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Juan D. Díaz-Rosales, Lilia Ortiz-Morales, Omar F. Loera
Medical Students
Medical School, Research Department
Instituto de Ciencias Biomédicas/Universidad Autónoma de Cd. Juárez
Corresponding author: Juan D. Díaz-Rosales at Estocolmo y Pronaf s/n CP 32310 Cd. Juárez, Chih. Méx.
Ph (52) 656 1675751 or at juandedios@salud.gob.mx
Abstract:
Cystinosis
is an autosomal-recessive lysosomal storage disease caused by
defective transport of amino acid cystine out of lysosomes.
There are three phenotypes, infantile, late-onset, and benign.
Infantile nephropathic cystinosis is the most common and the
most severe and consist of the development of renal failure by
cystine crystal in renal tissue. This disease is characterized
by renal Fanconi syndrome. The gene of the cystinosis (CTNS) was
isolated in 1998. There are many mutations in specific sites
causing all kinds of cistinosis. Currently, the treatment with
cystemine drug can deplete levels of intralisosomal cystine. In
this review we discussed the recent concepts in this enigmatic
disease.
Keywords:
cystinosis, cystinosin, CTNS, cystine.
Introduction
Cystinosis
is an autosomal recessive disorder with an estimated incidence
of 1 case per 100,000 to 200,000 live births (1). There are
various clinical forms, infantile, juvenile or late-onset, and
ocular or benign, based on age of onset and severity of symptoms
(4). Infantile nephropathic cystinosis is characterized by an
accumulation of cystine crystals within most body tissues (2).
Crystals of cystine are present in lysosomes, bone marrow
aspiration, leukocytes, cornea and conjunctiva (7).
Infantile
nephropathic cystinosis is the most common inherited cause of
the renal Fanconi syndrome. Late-onset (juvenile or adolescent)
cystinosis is much rare form of the disorder, is characterized
by onset of symptoms between 12 at 15 years of age (8). The
ocular form consists solely of a mild photophobia (9).
The gene causative of cystinosis (CTNS)
was isolated in 1998, this gene is on chromosome 17 and codes
for the cystine transport protein, cystinosin (7). This protein
transporter is in lysosomal membrane. The defect in the
transport of cystine caused an accumulation of this molecule,
and, the stored cystine is poorly soluble and crystallizes
within the lisosomes of many cell types.
Cystine
levels are directly proportional to disease severity; however
there is a substantial overlap between cystine content in
patients with all three forms (9).The course of the research on
cystinosis has been altered by two medical innovations. First,
renal transplantation and Second, cysteamine (β-mercaptoethylamine),
the last one has dramatically improved the prognosis for
children with nephropathic cystinosis (1).
Clinical
characteristics
The
disease manifests itself in raised intracellular levels of the
essential amino acid cystine to 50 to 100 times normal level
(7). The initial manifestations of infantile nephropathic
cystinosis generally appear several months after birth (1). The
sing and symptoms are protean, but kidney involvement remains
the foremost clinical characteristic of the disorder (1).
Nephropathic
cystinosis is characterized by renal Fanconi syndrome
(impairment in proximal tubular function), polyuria (excessive
urination), polydipsia (excessive thirst), hypokalaemia (low
levels K+), hypophosphataemia (low levels of PO4), crystals of
cystine present in lysosomes, bone marrow aspirated, leukocytes,
cornea and, conjunctiva, photophobia, headaches, burning or
itching of eyes, growth retardation, rickets, muscle myopathy,
central nervous system involvement, hypothyroidism, and hepatic
and gastrointestinal complications (7). Assadi et al. (14)
reported a case with primary and persistent constipation as an
initial manifestation of cystinosis.
The
majority of children had a history of feeding difficulties and
could present hypotonia, abnormal gag reflex, and throaty or
congested voice (oral motor dysfunction). The feeding
difficulties and oral motor dysfunction are common in children
with cystinosis and appear to correlate with the general degree
of neurologic dysfunction (17).
Patients
with late-onset cystinosis usually present with proteinuria,
glomerular renal impairment but do not do suffer from such
profound tubular dysfunction nor grown retardation (8).
Leukocyte
cystine concentrations in untreated patients are generally
higher in infantile nephropathic cystinosis than late-onset
cystinosis (8).
Many
of the end-organ effects of cystinosis are known to be risk
factors for osteopenia; these include deposition of cystine
crystals in bone, hypothyroidism, diabetes mellitus, primary
hypogonadism, urinary phosphate wasting, and chronic renal
failure (3). While transplantation may correct the latter, it
exposes the child to other risk factors for diminished bone
mass, notably the use of high-dose glucocorticoids (3). That the
majority of cystinotic patients post renal transplant do not
experience reduced bone mineral content as measured by DEXA
(dual-energy X-ray absorptiometry) (3).
Renal
transplantation and oral cysteamine have improved the general
prognosis of the disease, and ocular manifestations are now the
most common complication (2).
