5; ADHH is associated with activating mutations of the CaSR, which is a G-protein-coupled receptor; hypophosphatemic hypercalciuric nephrolithiasis associated with rickets is due to mutations in the type 2c sodium-phosphate co-transporter (NPT2c); and familial hypomagnesemia with hypercalciuria is due to mutations of paracellin-1, which is a member of the claudin family of membrane proteins that form the intercellular tight junction barrier in a variety of epithelia. These studies have provided valuable insights into the renal tubular pathways that regulate calcium reabsorption and predispose to hypercalciuria and nephrolithiasis.

New insights into the pathogenesis of idiopathic hypercalciuria

Idiopathic hypercalciuria (IH) is the most common metabolic abnormality in patients with calcium kidney stones. It is characterized by normocalcemia, absence of diseases that cause increased urine calcium, and calcium excretion that is greater than 250 mg/d in women and 300 mg/d in men. Subjects with IH have a generalized increase in calcium turnover, which includes increased gut calcium absorption, decreased renal calcium reabsorption, and a tendency to lose calcium from bone. Despite the increase in intestinal calcium absorption, a negative calcium balance is seen commonly in balance studies, especially on a low-calcium diet. The mediator of decreased renal calcium reabsorption is not clear; it is not associated with either an increase in filtered load of calcium or altered parathyroid hormone levels. There is an increased incidence of hypercalciuria in first-degree relatives of those with IH, but IH appears to be a complex polygenic trait with a large contribution from diet to expression of increased calcium excretion. Increased tissue vitamin D response may be responsible for the manifestations of IH in at least some patients.

Predisposition of genetic polymorphism with the risk of urolithiasis

Urolithiasis is a relevant clinical problem with a subsequent burden for health system. The aim of this review is to provide recent progress made using genetic polymorphisms to define pathophysiology, to identify persons at risk for kidney stone disease and to predict treatment response. Population case-control studies are useful both as an alternative and an adjunct as compared to family studies. These involve either whole genome scanning or candidate gene approaches. While whole genome scanning is likely to be widely used in future, at present, candidate gene studies are more feasible. When performing candidate gene case-control studies factors such as study design, methods for recruitment of case and controls, selection of candidate genes, functional significance of polymorphisms chosen for study and statistical analysis require close attention to ensure that only genuine associations are detected. Some of the significant genes that play role in stone formation include calcitonin receptor gene (CTR), vitamin D receptor (VDR), Urokinase, Interleukin, (IL-1β, IL-Ra), E-Cadherin, Androgen & oestrogen receptor gene, vascular endothelial growth factor (VEGF) and Arginine p21. In our case-control study we studied CTR, VDR, Urokinase, IL-1β(-511 and +3954), IL-Ra from north India and predict that VDR, IL-β (-511) and IL-1Ra gene may be used as a possible genetic marker for earlier detection in patients who are at risk for calcium oxalate stone disease. Further, linkage disequilibrium and haplotype structure of a certain candidate gene is important for association analysis. When a certain polymorphic allele has been found to be associated with disease, it is further explained on basis of LD and haplotype structure by one or more other alleles. Once it is determined which haplotype carries the risk allele, by means of molecular biological functional analyses, the variants on that haplotype allele truly causing the effect can be determined.

Mechanisms and regulation of epithelial Ca2+ absorption in health and disease

Ca2+ is essential for numerous physiological functions in our bodies. Therefore, its homeostasis is finely maintained through the coordination of intestinal absorption, renal reabsorption, and bone resorption. The Ca2+-selective epithelial channels TRPV5 and TRPV6 have been identified, and their physiological roles have been revealed: TRPV5 is important in final renal Ca2+ reabsorption, and TRPV6 has a key role in intestinal Ca2+ absorption. The TRPV5 knockout mice exhibit renal leak hypercalciuria and accordingly upregulate their intestinal TRPV6 expression to compensate for their negative Ca2+ balance. In contrast, despite their severe negative Ca2+ balance, TRPV6-null mice do not display any compensatory mechanism, thus resulting in secondary hyperparathyroidism. These results indicate that the genes for TRPV5 and TRPV6 are differentially regulated in human diseases associated with disturbed Ca2+ balance such as hypercalciuria, osteoporosis, and vitamin D-resistant rickets.

