Amino acid imbalance in cystinuria

Bone mineral density assessment in patients with cystinuria

BACKGROUND

Cystinuria is a rare genetic disease characterized by impaired tubular transport of cystine. Clinical features of cystinuria mainly include nephrolithiasis and its complications, although cystinuric patients may present with other comorbidities. There are currently no data on bone features of patients with cystinuria. Our aim is to characterize bone mineral density (BMD) in cystinuria.

METHODS

Our study included adult cystinuric patients with estimated glomerular filtration rate (eGFR) ≥ 60 mL/min/1.73 m2 followed at 3 specialized outpatient clinics in Italy (Rome, Naples and Verona). Markers of bone turnover were analyzed in a centralized laboratory. Clinical, biochemical and dual-energy X-ray absorptiometry (DEXA) data were collected from September 2021 to December 2022. Linear regression models were used to evaluate statistically significant deviations from zero of Z-scores.

RESULTS

Twenty-seven patients were included in the study. Mean (SD) age was 37 (15) years, 41% were women. Mean estimated glomerular filtration rate was 99 mL/min/1.73 m2. Serum parameters associated with bone turnover (parathyroid hormone, FGF23, calcium and phosphate) were all in the normal range, with only 4 patients showing mild hypophosphatemia. Prevalence of low bone mineral density, defined as Z-score ≤  - 2 at any site, was 15%. Average Z-scores were negative across most sites.

CONCLUSIONS

Our study suggests that cystinuric patients have lower bone mineral density compared with individuals of the same sex and age, even when their kidney function is normal.

Cystinuria: genetic aspects, mouse models, and a new approach to therapy

Cystinuria, a genetic disorder of cystine transport, is characterized by excessive excretion of cystine in the urine and recurrent cystine stones in the kidneys and, to a lesser extent, in the bladder. Males generally are more severely affected than females. The disorder may lead to chronic kidney disease in many patients. The cystine transporter (b^0,+) is a heterodimer consisting of the rBAT (encoded by SLC3A1) and b^0,+AT (encoded by SLC7A9) subunits joined by a disulfide bridge. The molecular basis of cystinuria is known in great detail, and this information is now being used to define genotype-phenotype correlations. Current treatments for cystinuria include increased fluid intake to increase cystine solubility and the administration of thiol drugs for more severe cases. These drugs, however, have poor patient compliance due to adverse effects. Thus, there is a need to reduce or eliminate the risks associated with therapy for cystinuria. Four mouse models for cystinuria have been described and these models provide a resource for evaluating the safety and efficacy of new therapies for cystinuria. We are evaluating a new approach for the treatment of cystine stones based on the inhibition of cystine crystal growth by cystine analogs. Our ongoing studies indicate that cystine diamides are effective in preventing cystine stone formation in the Slc3a1 knockout mouse model for cystinuria. In addition to crystal growth, crystal aggregation is required for stone formation. Male and female mice with cystinuria have comparable levels of crystalluria, but very few female mice form stones. The identification of factors that inhibit cystine crystal aggregation in female mice may provide insight into the gender difference in disease severity in patients with cystinuria.

Adverse gastrointestinal effects of arginine and related amino acids

Oral supplements of arginine and citrulline increase local nitric oxide (NO) production in the small intestine and this may be harmful under certain circumstances. Gastrointestinal toxicity was therefore reviewed with respect to the intestinal physiology of arginine, citrulline, ornithine, and cystine (which shares the same transporter) and the many clinical trials of supplements of the dibasic amino acids or N-acetylcysteine (NAC). The human intestinal dibasic amino acid transport system has high affinity and low capacity. L-arginine (but not lysine, ornithine, or D-arginine) induces water and electrolyte secretion that is mediated by NO, which acts as an absorbagogue at low levels and as a secretagogue at high levels. The action of many laxatives is NO mediated and there are reports of diarrhea following oral administration of arginine or ornithine. The clinical data cover a wide span of arginine intakes from 3 g/d to>100 g/d, but the standard of reporting adverse effects (e.g. nausea, vomiting, and diarrhea) was variable. Single doses of 3-6 g rarely provoked side effects and healthy athletes appeared to be more susceptible than diabetic patients to gastrointestinal symptoms at individual doses>9 g. This may relate to an effect of disease on gastrointestinal motility and pharmacokinetics. Most side effects of arginine and NAC occurred at single doses of >9 g in adults (>140 mg/kg) often when part of a daily regime of approximately>30 g/d (>174 mmol/d). In the case of arginine, this compares with the laxative threshold of the nonabsorbed disaccharide alcohol, lactitol (74 g or 194 mmol). Adverse effects seemed dependent on the dosage regime and disappeared if divided doses were ingested (unlike lactitol). Large single doses of poorly absorbed amino acids seem to provoke diarrhea. More research is needed to refine dosage strategies that reduce this phenomenon. It is suggested that dipeptide forms of arginine may meet this criterion.