In
adulthood patients, with infantile nephropathic cystinosis, more
severe ophthalmic manifestations (corneal crystals, superficial
punctate keratopathy, filamentary keratopathy, severe peripheral
corneal neovascularization, band keratopathy, and posterior
synechiae) are evident (15).
Into
the pulmonary dysfunction in patients with nephropathic
cystinosis after renal transplantation. The distal myopathy
characteristic of nephropathic cystinosis results in an
extraparenchymal pattern of restrictive lung disease in adults
who have not received long-term cystine depletion (18).
Characterization
in cystinosis not is always clear cut and there are patients
whose phenotype does not fit the classical description of either
form (8).
Pathophysiology
Glomerular
damage is present at the time of diagnosis (approximately one
year) is irreversible and may result in the need for renal
transplant (7).
Ingested
protein enters the lysosome, where acid hydrolases degrade it to
its component amino acids, including cysteine. Cysteine is
already oxidized within lisosomes the lysosome to cystine.
Cystine is the disulfide of the amino acid cysteine (4).
Cysteine and cystine can normally enter the cytoplasm, in
cytoplasm; cystine is converted to cysteine by a reducing agent,
glutathione. Cytoplasmic cystine is incorporated into protein or
degraded to inorganic sulfate for excretion. In cystinosis,
cystine can’t leave the lysosomes, because, the protein
transporter is defective.
Proximal
tubules loaded with cystine have a generalized proximal tubule
transport defect characteristic of the Fanconi syndrome (11).
The proximal tubulophaty seen in nephropathic cystinosis may be
a secondary metabolic consequence rather than a direct effect to
cystine storage (9). The
decrease in proximal tubular transport with cystine loading is
due to a decrease in active transport (11).
Previous studies have shown that lysosomes of cystine-loaded
rabbit proximal tubules display a significant reduction in
intracellular ATP concentration, leading to inhibition
NaK-ATPase activity. These data suggest that the mitochondrial
oxidative phosphorylation process responsible for ATP synthesis
is impaired in cystinosis proximal tubular cells (9).
Preservation of intracellular
phosphate at control levels prevents the decrease in
intracellula, ATP and the proximal tubule respiratory
dysfunction with cystine loading (11). The “swan neck”
deformity in cystinosis is an acquired lesion. The
manifestations of the Fanconi syndrome are correlated with the
stages of development of the "swan neck" lesion (10).
In
cystinotic patients, encephalopathy has been reported from 15
years of age, and cystine crystal detected in all portions of
the central nervous system. Myopathy has been described in young
adults, and cystine crystal reported within cells adjacent to
the myocytes (9).
The
cystine accumulation in central nervous system and muscular
tissue could lead to fiber atrophy or necrosis (9).
In
cystinotic children, bone abnormalities are consequent of
hypophosphatemic rickets and hyperparathyroidism secondary to
renal failure. Bone defect due to cystine storage may also
partially explain the poor growth of cystinotic children, which
is more severe than for children with tubulopathy or renal
failure from other etiologies (9).
How
lysosomal cystine causes this multisystemic disorder culminating
in end-stage renal disease is not known. Park et al. (12)
concluded that enhanced apoptosis (in cultured fibroblasts and
renal tubular epithelial cells) resulting from lysosomal cystine
storage may lead to inappropriate cell death and decreased cell
numbers in many tissues and hence contribute to the nephropathic
cystinotic phenotype.
Cetinkaya
et al. (13) suggest that phosphate depletion and dissipation of
the Na(+)-gradient are involved in the development of the
Fanconi syndrome of cystinosis.
The
variant forms may represent co-segregation or linkage of rare
alleles that confer resistance to apoptosis, moderating the cell
loss and causing the milder disease expression (12).
Genetic
In
1998 the causative gene, CTNS (cystine transport nephrotic
syndrome), was identified (4). CTNS is a gene and consist of 12
exons with the first methionine in exon 3. The gene is located
in chromosome 17p and spans 23 kb of genomic DNA (8).
CTNS
encodes cystinosin, a novel seven transmembrane domain (TM)
protein. Cystinosin is a lysosomal membrane protein that
requires two lysosomal targeting signals: a classic GYDQL motif
in its C-terminal tail and a novel conformational motif, the
core of which is YFPQA, situated in the fifth inter-TM loop.
Cystinosin is the lysosomal cystine transporter and its activity
is H(+)-driven (4).
Mutation
analysis of the CTNS gene of Caucasian patients revealed a
common 57-kb deletion, and several other mutations spread
throughout the entire gene (5). Complementary studies and
linkage analysis have demonstrated that infantile and late-onset
cystinosis are allelic, suggesting that phenotype would be
determinated predominantly by different mutations (8).