Update on primary hypercalciuria from a genetic perspective

PURPOSE

This review provides a brief update on genetic studies of primary hypercalciuria. We consider their possible implications for the pathogenesis and complications of primary hypercalciuria.

MATERIALS AND METHODS

Using the PubMed, MEDLINE and Scopus databases we reviewed the literature on pathogenesis and the complications of hypercalciuria, giving particular attention to genetic studies in humans.

RESULTS

Primary hypercalciuria is a defect occurring in 5% to 10% of the general population and it is most commonly detected in patients with calcium kidney stones or osteoporosis. In children it is associated with hematuria, renal stones or nocturnal enuresis. Although high penetrance, autosomal dominant inheritance cannot be ruled out, hypercalciuria is probably a polygenic disorder. A number of genes have been suggested as candidates in the pathogenesis of common idiopathic calcium nephrolithiasis and hypercalciuria, ie soluble adenylate cyclase, calcium sensing receptor, vitamin D receptor, chloride channel-5, sodium-phosphate cotransporter-2 and claudin-16. These genes may also have a role in complications of hypercalciuria.

CONCLUSIONS

The classic distinction among absorptive, renal and resorptive hypercalciuria seems insufficient to explain the many cellular and tissue modifications observed in patients with primary hypercalciuria. The condition seems to be a separate disorder, characterized by altered calcium transport in the intestine, kidney and bone, and caused by various combinations of multiple genetic and dietary changes.

Association of vitamin D receptor (Fok-I) polymorphism with the clinical presentation of calcium urolithiasis

OBJECTIVE

To investigate the effect of the vitamin D receptor (VDR) FokI polymorphism on the clinical presentation of calcium urolithiasis, as a FokI polymorphism in the VDR gene was recently reported to be associated with calcium metabolism disorders.

PATIENTS, SUBJECTS AND METHODS

In all, 235 patients with calcium urolithiasis and 231 age- and sex-matched healthy controls were recruited from Kaohsiung Medical University Hospital between June 2003 and February 2005. Clinical information on the age at first onset, stone episodes, stone severity and presence of family history were collected from patients with stones. Any VDR FokI polymorphism was detected using polymerase chain reaction-based restriction analysis.

RESULTS

The frequency of VDR FokI genotypes between the patients and the healthy controls was not significantly different. However, among patients, those with the FF genotype had a significantly higher risk of having more stone episodes (adjusted odds ratio 2.15, 95% confidence interval 1.02-4.54, P = 0.044) and were younger at the first onset (3.23, 1.08-9.63, P = 0.036) than those with the ff genotype.

CONCLUSION

The VDR FokI polymorphism might be important in the clinical presentation of patients with calcium urolithiasis, especially for the frequency of stone episodes and age at first onset, although it is not associated with the formation of stones.

Isolation and confirmation of a calcium excretion quantitative trait locus on chromosome 1 in genetic hypercalciuric stone-forming congenic rats

Hypercalciuria is the most common risk factor for kidney stones and has a substantial genetic component. The genetic hypercalciuric stone-forming (GHS) rat model displays complex changes in physiology involving intestine, bone, and kidney and overexpression of the vitamin D receptor, thereby reproducing the human phenotype of idiopathic hypercalciuria. Through quantitative trait locus (QTL) mapping of rats that were bred from GHS female rats and normocalciuric Wistar Kyoto (WKY) male rats, loci that are linked to hypercalciuria and account for a 6 to eight-fold phenotypic difference between the GHS and WKY progenitors were mapped. GHS x WKY rats were backcrossed to breed for congenic rats with the chromosome 1 QTL HC1 on a normocalciuric WKY background. Ten generations of backcrosses produced N10F1 rats, which were intercrossed to produce rats that were homozygous for GHS loci in the HC1 region between markers D1Mit2 and D1Mit32. On a high-calcium diet (1.2% calcium), significantly different levels of calcium excretion were found between male congenic (1.67 +/- 0.71 mg/24 h) and male WKY control rats (0.78 +/- 0.19 mg/24 h) and between female congenic (3.11 +/- 0.90 mg/24 h) and female WKY controls (2.11 +/- 0.50 mg/24 h); the congenics preserve the calcium excretion phenotype of the GHS parent strain. Microarray expression analyses of the congenic rats, compared with WKY rats, showed that of the top 100 most changed genes, twice as many as were statistically expected mapped to chromosome 1. Of these, there is a clear bias in gene expression change for genes in the region of the HC1. Of >1100 gene groups analyzed, one third of the 50 most differentially expressed gene groups have direct or secondary action on calcium metabolism or transport. This is the first QTL for hypercalciuria to be isolated in a congenic animal.

