Download Lysinuric protein intolerance: one gene, many

Am J Physiol Cell Physiol 293: C540–C541, 2007;
doi:10.1152/ajpcell.00166.2007.
Editorial Focus
Lysinuric protein intolerance: one gene, many problems
Stefan Bröer
School of Biochemistry and Molecular Biology, Australian National University, Canberra, Australia
Address for reprint requests and other correspondence: S. Bröer, School of
Biochemistry and Molecular Biology, Australian National Univ., Canberra
ACT 0200, Australia (e-mail: [email protected]).
C540
lysine plus citrulline supplementation to treat LPI unfortunately have remained inconclusive due to the small number of
LPI patients (13).
A breakthrough was achieved in 1999 with the identification
of the LPI gene by two independent groups (1, 15). The
transporter belongs to a larger family of heteromeric amino
acid transporters, which consist of a heavy subunit (called 4F2
or CD98, SLC3A2) and a light subunit (7, 14). The light
subunit forms the translocation pore, whereas the heavy subunit is essential for the trafficking of the assembled protein to
the plasma membrane. The heavy subunit 4F2 can associate
with a variety of light chains forming amino acid transporters
of different substrate specificity (19, 20). LPI is exclusively
caused by mutations in the so-called y⫹LAT1 light chain
(SLC7A7). Mutations in the 4F2 heavy chain have not been
observed and would affect a variety of essential amino acid
transport systems. In fact, 4F2 knockout mice die early during
embryogenesis (17). In addition to renal and intestinal epithelia, the y⫹LAT1 light chain is also expressed in lymphocytes,
alveolar macrophages, and epithelial cells of the lung, but not
in the liver (10). The related transporter y⫹LAT2 (SLC7A6)
also forms a heterodimer with 4F2 and is ubiquitously expressed (2). Both transporters export cationic amino acids from
the cell in exchange for neutral amino acids. Due to the low
expression levels of y⫹LAT2 in the intestine and kidney, it
cannot replace the function of y⫹LAT1 (11). LPI is more
frequent in Finland than in other countries. All Finnish LPI
patients carry a founder mutation that is persistent in this
population (1, 15). Despite their genetic commonality, Finnish
LPI patients show large variability of their clinical symptoms,
ranging from normal growth and little protein aversion to cases
with severe protein intolerance, an enlarged liver and spleen,
osteoporosis, and alveolar proteinosis.
The expression pattern of y⫹LAT1 suggests explanations for
some of the clinical symptoms of LPI. Reduced clearance of
lung surfactant by alveolar macrophages has been implicated in
the generation of alveolar proteinosis (16). If y⫹LAT1 expression is essential for alveolar macrophage function, mutations
would incapacitate macrophages and allow the build up of lung
surfactant proteins. The y⫹LAT1 protein is also found in blood
lymphocytes. The expression in these cells could be related to
the enlarged spleen observed in LPI patients, but the precise
cause is unknown. Many patients with LPI show signs of
disturbed immune function. Humoral immune responses are
defective in some patients, and Varicella viral infections may
be exceptionally severe (13).
The unresolved questions of LPI pathology, together with
the possibility of controlled studies using different dietary
supplements, call for a mouse model of this disorder. Sperandeo et al. (12a) reported on the generation of a slc7a7
nullizygous mouse. In contrast to humans, slc7a7⫺/⫺ mice
experience intrauterine growth restriction, resulting in small
pups, which were mostly cannabalized by their mothers. As a
result, only two pups could be rescued and studied further.
These two animals showed symptoms that closely resembled
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AMINO ACIDS are efficiently absorbed in intestine and kidney by
epithelial cells endowed with transporters for specific groups of
amino acids. Inherited renal aminoacidurias arise as a result of
mutations inactivating apical or basolateral transport systems.