The
commonest mutation in cystinosis patients is an ~65 kb deletion
encompassing 5’ end of the gene (8). On the basis of
sequencing and PCR analysis, patients with this deletion have an
identical 5’ breakpoint upstream of the CTNS gene and have a
3’ breakpoint in exon 10 of the gene. Individuals who are
homozygous for this deletion are easily detected by PCR
analysis, and all such patients have classical infantile
nephropathic cystinosis, with early onset symptoms and severe
disease (8).
Attard
et al. (8) identified 23 diferent mutations for cystinosis. One
of these mutations, the major~65 kb deletion was identified in
previous studies. Eight of these mutations were identified
previously by Shotelersuk et al. (8) and 14 of these mutation
are new.
Mutations
in the CTNS gene are consistent with the very severe phenotype
of the disease and whit the observations that patients have
completely defective lysosomal cystine transport whereas
obligate heterozygotes exhibit 50% of normal transport activity.
Attard et al. (8) found that all patients that have two
truncating mutations have infantile nephropathic cystinosis.
Of
the two mutations associated with late-onset cystinosis, one is
found near the N-terminus and the other adds three residues to a
cytosolic domain. It has been recognized that there are
individuals for whom the progression of the disease does not
conform to the classical definitions. Presumably, the missense
mutations permit the production of some functional protein,
which would account for the milder phenotypes (8).
The
identifications of the mutations in the CTNS gene in patients
with infantile nephropathic and late-onset cystinosis confirms
earlier work which suggested that these diverse forms were
allelic.
Patients
with two truncating mutations or mutations affecting conserved
amino acids associated with trans-membrane regions of the
protein have the severe, infantile nephropathic cystinotic
phenotype. The late-onset phenotype with generally milder
symptoms and a better prognosis for the patient is associated
with mutations that are located in functionally less important
regions of the protein (8).
Patients,
whose clinical course cannot easily be classified into the usual
phenotypes, have at least one mutation which will result in the
production of some functional protein (8).
The
diagnosis of cystinosis should be entertained in African
Americans with symptoms of the disease, and mutation analysis
for the 57-kb deletion should be considered in this group of
patients (16).
The
discover of CTNS offers the possibility of improved diagnosis
for the disease (through mutation delection) and work is under
way in Europe and in the United States to establish a knock-out
mouse model (an animal with an engineered genetic defect that
results in it displaying the symptoms of cystinosis) (7). A
“mouse with cystinosis” will enable novel therapies to be
evaluated quickly. These animals could be potentially target for
gene therapy; work can begin on correction the defect using a
proper functioning CTNS gene (7). Successful genetic correction
of kidney dysfunction in the cystinotic mouse would be a
necessary preamble to development to gene therapy to human
patients with cystinosis (7).
Diagnosis
A
definitive diagnosis of cystinosis is obtained by assaying for
increased intracellular cystine levels in peripheral blood
leukocytes or fibroblasts (9).
Treatment
Nephropathic
cystinosis is fatal if not treated and death occurs in second
decade of life. Treatment begun just after birth can attenuate
rate of renal failure. Treatment of cystinosis involves
administration of glucose and electrolytes to reverse of effects
of Fanconi syndrome, as well as corneal and renal transplant
(7).
Indomethacin
is administered for its sodium, potassium and water retaining
action. In some patients, carnitine is used to combat the
effects muscle weakness brought about for urinary loss of free
carnitine and subsequent reduction in the transport of the fatty
acids into muscle tissue. Growth hormone may also help improve
growth velocity in children with nephrophatic cystinosis (7).
The
numbers of medicines taken daily by cystinosis patient is often
considerable, especially when serious medical conditions such as
epilepsy or diabetes are present. These conditions may arise
subsequent to cystinosis or may be unrelated to the condition.
The
main drug treatment for cystinosis is administration of the
aminothiol, cysteamine (mercaptamine, as bitarltrate salt,
Cystargon) (7).
Cystargon
acts to lower intracellular levels of cysteine by forming
cysteamine-cysteine mixed disulphide within cells, which is
structurally similar to the amino acid lysine and can egress the
lysosome using the pathway for lysine excretion (1). Cysteamine
possesses an offensive taste and smell and irritates
gastrointestinal tract, leading to nausea, and vomiting
following administration (7). Cysteamine is excreted in breath
and sweat, which leads to halitosis and body odour (1). Some
patient exhibit more serous side effects, such as neutropenia
(7).
Gastrointestinal
symptoms in children with cystinosis receiving cysteamine are
often acid-mediated and improve with omeprazole. Cystine
crystals were detected in the gastrointestinal tract and may
signify inadequate treatment with cysteamine (6).
Photophobia
and corneal infiltration, although generally severe after 15
years of age, could be treated with topical cysteamine and
corneal transplantation (2).
What
else
Cherqui
et al. (9) reported generation of the first known animal model
for cystinosis, this animal model is an essential and unique
tool for testing the ability of emerging therapies to reduce
cystine levels and for comparing their efficiency with that of
the treatment currently in use.
Acknowledgements
We
thank Research Department of UACJ and Dra. Leticia Belmont (Cystinosis
Foundation México).
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