Kidney stones: pathophysiology and medical management

The formation of stones in the urinary tract stems from a wide range of underlying disorders. That clinicians look for the underlying causes for nephrolithiasis is imperative to direct management. There are many advances in genetics, pathophysiology, diagnostic imaging, medical treatment, medical prevention, and surgical intervention of nephrolithiasis. Here, I provide a brief general background and focus mainly on pathophysiology and medical treatment of kidney stones. Although important advances have been made in understanding nephrolithiasis from single gene defects, the understanding of polygenetic causes of kidney stones is still largely elusive. A substantial proportion of data that resulted in new methods of treatment and prevention, which can be empirical or definitive, has focused on urinary luminal chemical composition of the precipitating solutes. Manipulation of inhibitors and epithelial factors is important and needs further investigation. Advances in the management of nephrolithiasis depend on combined efforts of clinicians and scientists to understand the pathophysiology.

Vitamin D receptor gene polymorphisms in patients with urolithiasis

Urolithiasis is a multifactorial disease, the onset and severity of which is influenced by both genetic and environmental factors. This study represents an investigation of the role of vitamin D receptor (VDR) gene polymorphisms (ApaI, BsmI, and TaqI) and combined genotypes in urolithiasis in a Turkish population. We studied 110 patients with urinary stones and 150 control subjects. The polymorphic regions were amplified using polymerase chain reaction, followed by digestion with restriction enzymes BsmI, ApaI, and TaqI, and analyzed electrophoretically. Genotype and allele frequencies were calculated, and the association with urolithiasis, family history, and recurrence of stone was investigated. Our data provide no evidence for an association between urolithiasis and VDR ApaI, BsmI, and TaqI genotypes. We also analyzed the effects of VDR ApaI, BsmI, and TaqI genotypes in combination; the "GTT" VDR haplotype, constructed from three adjacent restriction fragment length polymorphisms was overrepresented among the urolithiasis patients. However, no significant differences between heterozygous carriers (OR 1.302; 95% CI 0.527-3.215) and homozygous carriers (OR 3.39; 95% CI 0.719-15.985) were observed in our study population. A significant association was found only between the ApaI polymorphism and family history (P=0.017; chi (2)=5.657). Our data indicate that the VDR ApaI, BsmI, and TaqI polymorphisms do not confer a significant risk for urolithiasis.

Les hypercalciuries

The frequency of hypercalciuria is increasing in western countries with an incidence of nephrolithiasis which can reach 13%. Hypercalciuria appears as an alteration of the calcium transport system (kidney, bowel, bone) which is regulated by calcitriol and parathormone. The aim of this review was to screen etiologies of hypercalciuria taking into account recent genetic advances (calcium epithelial channel and calcium sensing receptor). Hypercalciuria may be favored by nutritional causes (diet rich in calcium, sodium, carbohydrates, proteins, poor in phosphates and potassium). It may also be related to an increase in calcium absorption (vitamin D excess, primary hyperparathyroidism, sarcoidosis, lymphoma, estrogens, and certain genetic causes), an increase in osteoresorption (bone metastasis, myeloma, Paget, hyperthyroidism, immobilization, hypercortisolism and corticosteroid therapy), or a decrease of kidney tubular resorption (diuretics, Cacci and Ricci, acromegally, Bartter, familial dominant hypocalcemia, Fanconi, Dent, familial hypomagnesemia-hypercalciuria syndrome, type 1 distal tubular acidosis, pseudohypoaldosteronism, diabetes). If no cause is identified, persistence of hypercalciuria after instituting a correct diet is defined as idiopathic hypercalciuria. Treatment of the cause is essential in secondary hypercalciuria, in addition to diet (low sodium intake, normocalcic diet, hydration), associated with thiazide diuretics and biphosphonates if necessary.

