Genetic Hypercalciuria

Diagnosis and management of hypercalciuria in children

PURPOSE OF REVIEW

In this review, recent advances in the epidemiology, genetics, clinical associations and management of idiopathic hypercalciuria will be discussed.

RECENT FINDINGS

A significant genetic contribution exists in the pathophysiology of hypercalciuria. Although several candidate genes and genetic alterations have been proposed, identification of precise gene(s) responsible remains elusive. Decreased bone density has been increasingly associated with hypercalciuria. Recent publications have suggested that bisphosphonates may play a role in the management in patients in whom both hypercalciuria and decreased bone density are present.

SUMMARY

Idiopathic hypercalciuria is a common disorder in children and can present with a range of clinical presentations such as hematuria, voiding dysfunction, flank pain, abdominal pain, nephrolithiasis, urinary tract infection and decreased bone mineral density. Dietary modifications are often sufficient in the management of hypercalciuria. If the symptoms persist or a rare monogenic disorder is present, consideration should be given to medical treatment with a thiazide diuretic and/or citrate therapy.

TRPV5 gene polymorphisms in renal hypercalciuria

BACKGROUND

Kidney stone formation is a major socioeconomic problem in humans, involving pain, recurrent treatment and renal insufficiency. As most renal precipitates contain calcium as a major component, hypercalciuria is the main risk factor for renal stone formation. Different forms of hypercalciuria can be classified, which primarily arise from defects in the main organs involved in calcium homeostasis. A distinction can be made between renal, absorptive and resorptive hypercalciuria, originating from disturbed calcium handling in kidney, intestine and bone, respectively. A positive family history predisposes individuals to an increased risk of stone formation, which strongly indicates the involvement of genetic susceptibility factors. TRPV5 is the renal epithelial calcium channel that is the gatekeeper protein in active calcium reabsorption in the kidney. TRPV5 gene ablation in mice leads to severe hypercalciuria, implying that TRPV5 is an interesting candidate gene for renal hypercalciuria in humans. This study aims to identify and functionally characterize TRPV5 gene aberrations in patients with renal hypercalciuria.

METHODS

The TRPV5 coding region and intron-exon boundaries were screened for gene mutations in 20 subjects displaying renal hypercalciuria after which identified non-synonymous polymorphisms were functionally characterized by patch-clamp analysis. Wild-type and TRPV5 channels including polymorphisms were transiently expressed in human embryonic kidney (HEK) 293 cells and functionally characterized by path-clamp analysis.

RESULTS

Genotyping TRPV5 in renal hypercalciuria patients revealed three non-synonymous and five synonymous polymorphisms. Electrophysiological characterization of the TRPV5 mutants did not reveal significant functional changes compared to wild-type TRPV5 channel recordings.

CONCLUSIONS

In this specific patient cohort, our data do not support a primary role for TRPV5 in the pathogenesis of renal hypercalciuria. However, TRPV5 cannot be excluded as a candidate gene in hypercalciuria.

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.

[Inherited monogenic kidney stone diseases: recent diagnostic and therapeutic advances]

Hereditary monogenic kidney stone diseases are rare diseases, since they account for nearly 2% of nephrolithiasis cases in adults and 10% in children. Most of them are severe, because they frequently are associated with nephrocalcinosis and lead to progressive impairment of renal function unless an early and appropriate etiologic treatment is instituted. Unfortunately, treatment is often lacking or started too late since they are often misdiagnosed or overlooked. The present review reports the genotypic and phenotypic characteristics of monogenic nephrolithiases, with special emphasis on the recent advances in the field of diagnosis and therapeutics. Monogenic stone diseases will be classified into three groups according to their mechanism: (1) inborn errors of the metabolism of oxalate (primary hyperoxalurias), uric acid (hereditary hyperuricemias) or other purines (2,8-dihydroxyadeninuria), which, in addition to stone formation, result in crystal deposition in the renal parenchyma; (2) congenital tubulopathies affecting the convoluted proximal tubule (such as Dent's disease, Lowe syndrome or hypophosphatemic rickets), the thick ascending limb of Henlé's loop (such as familial hypomagnesemia and Bartter's syndromes) or the distal past of the nephron (congenital distal tubular acidosis with or without hearing loss), which are frequently associated with nephrocalcinosis, phosphatic stones and extensive tubulointerstitial fibrosis; (3) cystinuria, an isolated defect in tubular reabsorption of cystine and dibasic aminoacids, which results only in the formation of stones but requires a cumbersome treatment. Analysis of stones appears of crucial value for the early diagnosis of these diseases, as in several of them the morphology and composition of stones is specific. In other cases, especially if nephrocalcinosis, phosphatic stones or proteinuria are present, the evaluation of blood and urine chemistry, especially with regard to calcium, phosphate and magnesium, is the key of diagnosis. Search for mutations is now increasingly performed in as much as genetic counselling is important for the detection of heterozygotes in autosomic recessive diseases and of carrier women in X-linked diseases. In conclusion, better awareness to the rare monogenic forms of nephrolithiasis and/or nephrocalcinosis should allow early diagnosis and treatment which are needed to prevent or substantially delay progression of end-stage renal disease. Analysis of every first stone both in children and in adults should never be neglected, in order to early detect unusual forms of nephrolithiasis requiring laboratory evaluation and deep etiologic treatment.

