MO038: Mineral bone density assessment in patients affected by cystinuria
Authors: D'ambrosio Viola , Ambrogio Martina , Gremese Elisa , Varriano Valentina , Manuel Ferraro Pietro 
Published: May 2022

Abstract

BACKGROUND AND AIMS
Cystinuria is a rare genetic disease characterized by impaired tubular transport of cystine and other dibasic amino acids that leads to cystine renal calculi. Although being a rare condition, cystinuria is the most frequent monogenic cause of nephrolithiasis accounting for 1% of kidney stones in adults and 8% in children with an average prevalence of 1 in 7000 births. Up to date, clinical features of cystinuria are believed to be attributable only to nephrolithiasis although cystinuric patients may present other comorbidities (chronic kidney disease, progression toward end-stage kidney disease and hypertension). All these features may be partially explained by recurrent nephrolithiasis. There are currently no data on bone features of patients with cystinuria. Data on a murine model demonstrated the presence of reduced bone mineral density irrespective of the presence or absence of uremia, suggesting bone metabolic abnormalities in cystinuria even in the absence of reduced glomerular filtration rate (GFR). Our aim is to characterize bone mineral density (BMD) in patients affected by cystinuria.

METHOD
The present study included 13 patients affected by cystinuria followed at the Rare Kidney Diseases outpatient clinic of our Institution. All eligible patients were more than 18 years of age with an estimated GFR ≥15 mL/min/1.73 m2 and provided written informed consent. Patients who did not sign informed consent, pregnant women, patients undergoing chronic hemodialysis or with a previous kidney transplant were excluded. History and laboratory data were collected during scheduled outpatient visits from September 2021 to December 2021, and all patients underwent a dual-energy X-ray absorptiometry (DEXA). One sample t-tests were used to assess whether BMD Z-scores were significantly different from 0.

RESULTS
Patients’ characteristics are summarized in Table 1. Of the enrolled patients, six (46%) were female and four of them (67%) were in menopause, four (31%) had active nephrolithiasis described as at least two episodes in the last 3 months, and two (15%) had a self-reported diagnosis of osteoporosis. Moreover, six patients (46%) had a positive history for bone fractures that were all self-reported as traumatic. Femoral Z-scores were significantly lower than 0 overall (P = 0.015) and across femoral sites, whereas lumbar Z-scores, although low on average, did not meet statistical significance (Figure 1). Interestingly, reduced BMD was very prevalent, with 10 patients (77%) showing osteopenia and 3 (23%) showing osteoporosis.

CONCLUSION
Our data show a high prevalence of osteopenia and osteoporosis in cystinuric patients, especially when considering that our population was rather young and with a normal or moderately impaired kidney function. Of further interest appears also the differential distribution of decreased BMD that seems to be affecting the femur rather than the lumbar spine. To our knowledge, these are the first data assessing the prevalence and the characteristics of BMD disorder in cystinuric patients.