Association of vitamin D receptor-gene (FokI) polymorphism with calcium oxalate nephrolithiasis

BACKGROUND AND PURPOSE

Formation of kidney stones is still not understood but is hypothesized to be associated with the vitamin D receptor gene (VDR). In order to assess the eventual role of VDR start-codon FokI polymorphism in stone formation, we evaluated the association between calcium stone disease and this polymorphism in a North Indian population.

PATIENTS AND METHODS

A control group comprised of 166 healthy individuals (age range 22-58 years) and a group of 138 patients with calcium oxalate stones (age range 21-72 years) were examined. The polymorphism was detected using polymerase chain reaction-based restriction analysis. An unexcisable length of 265 bp (CC) and two fragments of 169 bp and 96 bp (TT) were obtained by FokI restriction digestion.

RESULTS

There was a statistically significant difference between the control and patient groups (X2 test, P<0.001) for the genotype of the VDR FokI start-codon polymorphism. The odds ratio (with 95% CI) for the C allele in those at risk of stone disease was 1.654 (1.041, 2.628). The VDR frequency distribution was also statistically significant (P<0.001) in case of male sex. The frequency distribution for this genetic polymorphism was not statistically different in normocalciuric and hypercalciuric stone patients (P=0.355).

CONCLUSION

The VDR FokI polymorphism may be a good candidate for a marker for calcium oxalate-stone disease. These findings may contribute a small piece to the understanding of the pathogenesis of urinary calculi.

Association of vitamin-D and calcitonin receptor gene polymorphism in paediatric nephrolithiasis

We investigated the role of vitamin-D receptor gene (VDR) and calcitonin receptor (CTR) gene polymorphism in childhood nephrolithiasis in the north Indian population. A control group of 60 healthy paediatric individuals (age range 4-16 years) and 50 paediatric patients (age range 2-14 years) with kidney stones were examined. Polymorphism in both genes (VDR and CTR) was detected by using PCR-based restriction analysis. There was a statistically significant difference between the two groups for the genotypes of the VDR gene Fok-I polymorphism (P =0.007) and the CTR gene (P =0.048) polymorphism. The odds ratio (95% Confidence Interval) for the C allele in those at risk of stone disease was 1.83 (0.82-4.09) in VDR gene polymorphism and 1.99 (0.90-4.39) in the case of CTR gene polymorphism. Our results suggest that the effects of VDR (Fok-I) and CTR gene polymorphism contribute to the understanding of the pathogenesis of urinary calculi. It is also suggestive of a potential candidate gene in the search for genetic causes of paediatric calcium oxalate nephrolithiasis.

Genetics of hypercalciuria and calcium nephrolithiasis: from the rare monogenic to the common polygenic forms

Idiopathic calcium nephrolithiasis is a multifactorial disease with a pathogenesis that involves a complex interaction of environmental and individual factors. This review discusses what is known about monogenic renal calcium stone-related disorders, provides an update on genetic research in calcium nephrolithiasis and such intermediate phenotypes as idiopathic hypercalciuria, discusses the problems that these conditions pose to clinicians and geneticists interested in their pathogenesis, and proposes some method tools potentially useful in this research frame of reference.

A twin study of genetic and dietary influences on nephrolithiasis: A report from the Vietnam Era Twin (VET) Registry

BACKGROUND

Nephrolithiasis is a complex phenotype that is influenced by both genetic and environmental factors. We conducted a large twin study to examine genetic and nongenetic factors associated with stones.

METHODS

The VET Registry includes approximately 7500 male-male twin pairs born between 1939 to 1955 with both twins having served in the military from 1965 to 1975. In 1990, a mail and telephone health survey was sent to 11,959 VET Registry members; 8870 (74.2%) provided responses. The survey included a question asking if the individual had ever been told of having a kidney stone by a physician. Detailed dietary habits were elicited. In a classic twin study analysis, we compared concordance rates in monozygotic (MZ) and dizygotic (DZ) twins. We also conducted a cotwin control study of dietary risk factors in twins discordant for stones.