Calcium oxalate crystal deposition in kidneys of hypercalciuric mice with disrupted type IIa sodium-phosphate cotransporter

The most common theories about the pathogenesis of idiopathic kidney stones consider precipitation of calcium phosphate (CaP) within the kidneys critical for the development of the disease. We decided to test the hypothesis that a CaP substrate can promote the deposition of calcium oxalate (CaOx) in the kidneys. Experimental hyperoxaluria was induced by feeding glyoxylate to male mice with knockout (KO) of NaP(i) IIa (Npt2a), a sodium-phosphate cotransporter. Npt2a KO mice are hypercalciuric and produce CaP deposits in their renal tubules. Experimental hyperoxaluria led to CaOx crystalluria in both the hypercalciuric KO mice and the normocalciuric control B6 mice. Only the KO mice produced CaOx crystal deposits in their kidneys, but the CaOx crystals deposited separately from the CaP deposits. Perhaps CaP deposits were not available for a CaOx overgrowth. These results also validate earlier animal model observations that showed that CaP substrate is not required for renal deposition of CaOx and that other factors, such as local supersaturation, may be involved. The absence of CaOx deposition in the B6 mice despite extreme hyperoxaluria also signifies the importance of both calcium and oxalate in the development of CaOx nephrolithiasis.

Laboratory assessment

Nephrolithiasis has a multifactorial origin, and several disorders may coexist in the same patient. We made a basic and a specific laboratory evaluation. The complete metabolic evaluation consisted of a serum chemistry panel: blood sugar level, complete hemogram, serum electrolytes, GOT, GPT, calcium, phosphate, uric acid, and creatinine levels and RIA dosage of PTH, vitamin D3, cAMP, FT4, FT3 and TSH. The complete analyses of random urinalysis and culture are: (1) dip-stick test: pH, leukocytes/bacteria and Brand's test, and (2) 24-hour urine collection: calcium, magnesium, oxalate, phosphate, citrate, urea, urate, sodium, creatinine, chloride, potassium. It is possible with these tests to identify secondary causes of nephrolithiasis and uncover coexisting problems that may have an impact on patient management. The future for diagnosis, prevention and therapy will be the identification of genetic alterations and related specific dosage.

Idiopathic hypercalciuria

Hypercalcuria is the most common metabolic disorder found in patients with nephrolithiasis. As the prevalence of kidney stones rises in industrialized nations, understanding the pathogenesis and treatment of hypercalciuria becomes increasingly important. Idiopathic hypercalciuria (IH), defined as an excess urine calcium excretion without an apparent underlying etiology, is the most frequent cause of hypercalciuria and will be the focus of this paper. Calcium homeostasis is tightly controlled and slight disturbances in transport at the level of the intestine, bone, and/or kidney can lead to excessive urine calcium excretion and promote stone formation. IH is a systemic disorder with dysregulation of calcium transport at a combination of these calcium regulatory sites. The goal of treatment is to prevent stone formation and relies on a combination of dietary and pharmaceutical interventions. Dietary management includes increasing fluid intake, salt restriction, animal protein restriction, and maintaining a normal calcium intake. Thiazide diuretics have proven effective in preventing calcium stone formation by reducing the urinary excretion of calcium. It is important to note that while decreasing urinary calcium excretion is important the clinician should focus primarily on reducing the supersaturation of calcium oxalate as this determines the true tendency for stone formation.

Genetics of hypercalciuric stone forming diseases

With a lifetime incidence of up to 12% in man and 6% in woman, nephrolithiasis is a major health problem worldwide. Approximately, 80% of kidney stones are composed of calcium and hypercalciuria is found in up to 40% of stone-formers. Although the mechanisms resulting in precipitation and growth of calcium crystals in the urinary tract are multiple and not fully understood, hypercalciuria per se is recognized as an important and reversible risk factor in stone formation. In this brief review, we summarize the studies assessing the heritability of hypercalciuria and pinpoint recently identified human genetic disorders as well as relevant animal models that provided new insights into the segment-specific tubular handling of calcium and the pathophysiology of renal hypercalciuria and kidney stones. We also discuss novel strategies that may help to unravel the genetic bases of such complex conditions.

Pathophysiology and Management of Calcium Stones

Nephrolithiasis is a common disorder that accounts for significant cost, morbidity, and loss of work. There is a one in eight lifetime chance of being diagnosed with urinary stones. Calcium is the most common component of renal stones in individuals in industrialized nations. Calcium stones form as a result of a variety of environmental and metabolic abnormalities that change the urinary environment and increase supersaturation of stone-forming salts. Understanding the pathophysiology of stone disease can help direct treatment toward correction of the underlying abnormalities. Current medical and dietary therapeutic regimens have been shown to significantly reduce the risk of recurrent stone formation.

