Cystinuria in childhood and adolescence: recommendations for diagnosis, treatment, and follow-up

[SLC7A9 gene variation: impact of 13 frequent mutations in the etiology of cystinuria in a Spanish Mediterranean population]

BACKGROUND AND OBJECTIVE

The aim of this study was to investigate the presence of the most prevalent mutation in the SLC7A9 gene in families of the Mediterranean Spanish population and their association with clinical phenotypes.

PATIENTS AND METHOD

Twenty cystinuria families were studied (6 type I, 12 non type I, and 2 unknown type), including 48 cystinuria patients and 44 relatives. DNA was isolated and molecular analysis of 13 variations (P52L, N58_G79del22, G63R, G105R, T123M, V170M, A182T, V188M, c.614dupA, G259R, L283F, A316V and R333W) in the SLC7A9 gene was undertaken. Association studies between these mutations and urinary aminoacid concentrations, stones, urinary infections, colics and other clinical traits were carried out.

RESULTS

Of the 13 investigated mutations, the most prevalent mutation in cystinuria patients was c.614dupA (17.1%), which was found in 13 patients in heterozygous state (17.1%) and in 2 relatives, all of them belonging to 4 non type I families. Mutations G105R (9.2%), T123M (3.9%) and N58_G79del22 (2.6%) were detected only in non type I cystinuria patients. Meanwhile, a R333W carrier allele was found in a patient of a unknown family, and a G105R allele in a relative of a non type I family. No mutation was found in type I families and no patients with mutations in both SLC3A1 and SLC7A9 genes were found in any family.

CONCLUSIONS

Although we have not carried out the whole screening of SLC7A9 gene, the detection rate of variations in SLC7A9 gene suggests a greater impact of this gene in the etiology of cystinuria in our population than variations in the previously screened SLC3A1 gene. The wide variation of phenotypical traits in subjects of families with the same mutations suggests that further investigation of other genetic and/or environmental factors should be carried out.

Reduced renal function and benefits of treatment in cystinuria vs other forms of nephrolithiasis

A significant study from the USA compares cystine stone formers and routine stone formers; the former group had a higher requirement for therapeutic procedures, but this was less if they took chelating agents, although remaining higher than in the latter group. Other interesting findings are also presented.

OBJECTIVE

First, to compare two types of stone formers (SF), those with cystinuria and those without, for effects of treatments for stones, as cystinuria leads to recurrent stones that are difficult to fragment with shock-wave lithotripsy, and there is disagreement about the efficacy of current treatments. Second, to compare these two groups with respect to blood pressure (BP) and renal function, as cystine stones may be associated with more morbidity than are routine stones.

PATIENTS AND METHODS

Fifty-two cystinuric patients (cystine SF) entering our programme since 1970 were compared with 3215 SF without cystinuria (routine SF), of whom 114 had a single functioning kidney (routine SF + nephrectomy). All patients had three 24-h urine and blood samples taken to determine the risk of stones before their first clinic visit; these studies were repeated after therapy was initiated, and at regular intervals to monitor therapy. Cystine was measured in the urine samples of the cystine SF. All stone-related procedures were recorded, and BP measured at clinic visits. Creatinine clearances (CCr) were calculated from each set of serum and urine values. Cystine supersaturation (SS) was directly measured in 16 urine samples collected before treatment and 13 afterward.

RESULTS

Patients were treated with increased fluid intake, potassium alkali and chelating agents such as alpha-mercapto-propionyl-glycine, as needed. The mean (sd) CCr, corrected for age and gender, was significantly lower at entry in cystine SF than in routine SF, at 91 (6) vs 160 (1) L/day, respectively (P < 0.001), and remained so at the last CCr. Neither systolic nor diastolic BP, similarly corrected, differed between the groups, but cystine SF had significantly more procedures, corrected for time at risk, before treatment than did routine SF, at 4.0 (0.4) vs 1.86 (0.06), respectively (P < 0.001); time-adjusted procedures decreased significantly in both groups during treatment, but remained higher in cystine SF, at 0.88 (0.14) vs 0.23 (0.02), respectively, (P < 0.001). Urine volume and pH were significantly higher in cystine SF than in routine SF, both before and during treatment. Cystine SS decreased during treatment, consistent with the increase in urine volume and decline in procedure rates during treatment.

CONCLUSION

Cystine SF have significantly higher procedure rates than routine SF, but procedure rates decline during therapy, although they remain higher than in routine SF. The lower CCr in cystinurics suggests that treatment to prevent stone recurrence and the need for procedures is particularly important, and emphasizes the need for a close follow-up. Use of cystine SS measurements may allow closer monitoring of the effect of treatment on the risk of stone recurrence.

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.

Hyper-echogenic colon: a prenatal sign of cystinuria?

We suggest the term 'hyper-echogenic colon' to describe a hyperechoic foetal colonic content with no other intestinal abnormality. This is a rare pattern, which to our knowledge, has never been correlated with a specific pathology. The accidental observation of a cystine kidney stone in an infant who presented with this sign during the prenatal period made it possible to diagnose this disease retrospectively in two other children.