Ontogeny modifies manifestations of cystinuria genes: implications for counseling

Enhancement of β-Lactam-Mediated Killing of Gram-Negative Bacteria by Lysine Hydrochloride

Widespread bacterial resistance among Gram-negative bacteria is rapidly depleting our antimicrobial arsenal. Adjuvants that enhance the bactericidal activity of existing antibiotics provide a way to alleviate the resistance crisis, as new antimicrobials are becoming increasingly difficult to develop. The present work with Escherichia coli revealed that neutralized lysine (lysine hydrochloride) enhances the bactericidal activity of β-lactams in addition to increasing bacteriostatic activity. When combined, lysine hydrochloride and β-lactam increased expression of genes involved in the tricarboxylic acid (TCA) cycle and raised reactive oxygen species (ROS) levels; as expected, agents known to mitigate bactericidal effects of ROS reduced lethality from the combination treatment. Lysine hydrochloride had no enhancing effect on the lethal action of fluoroquinolones or aminoglycosides. Characterization of a tolerant mutant indicated involvement of the FtsH/HflkC membrane-embedded protease complex in lethality enhancement. The tolerant mutant, which carried a V86F substitution in FtsH, exhibited decreased lipopolysaccharide levels, reduced expression of TCA cycle genes, and reduced levels of ROS. Lethality enhancement by lysine hydrochloride was abolished by treating cultures with Ca2+ or Mg2+, cations known to stabilize the outer membrane. These data, plus damage observed by scanning electron microscopy, indicate that lysine stimulates β-lactam lethality by disrupting the outer membrane. Lethality enhancement of β-lactams by lysine hydrochloride was also observed with Acinetobacter baumannii and Pseudomonas aeruginosa, thereby suggesting that the phenomenon is common among Gram-negative bacteria. Arginine hydrochloride behaved in a similar way. Overall, the combination of lysine or arginine hydrochloride and β-lactam offers a new way to increase β-lactam lethality with Gram-negative pathogens. IMPORTANCE Antibiotic resistance among Gram-negative pathogens is a serious medical problem. The present work describes a new study in which a nontoxic nutrient increases the lethal action of clinically important β-lactams. Elevated lethality is expected to reduce the emergence of resistant mutants. The effects were observed with significant pathogens (Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa), indicating widespread applicability. Examination of tolerant mutants and biochemical measurements revealed involvement of endogenous reactive oxygen species in response to outer membrane perturbation. These lysine hydrochloride-β-lactam data support the hypothesis that lethal stressors can stimulate the accumulation of ROS. Genetic and biochemical work also revealed how an alteration in a membrane protease, FtsH, abolishes lysine stimulation of β-lactam lethality. Overall, the work presents a method for antimicrobial enhancement that should be safe, easy to administer, and likely to apply to other nutrients, such as arginine.

Newborn screening in Spain, with particular reference to Galicia: Echoes of Louis I. Woolf

After briefly recalling the main events leading to the establishment of newborn screening programmes, this paper details the early history of their introduction in Spain and sketches their expansion to cover the whole Spanish population. Spain is exceptional in that its screening methods have in general been based on planar chromatographic techniques developed or inspired by Louis I. Woolf, rather than on bacterial inhibition tests, as is illustrated by the practice of the newborn screening laboratory of Galicia (N.W. Spain).

Cystinuria

Cystinuria is an inherited disorder characterized by the impaired reabsorption of cystine in the proximal tubule of the nephron and the gastrointestinal epithelium. The only clinically significant manifestation is recurrent nephrolithiasis secondary to the poor solubility of cystine in urine. Although cystinuria is a relatively common disorder, it accounts for no more than 1% of all urinary tract stones. Thus far, mutations in 2 genes, SLC3A1 and SLC7A9, have been identified as being responsible for most cases of cystinuria by encoding defective subunits of the cystine transporter. With the discovery of mutated genes, the classification of patients with cystinuria has been changed from one based on phenotypes (I, II, III) to one based on the affected genes (I and non-type I; or A and B). Most often this classification can be used without gene sequencing by determining whether the affected individual's parents have abnormal urinary cystine excretion. Clinically, insoluble cystine precipitates into hexagonal crystals that can coalesce into larger, recurrent calculi. Prevention of stone formation is the primary goal of management and includes hydration, dietary restriction of salt and animal protein, urinary alkalinization, and cystine-binding thiol drugs.

Heterogeneous mutations in the SLC3A1 and SLC7A9 genes in Chinese patients with cystinuria

Cystinuria is a recessively inherited aminoaciduria that leads to recurrent urolithiasis. It is caused by the defective transport of cystine and dibasic amino acids in the proximal renal tubules and intestinal epithelium. Two genes responsible for this, SLC3A1 and SLC7A9, are known. Patients with two SLC3A1 mutations are classified as type A cystinuria, whereas patients with two SLC7A9 mutations are classified as type B cystinuria. Few clinical and molecular data have been reported for Asian cystinuria patients. In this study, we determined the molecular basis of cystinuria in eight unrelated Chinese subjects. Coding exons and flanking introns of the SLC3A1 and SLC7A9 genes were directly sequenced after amplification by polymerase chain reaction. Five different SLC3A1 mutations were found. Two missense mutations, D210G and S547L, were novel. The other three SLC3A1 mutations (IVS6+2T>C, R181Q and R365W) have been described previously. In addition, four novel SLC7A9 mutations, C137R, c.730delG, IVS10+2_3delTG and IVS12+3insT, together with two previously reported mutations (A70V and G195R) were found. All patients except one carried compound heterozygous mutations. IVS12+3insT was detected in patients from two families. This is the first molecular genetic study on Chinese cystinuria patients. Three patients with type A cystinuria, two with type B cystinuria, and three carriers of type B cystinuria were identified. Our results suggest that the molecular basis of cystinuria is heterogeneous in our local population.