Although aminoacidurias are diagnosed by urine amino acid
analysis, most of the disorders affect both intestinal and renal
transport. In the intestine, peptide transporters and amino acid
transporters work in parallel in the apical membrane to aid in
protein absorption. Mutations in apical transporters, as a result,
show predominantly renal phenotypes because intestinal malabsorption is compensated by peptide transport (5). Lysinuric
protein intolerance (LPI), by contrast, is an autosomal recessive disorder affecting the basolateral transporter for cationic
amino acids in the kidney and intestine (12). It is generally
assumed that a block of basolateral membrane transport is
more severe than mutations affecting apical transport because
peptides are largely hydrolyzed inside epithelial cells and,
therefore, no shunt pathway exists in protein absorption across
the basolateral membrane (8). The phenotype of LPI has
puzzled researchers for many years. It is a multiorgan disorder
with a variety of clinical symptoms. Nausea and vomiting are
observed as acute symptoms after protein ingestion. Chronic
symptoms include an enlarged liver and spleen, growth retardation, muscle weakness, and osteoporosis. Some patients also
develop renal insufficiency and pulmonary alveolar proteinosis, a diffuse lung disorder characterized by lipoprotein deposits derived from surfactant. LPI is diagnosed by the presence of
excessive amounts of dibasic amino acids in the urine combined with high plasma ammonia levels, particularly after
protein ingestion. Lysine levels in the urine are more elevated
than other cationic amino acids (6). A variety of mechanisms
are discussed to explain the multiplicity of symptoms. The
poor intestinal absorption of arginine, ornithine, and lysine
combined with their loss in the kidney causes low levels of
these amino acids in the blood of LPI patients. This, in turn,
results in low activity of the urea cycle, causing hyperammonemia. The above-mentioned acute symptoms after protein ingestion, such as nausea and vomiting, are likely to be caused by
this mechanism. LPI is therefore treated by strict avoidance of
protein-rich food and supplementation of citrulline. Citrulline
is absorbed by neutral amino acid transporters and can subsequently replenish the urea cycle (9). The reduced capacity of
the urea cycle probably also causes a compensatory enlargement of the liver. Reduced growth, alveolar proteinosis, and
osteoporosis are more difficult to explain. These symptoms are
not alleviated by citrulline supplementation, and it is rather
likely that they are related to reduced lysine availability (13).
Lysine is important for the cross-linking of collagen fibrils, and
low plasma lysine concentrations appear also to be correlated
with reduced levels of growth hormone (4). Studies using
Editorial Focus
C541
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those of LPI. Using a protein-reduced diet and citrulline, the
animals survived, and the female mouse could even produce
offspring. Returning the animals to a protein-rich diet caused
progressive hypotonia, tremors, and weight loss followed by
death. It appears likely that the death of these animals was
caused by acute hyperammonemia leading to brain edema and
coma (18). In humans with LPI, intrauterine development
appears to be normal. Symptoms are usually not observed
while breastfeeding but appear after weaning (6). Further
investigation of the intrauterine growth restriction in the mice
showed that expression of Igf-1 and Igf-2 was significantly
reduced in slc7a7⫺/⫺ animals. Growth hormone exerts many of
its effects via stimulation of Igf1–2 released from the liver and
other tissues. Interestingly, it appears that high concentrations
of cationic amino acids in blood plasma increase the secretion
of growth hormone. In agreement with this notion, growth
hormone deficiency has been reported in a patient with LPI (3).
Growth restriction in humans with LPI appears to be a postnatal event, whereas retarded growth of mice is already evident
in utero.
Microarray expression analysis revealed a number of compensatory changes in slc7a7⫺/⫺ mice. The cationic amino acid
transporter Cat-2 (slc7a2) was upregulated in liver tissue, most
likely improving capture of arginine and ornithine from the
blood plasma. The related y⫹LAT2 (slc7a6) transporter was
found to be upregulated, but this compensation did not prevent
LPI. Surprisingly, it was found that the intestinal Na⫹-Pi
cotransporter (slc34a2) was strongly downregulated, which
may contribute to osteoporosis and reduced growth. However,
the relevance of this finding needs further investigation in view
of normal plasma and urine phosphate levels in LPI patients
(12). The slc7a7 nullizygous mouse provides a promising start
to the investigation of LPI pathology and will now allow many
of these questions to be addressed experimentally. In the longer
term, this mouse model may allow improved treatment of
patients with LPI, but progress will be accelerated if the
severity of the mouse phenotype can be attenuated by breeding
onto a different genetic background or by generating a conditionally nullizygous mouse.