RESULTS

Among dizygotic twins, there were 17 concordant pairs and 162 discordant pairs for kidney stones. Among monozygotic twins, there were 39 concordant pairs and 163 discordant pairs. The proband concordance rate in MZ twins (32.4%) was significantly greater than the rate in DZ twins (17.3%) (chi(2)= 12.8; P < 0.001), consistent with a genetic influence. The heritability of the risk for stones was 56%. In the multivariate analysis of twin pairs discordant for kidney stones, we found a protective dose-response pattern of coffee drinking (P= 0.03); those who drank 5 or more cups of coffee were half as likely to develop kidney stones as those who did not drink coffee (OR = 0.4, 95% CI 0.2, 0.9). Those who drank at least 1 cup of milk per day were half as likely to report kidney stones (OR = 0.5, 95% CI 0.3, 0.8). There were also marginally significant protective effects of increasing numbers of cups of tea per day and frequent consumption of fruits and vegetables. Other factors such as the use of calcium supplements, alcohol drinking, consumption of solid dairy products, and the amount of animal protein consumed were not significantly related to kidney stones in the multivariate model.

CONCLUSION

These results confirm that nephrolithiasis is at least in part a heritable disease. Coffee, and perhaps tea, fruits, and vegetables were found to be protective for stone disease. This is the first twin study of kidney stones, and represents a new approach to elucidating the relative roles of genetic and environmental factors associated with stone formation.

Genetic Hypercalciuria

Hypercalciuria is an important, identifiable, and reversible risk factor in stone formation. The foremost and most fundamental step in dissecting the genetics of hypercalciuria is understanding its pathophysiology. Hypercalciuria is a complex trait. This article outlines the various factors that compromise the attempt to dissect the genetics of hypercalciuria, summarizes the clinical and experimental monogenic causes of hypercalciuria, and outlines the initial results from attempts in studying polygenic hypercalciuria. Finally, the problem is set in perspective of the current database, technologic advances and limitations are highlighted, and prospects of further advances in the field are speculated upon.

Association between vitamin D receptor gene polymorphisms and fasting idiopathic hypercalciuria in recurrent stone-forming patients

OBJECTIVES

To investigate the association between fasting idiopathic hypercalciuria (IHc), defined as IHc in the fasting state associated with normal parathyroid function, and ApaI, BsmI, and FokI polymorphisms of the vitamin D receptor (VDR) gene in 159 hypercalciuric recurrent stone formers. IHc contributes to the formation of calcium kidney stones in more than one half of reported cases.

METHODS

We examined 62 patients with fasting IHc (24 women, mean age 42.8 +/- 11.1 years, body mass index 25.7 +/- 4.8 kg/m2), 97 patients with absorptive IHc (41 women, mean age 43.5 +/- 10.8 years, body mass index 26.1 +/- 4.4 kg/m2), and 124 healthy control subjects (52 women, mean age 41.9 +/- 10.4 years, body mass index 25.4 +/- 5.1 kg/m2) without a history of nephrolithiasis and without IHc. The bone mass density and VDR genotype and haplotype frequencies were determined in the studied populations.

RESULTS

A reduced bone mass density was observed in fasting IHc patients compared with absorptive IHc patients (P = 0.009) and control subjects (P = 0.006). The prevalence of ApaI and BsmI VDR genotypes and alleles in patients with fasting IHc was significantly different statistically (P <0.05) from that observed in patients with absorptive IHc and control subjects, and the ba haplotype was overrepresented in these patients. No statistically significant difference in the distribution of FokI VDR genotypes and alleles was found between the studied groups.

CONCLUSIONS

Our results suggest a genetic association between 3' VDR alleles, fasting IHc, and reduced bone mass density in patients with recurrent stone formation.

Vitamin D Receptor mRNA Measured in Leukocytes with the TaqMan Fluorogenic Detection System: Effect of Calcitriol Administration

Background: The aim of the present study was to investigate the interactions between the circulating concentrations of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and the mRNA concentration of its specific nuclear receptor in human leukocytes.

Methods: We measured vitamin D receptor (VDR) mRNA extracted from leukocytes by use of TaqMan fluorescence analysis applied to the reverse transcription-PCR (RT-PCR) technique in 16 volunteers before and after calcitriol administration. VDR mRNA was also measured in leukocytes from calcium-stone-formers (37 hypercalciuric and 34 normocalciuric patients). The relationship between VDR mRNA concentrations and genetic VDR polymorphisms was analyzed in these patients.

