Severity of phenotype in cystinosis varies with mutations in the CTNS gene: predicted effect on the model of cystinosin

Abstract

Infantile nephropathic cystinosis is a rare, autosomal recessive disease caused by a defect in the transport of cystine across the lysosomal membrane and characterized by early onset of renal proximal tubular dysfunction. Late-onset cystinosis, a rarer form of the disorder, is characterized by onset of symptoms between 12 and 15 years of age. We previously characterized the cystinosis gene, CTNS, and identified pathogenic mutations in patients with infantile nephropathic cystinosis, including a common, approximately 65 kb deletion which encompasses exons 1-10. Structure predictions suggested that the gene product, cystinosin, is a novel integral lysosomal membrane protein. We now examine the predicted effect of mutations on this model of cystinosin. In this study, we screened patients with infantile nephropathic cystinosis, those with late-onset cystinosis and patients whose phenotype does not fit the classical definitions. We found 23 different mutations in CTNS; 14 are novel mutations. Out of 25 patients with infantile nephropathic cystinosis, 12 have two severely truncating mutations, which is consistent with a loss of functional protein, and 13 have missense or in-frame deletions, which would result in disruption of transmembrane domains and loss of protein function. Mutations found in two late-onset patients affect functionally unimportant regions of cystinosin, which accounts for their milder phenotype. For three patients, the age of onset of cystinosis was <7 years but the course of the disease was milder than the infantile nephropathic form. This suggests that the missense mutations found in these individuals allow production of functional protein and may also indicate regions of cystinosin which are not functionally important.

Mutational Spectrum of the CTNS Gene in Egyptian Patients with Nephropathic Cystinosis

“…Ten out of 15 patients had homozygous mutations. Six previously reported CTNS mutations were detected in our study: c.829dup; p.T277NfsX19 (Besouw et al 2012), c.922G>A; G308R (Shotelersuk et al 1998), 809_811delCCT; p.S270del (Attard et al 1999), c.15G>A; p.W5X , c.681G>A; E227E (Aldahmesh et al 2009), and c.1015G>A; p.G339R (Shotelersuk et al 1998). All these mutations were previously associated with infantile nephropathic cystinosis in various populations.…”

“…Likewise, the small deletion c.809_811del; p. S270del was tested functionally and resulted in almost complete loss of function (Kalatzis et al 2004). The most widely distributed among the six previously reported mutations in Egyptian patients was c.1015G>A; p. G339R, as it was reported in Turkey (Topaloglu et al 2012), Italy (Mason et al 2003), Germany (Kiehntopf et al 2002), France (Attard et al 1999), Spain (Macías-Vidal et al 2009), the USA (Shotelersuk et al 1998), and Canada (Rupar et al 2001), followed by c.922G>A; p.G308R reported in Saudi Arabia (Aldahmesh et al 2009), Italy (Mason et al 2003), France (Attard et al 1999), Spain (Macías-Vidal et al 2009), and the USA (Shotelersuk et al 1998). The mutation c.809_811del; p.S270del was also relatively widespread, as it was reported in France (Attard et al 1999) and India (Tang et al 2009) (Fig.…”

“…The CTNS genotype is definitely an important one (Attard et al 1999); however, delayed diagnosis in many of our patients, underdosage due to financial or logistical problems, and differences in response to cysteamine therapy may all be implicated (Soliman et al 2013). …”

“…The novel mutations found in this study include the frameshift mutation (c.260_261delTT; p.F87SfsX36) detected in patient 8, converting phenylalanine at position 87 to serine and resulting in a truncated protein at AA 123, completely abolishing the protein function. Likewise, the other novel frameshift mutations detected in sisters 11a and 11b (c.1032delCinsTG; p.F345CfsX19) resulted in the conversion of phenylalanine to cysteine at position 345 and a truncated protein at AA 364, disrupting the seventh and final transmembrane domain of cystinosin (Attard et al 1999). The nonsense mutation (c.734G>A; p.W245fsX) also detected in patients 11a and 11b led to the conversion of tryptophan at position 245 into a stop codon (TAG), thus immediately stopping the protein translation.…”

“…This mutation replaced the highly conserved glycine at position 362 at the C-terminus of the protein with the basic amino acid arginine. This charge alteration is expected to disrupt the cytosolic lysosomal targeting motif (Attard et al 1999). Although previous intentional sitedirected mutagenesis of C-terminus lysosomal targeting motif led to the redirection of most of cystinosin to the plasma membrane, still partial localization to lysosomes occurred, denoting the presence of a second lysosomal targeting signal, which was identified in the third cytoplasmic loop (YFPQA) extending from AA 281 to 285 (Cherqui et al 2001).…”