**4. Genetics of cystinuria**

Cystinuria is an inherited renal stone disorder. Traditionally, a classification system based upon phenotype grouped patients as type I, II or III according to the levels of urinary cystine excreted by their parents, known to be obligate heterozygotes [5]. However, identification of the individual mutations underlying cystinuria, in addition to recognised limitations of the phenotypebased classification, has led to a new genotype-based classification system being introduced [6].

The heterodimeric amino acid transporter responsible for cystine absorption in the renal proximal tubule is formed by two proteins, b0,AT and rBAT, which are joined by a disulphide bridge [7]. The *SLC3A1* gene, located on chromosome 2, encodes rBAT and mutations in both alleles (homozygous or compound heterozygous) of this gene lead to type AA cystinuria. The *SLC7A9* gene, located on chromosome 19, encodes b0,AT and homozygous mutations in this gene produce type B cystinuria. Due to digenic inheritance of two or more mutant alleles there exist much rarer forms, including type AB, type ABB and type AAB cystinuria [8]. Such mutations account for only 2% of cases [7] (**Table 1**). Interestingly, patients with underlying mutations in *SLC3A1* and *SLC7A9* may sometimes present with calcium stones [9]. Genetic screening in paediatric stone formers may allow for precise diagnosis and earlier opportunities for therapeutic and preventative measures to be adopted [10].

Recently, a further membrane protein has been identified which is involved in cystine transport in the S3 (distal) part of the proximal tubule; mutations in the gene encoding this protein could account for further cystinuria cases. The AGT1 protein is encoded by the *SLC7A13* gene, and forms a heterodimer with rBAT to facilitate cystine reabsorption in the S3 part of the proximal tubule [11]. The discovery of AGT1 working alongside rBAT in the S3 section of the proximal tubule may help explain the previously recognised paradox of b0,AT and rBAT being predominantly expressed in different segments of the proximal tubule. It is known that b0,AT expression is highest in the S1 (proximal) segment of the proximal tubule, whilst rBAT expression is mostly in the S3 (distal) segment [11].

The clinical phenotype does not vary between the three recognised subtypes of cystinuria, although male gender and early age of stone onset have been suggested to be poor prognostic signs [1, 4]. As treatment options do not vary between genetic subtypes of cystinuria individual genotyping has not been routinely performed in clinical practice [4].


**Table 1.** Known genetic causes of cystinuria.

crystals appear as flat hexagonal crystals [2]. The recurrent formation of the cystine stones can lead to development of chronic kidney disease [3]. Ornithine, lysine and arginine are more soluble and therefore their excretion in excess concentrations in the urine does not produce clinical sequelae. There are currently no known clinical consequences of impaired absorption

Cystinuria may present with renal calculi at any age, but most patients will present before 30 years of age. Historically, cystinuria was diagnosed mainly through renal calculi analysis and this lead to an underestimation of the incidence of the condition [1]. As methods developed to analyse the cystine concentration in urine samples it became possible to detect cases with confirmed accuracy, and also provided a valuable screening tool. The cyanide-nitroprusside reaction, developed by Brand et al. in 1930, provided a qualitative method of measuring excessive urinary excretion of cystine [1]. This test is positive when the urinary cystine level is greater

Modern diagnostic methods may utilise a combination of laboratory tests, stone composition analysis, radiological investigation and genetic testing. Urinary cystine levels may be precisely measured using mass spectrometry, and are significantly elevated in cystinuria patients [4]. Levels of urinary cystine has also been used historically to detect carrier status, but a molecular genetic diagnosis is more reliable in the modern era. On urine microscopy, hexagonal crystals can be visualised which are pathognomonic [3]. Cystine stones are faintly radioopaque and have a homogenous, ground-glass appearance [4], but may be missed on plain X-ray imaging. Cystine stones are often 100% cysteine but may contain variable amounts of calcium. CT-scanning allows accurate detection and localisation of cystine stones within the

Cystinuria is a rare inherited metabolic disorder, with an estimated incidence of 1:7000 live births [3]. The condition requires lifelong treatment, with the aim of minimising urinary calculi formation. However, the treatments involving high fluid intake, dietary modification, and urinary alkalinisation may be burdensome for some patients and compliance may be difficult. In those patients who continue to form recurrent cystine stones there may be an

Cystinuria is an inherited renal stone disorder. Traditionally, a classification system based upon phenotype grouped patients as type I, II or III according to the levels of urinary cystine excreted

of these four dibasic amino acids within the intestine.

than 75 mg/g creatinine [2].

kidney and urinary tract.

**3. Incidence and outcomes**

**4. Genetics of cystinuria**

associated decline in kidney function over time [5].

**2. Diagnosis of cystinuria: historical and modern**

70 Updates and Advances in Nephrolithiasis - Pathophysiology, Genetics, and Treatment Modalities
