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

[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.

Hypercalciuria revisited: one or many conditions?

Idiopathic hypercalciuria is a defect occurring in 5-10% of the general population and most commonly detected in patients with calcium kidney stones or osteoporosis. Although high-penetrance autosomal dominant inheritance cannot be ruled out, hypercalciuria is probably a polygenic phenomenon. Findings obtained in monogenic disorders characterized by renal calcium stones, and/or hypercalciuria, and/or nephrocalcinosis, have suggested a number of genes as candidate genes in the pathogenesis of idiopathic hypercalciuria, i.e. soluble adenylate cyclase, calcium sensing receptor, vitamin D receptor and 1-alpha hydroxylase, sodium-phosphate co-transporter-2, claudin-16, chloride channel 5, etc. All the genetic findings obtained so far do not support the idea of different types of idiopathic hypercalciuria, i.e. absorptive, renal, and resorptive. On the contrary, they support clinical observations, which suggest idiopathic hypercalciuria as a single disorder characterized by altered calcium transport in the intestine, kidney and bone, due to various different combinations of multiple genetic and dietary players.

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.

Four siblings with distal renal tubular acidosis and nephrocalcinosis, neurobehavioral impairment, short stature, and distinctive facial appearance: a possible new autosomal recessive syndrome

We report on four siblings (three males, one female) born to first cousin Arab parents with the constellation of distal renal tubular acidosis (RTA), small kidneys, nephrocalcinosis, neurobehavioral impairment, short stature, and distinctive facial features. They presented with early developmental delay with subsequent severe mental, behavioral and social impairment and autistic-like features. Their facial features are unique with prominent cheeks, well-defined philtrum, large bulbous nose, V-shaped upper lip border, full lower lip, open mouth with protruded tongue, and pits on the ear lobule. All had proteinuria, hypercalciuria, hypercalcemia, and normal anion-gap metabolic acidosis. Renal ultrasound examinations revealed small kidneys, with varying degrees of hyperechogenicity and nephrocalcinosis. Additional findings included dilated ventricles and cerebral demyelination on brain imaging studies. Other than distal RTA, common causes of nephrocalcinosis were excluded. The constellation of features in this family currently likely represents a possibly new autosomal recessive syndrome providing further evidence of heterogeneity of nephrocalcinosis syndromes.

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.

Idiopathic hypercalciuria in infants with renal stones

In children older than 2 years, hypercalciuria is the most common metabolic cause of renal stones. In infants, its prevalence is not well established. Since 1990, we observed five infants in whom renal stones or microcalculi were diagnosed between the age of 5 and 19 months. One of them was lost to follow up. In the present report, we describe the four patients who underwent metabolic evaluation and in whom idiopathic hypercalciuria (IH) was diagnosed. We conclude that IH is a frequent cause of microcalculi and renal stones in infants, and a screen for this condition is warranted in this age group.

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.

Mild Tubular Damage Induces Calcium Oxalate Crystalluria in a Model of Subtle Hyperoxaluria: Evidence that a Second Hit Is Necessary for Renal Lithogenesis

Environment and diet have a major role in calcium nephrolithiasis by affecting urine saturation, but this is not enough to cause lithogenesis; the crystals must adhere to the tubular epithelium (TE), but it is hard to say how environment and nutrition may be involved in this step. The hypothesis that TE damage (known to enhance crystal attachment) is lithogenic in mild hyperoxaluria was tested. Mild hyperoxaluria was induced in male Wistar rats using ethylene glycol (EG; 0.5% in water) for 21 d, and TE damage was induced by intraperitoneal administration of hexachloro-1:3-butadiene (HCBD; an industrial nephrotoxin) at 10, 25, and 50 mg/kg body wt on days 7 and 14. These EG and HCBD concentrations were chosen to span from suboptimal to very low doses as far as effects on crystalluria and TE damage are concerned. Enzymuria, proteinuria, oxaluria, crystalluria, and renal pathology were investigated. All HCBD dosages induced crystalluria in mildly hyperoxaluric rats, but no intrarenal crystals were found. EG alone induced very mild hyperoxaluria but no damage to the renal tubule observable on transmission electron microscopy, and it did not cause crystalluria or intrarenal crystals. HCBD with the concomitant administration of EG caused apoptosis of the TE at the two highest dosages after the second injection. Apoptosis did not correlate with crystalluria. A TE toxin is needed for crystallogenesis to occur in borderline metabolic conditions. It may take more than just a metabolic predisposition for calcium nephrolithiasis to occur, and the second hit could come from an environmental pollutant such as HCBD.

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.

Insuffisance rénale terminale d'origine lithiasique : fréquence, causes et prévention

Nephrolithiasis still remains a too frequent - and under-appreciated - cause of end-stage renal disease (ESRD), and this is all the most unfortunate since such an untoward course is now preventable in most cases. Among 1391 patients who started maintenance dialysis at Necker hospital between 1989 and 2000, nephrolithiasis was identified as the cause of ESRD in 45 of them, an overall prevalence of 3.2%. Infection stones accounted for 42.2% of cases, calcium stones for 26.7%, uric acid stones for 17.8% and hereditary diseases for 13.3%. The proportion of nephrolithiasis-associated ESRD declined from 4.7% to 2.2% from the 1989-1991 to the 1998-2000 period, as a result of the decreased incidence of ESRD in patients with infection and calcium nephrolithiasis. Based on our observations and on published reports, it emerges that most cases of nephrolithiasis-associated ESRD were due to sub-optimal management (especially in the case of infection or cystine stones) or to late (or erroneous) etiologic diagnosis, precluding early institution of appropriate therapeutic measures. In particular, several patients with primary hyperoxaluria or 2,8-dihydroxyadeninuria were diagnosed while already on dialysis or after unsuccessful kidney transplantation, due to wrong initial diagnosis. In conclusion, thanks to recent advances in diagnosis and management of stone formers, ESRD should now be prevented in the great majority of patients, at the condition of early etiologic diagnosis based on accurate morphoconstitutional analysis of calculi and metabolic evaluation, and early implementation of appropriate preventive medical treatment.