Pathophysiology of hypercalciuria in children

Urinary excretion of calcium is the result of a complex interplay between three organs-namely, the gastrointestinal tract, bone, and kidney-which is finely orchestrated by multiple hormones. Hypercalciuria is believed to be a polygenic trait and is influenced significantly by diet. This paper briefly reviews calcium handling by the renal tubule in normal and in hereditary disorders as it relates to the pathophysiology of hypercalciuria. The effects of dietary sodium, potassium, protein, calcium, and phosphate on calcium excretion, and the association of hypercalciuria with bone homeostasis is discussed, leading to recommendations on means to address excessive urinary calcium excretion.

Bone disease in idiopathic hypercalciuria

PURPOSE OF REVIEW

Decreased bone mineral density and increased prevalence of bone fractures have been found in patients with idiopathic hypercalciuria. The purpose of this review is to summarize the recent published evidence that supports a potential role of the bone, and its link to the kidney and intestine, in the pathogenesis of idiopathic hypercalciuria. The effects of hypercalciuria on bone and the implications for treatment are also reviewed.

RECENT FINDINGS

Evidence suggests that the incidence of a first fracture in kidney stone patients is fourfold higher than the control population. Support for the role of bone in the pathophysiology of hypercalciuria has been corroborated. New studies have detailed the effects of several cytokines - increased number and sensitivity of vitamin D receptors, and increased acid production - upon the bone acting cells. Similarly, recent clinical and experimental studies have suggested that genetic factors confer a predisposition to the formation of renal calcium stones and bone demineralization.

SUMMARY

Whether hypercalciuria is the result of a primary bone disorder, a consequence of a persisting negative calcium balance or a combination of both still remains to be determined. Nevertheless, bone status must be evaluated and followed up in patients with idiopathic hypercalciuria.

Renal lithiasis and nutrition

Renal lithiasis is a multifactorial disease. An important number of etiologic factors can be adequately modified trough diet, since it must be considered that the urine composition is directly related to diet. In fact, the change of inappropriate habitual diet patterns should be the main measure to prevent kidney stones. In this paper, the relation between different dietary factors (liquid intake, pH, calcium, phosphate, oxalate, citrate, phytate, urate and vitamins) and each type of renal stone (calcium oxalate monohydrate papillary, calcium oxalate monohydrate unattached, calcium oxalate dihydrate, calcium oxalate dihydrate/hydroxyapatite, hydroxyapatite, struvite infectious, brushite, uric acid, calcium oxalate/uric acid and cystine) is discussed.

Randall's plaque: Pathogenesis and role in calcium oxalate nephrolithiasis

The purpose of these studies was to test the hypothesis that Randall's plaque develops in unique anatomical sites of the kidney and their formation is conditioned by specific stone-forming pathophysiologies. We performed intraoperative papillary biopsies from kidneys of idiopathic-calcium oxalate (CaOx), intestinal bypass for obesity, brushite (BR) and cystine stone formers (SF) during percutaneous nephrolithotomy. Tissues were examined by infrared analysis and light and electron microscopy. Our analysis revealed a distinct pattern of mineral deposition and papillary pathology for each type of SF. CaOx SF had interstitial apatite crystals beginning at thin loops of Henle. These deposits termed Randall's plaque are thought to serve as sites for stone attachment. No tubular injury was noted. Intestinal bypass patients possessed intraluminal apatite deposits in inner medullary collecting ducts (IMCD) with associated cell injury. BR SF showed the most severe form of cortical and medullary changes with sites of Randall's plaque, and yellowish intraluminal deposits of apatite in IMCD. Cystine SF had plugging of ducts of Bellini with cystine crystals and apatite deposits in IMCD and loops of Henle. Intratubular sites of crystalline deposits were always associated to adjacent regions of interstitial fibrosis. The metabolic, anatomic, and surgical pathologic findings in four distinct groups of SF clearly show that 'the histology of the renal papilla from a stone former, is particular to the clinical setting'. We believe our approach to studying stone disease will provide insights into the pathogenesis of stone formation for each type of SF that will lead to improved clinical treatment.

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.

Kidney stone disease

About 5% of American women and 12% of men will develop a kidney stone at some time in their life, and prevalence has been rising in both sexes. Approximately 80% of stones are composed of calcium oxalate (CaOx) and calcium phosphate (CaP); 10% of struvite (magnesium ammonium phosphate produced during infection with bacteria that possess the enzyme urease), 9% of uric acid (UA); and the remaining 1% are composed of cystine or ammonium acid urate or are diagnosed as drug-related stones. Stones ultimately arise because of an unwanted phase change of these substances from liquid to solid state. Here we focus on the mechanisms of pathogenesis involved in CaOx, CaP, UA, and cystine stone formation, including recent developments in our understanding of related changes in human kidney tissue and of underlying genetic causes, in addition to current therapeutics.