Results: Imprecision (CV) of RT-PCR was 1.3% within assay (n = 10) and 1.7% between assays (n = 4). Oral 1,25(OH)2D3 increased mean (SE) serum 1,25(OH)2D3 1.6 (0.3)-fold and VDR mRNA 1.6 (0.1)-fold 8 h after administration. The maximum VDR mRNA was reached 3.6 (1.3) h after 1,25(OH)2D3 ingestion. No differences in leukocyte VDR mRNA concentrations were found between normocalciuric and hypercalciuric stone-formers in the absence of stimulation. Finally, no association was found between VDR mRNA concentrations and genetic VDR polymorphisms in stone-formers.

Conclusions: The TaqMan RT-PCR assay is a rapid and accurate method to measure VDR mRNA, and leukocytes are a useful model to study VDR and 1,25(OH)2D3 interactions. In humans, VDR mRNA is increased by agonist 1,25(OH)2D3, a finding resembling previously reported results obtained in cellular and animal models.

The relationship of 3' vitamin D receptor haplotypes to urinary supersaturation of calcium oxalate salts and to age at onset and familial prevalence of nephrolithiasis

BACKGROUND

Idiopathic hypercalciuria (IHc) and idiopathic hypocitraturia are frequently associated with calcium nephrolithiasis. We investigated the relationship of vitamin D receptor (VDR) polymorphisms (BsmI, TaqI and FokI) to urinary supersaturation of calcium oxalate salts in recurrent calcium oxalate stone formers with IHc and the clinical relevance of this relationship.

METHODS

The study included 110 Caucasian stone formers with IHc and 127 unrelated healthy controls without history of nephrolithiasis. Age at onset of nephrolithiasis, familial history score (FHS) and the ion activity product of calcium oxalate salts in urine (AP(CaOx)) were tabulated. BsmI, TaqI and FokI VDR polymorphisms were evaluated in all participants.

RESULTS

Patients and controls were classified as homozygous (bbTT and BBtt) or heterozygous in relation to BsmI and TaqI polymorphisms. Compared with BBtt patients, bbTT homozygous stone formers showed lower citrate excretion (1.91+/-0.89 vs 3.46+/-1.39 mmol/24 h, P = 0.004) and higher AP(CaOx) (2.02+/-0.51 vs 1.53+/-0.53, P = 0.006). Among controls, there were similar differences in citrate excretion and AP(CaOx) between the two groups, but they were not statistically significant. Compared with BBtt, bbTT patients showed lower mean age at onset of nephrolithiasis (29.7+/-12.1 vs 38.1+/-12.7 years, P = 0.008) and higher values of FHS (2.45+/-1.9 vs 0.83+/-0.7, P = 0.006). Similar results were obtained for individual BsmI and TaqI alleles. The analysis of FokI alleles was not informative.

CONCLUSIONS

Recurrent calcium oxalate stone formers with IHc and the bT VDR haplotype have more aggressive kidney stone diseases as indicated by a higher familial incidence and lower mean age at onset. This clinical severity is associated with the higher urinary supersaturation of calcium oxalate salts and abnormalities of renal citrate handling.

Association of vitamin D receptor genotypes with calcium excretion in nephrolithiatic subjects in northern India

Our objective was to investigate the association between the vitamin D receptor (VDR) allelic variants (Bsm I and Fok I) and nephrolithiasis in northern India. A total of 150 nephrolithiatic patients and 100 age and sex matched controls were enrolled for study. A 10 ml blood sample was obtained for biochemical analysis and DNA isolation. In addition, 24 h urine samples were obtained from each patient for the estimation of calcium and creatinine. PCR was performed for the Bsm I and Fok I VDR variants. The association between Bsm I and Fok I VDR polymorphism and nephrolithiasis was investigated after digestion with restriction enzymes (3 U). The product was analysed on 3% agarose gel for Bsm I and 15% polyacrylamide gel for Fok I allelic variants. We did not observe any significant differences in the prevalence of either the Bsm I or Fok I VDR genotypes between stone formers and controls. The B allele was found to be more prevalent in hypercalciuric patients compared to controls and nephrolithiatic subjects. The subjects with the bb genotype exhibited a higher calcium excretion than the BB genotype. Patients with the F allele were also found to excrete higher urinary calcium. VDR genotypes may be associated with increased calcium excretion in hypercalciuric nephrolithiatic subjects.