Transient neonatal cystinuria

BACKGROUND

Cystinuria is an inherited disorder of luminal reabsorptive transport for cystine and dibasic amino acids in the renal proximal tubule. Two cystinuria genes have been identified. Mutations of SLC7A9, which encodes the luminal transport channel itself, tend to be dominant and mutations of SLC3A1 (rBAT), which encodes a transporter subunit, are always recessive. Patients who inherit two recessive mutations or two dominant mutations have equally severe forms of cystinuria. Heterozygotes excrete cystine in the normal (type I), moderate (type III), or high stone-forming (type II) range.

METHODS

Infants with cystinuria were identified via the Quebec Newborn Urinary Screening Program. In a subgroup of these infants, cystinuria was severe in the first months of life, but partially resolved by 2 to 4 years postnatally. We assigned each patient a final cystinuria phenotype at 3 to 4 years. In addition, we characterized SLC3A1 gene expression in fetal and postnatal human kidney.

RESULTS

Most infants with transient neonatal cystinuria are eventually classified as type III heterozygotes. All infants with mutant cystinuria genes have exaggerated neonatal cystine excretion except those who inherit two SLC3A1 mutations (type I/I cystinuria); these children have persistent severe cystinuria, implying that wildtype SLC3A1 is required for the maturational effect. Expression of SLC3A1 mRNA was found to be tenfold higher in postnatal vs. fetal kidney; SLC3A1 expression is doubled by the proximal tubule transcription factor, PAX8. rBAT is expressed in the proximal convoluted and straight tubules in both fetal and adult kidney.

CONCLUSION

Maturation of SLC3A1 gene expression between midgestation and 4.5 years postnatal age may account for transient neonatal cystinuria.

Pilot screening programme for cystinuria in the Valencian community

Cystinuria is an autosomal recessive disorder of the kidneys and small intestine, affecting a luminal transport mechanism shared by cystine, ornithine, arginine and lysine. When cystine exceeds its solubility at low pH, the risk of stone formation increases. The data reported in the literature show a variation for the incidence of cystinuria, from 1 in 600 to 1 in 17,000, depending on the definition of cystinuria and the method used for screening the population. We set up a pilot screening programme to determine the incidence of cystinuria in the population of the Valencian Community. Urine filter paper samples submitted for the neonatal screening programme from 33,995 newborns (5-10 days old) were used for the study. Thin layer chromatography (TLC) was performed to screen cystinuric patients. To confirm positive filter paper samples, liquid samples were requested and TLC as well as the cyanide-nitroprusside test (CNT) were performed. Final diagnosis was achieved by quantifying cystine, lysine, ornithine and arginine using high-performance liquid chromatography (HPLC) in children's urine samples which remained positive for TLC and CNT for more than 1 year. We conclude that the incidence of subjects at risk for cystine stones in the Valencian Community is 1:1887. TLC is shown as a reliable method to perform newborn screening in large population to detect cystinuric subjects. Additional studies, including characterization of appropriate haplotypes, should be carried out for a more precise identification of the frequency of the different types of cystinuria in our population.

Reference values of urinary excretion of cystine and dibasic aminoacids: classification of patients with cystinuria in the Valencian Community, Spain

OBJECTIVE

Cystinuria is an autosomal-recessive disorder of the kidneys and small intestine affecting a luminal transport mechanism shared by cystine, ornithine, arginine, and lysine. Three different types of cystinuria can be distinguished according to the excretion of these amino acids in urine samples. We propose cutoff values from our population as references and we present a classification of cystinuric patients using quantitative amino acid chromatography in first morning urine samples.

DESIGN AND METHODS

A random sample of forty healthy subjects belonging to general population of the Valencian Community were selected as control subjects. Cystine, lysine, arginine, and ornithine were quantified by reverse-phase HPLC. Seventy-two subjects, diagnosed previously as cystinuric by the cyanide-nitroprusside test were classified. Probands excreting more than 113.12 micromol cystine per mmol of creatinine (i.e., 1,000 micromol cystine per gram of creatinine) were classified as homozygotes. Parents of homozygotes in whom excretion of amino acids were normal were classified as heterozygotes type I. Those probands showing the excretion of at least one amino acid and the sum of urinary cystine plus the basic amino acids higher than the corresponding references ranges in our population were classified as heterozygotes type II or type III (heterozygotes non-type 1).

RESULTS

We identified 24 homozygotes, 39 non-type I heterozygotes and 3 type I heterozygotes. The remaining 6 probands could not be classified. Means for cystine, lysine, arginine ornithine and their sum in homozygotes and heterozygotes non-type I were significantly (p < 0.001) in excess of the respective reference ranges. Moreover, means values in homozygotes were statistically different (p < 0.001) from heterozygotes non-type I.

CONCLUSION

Urinary excretion of cystine per mmol creatinine allow us to distinguish heterozygotes from homozygotes. However, the best discriminator to distinguish non-type I heterozygotes from normal population might be the excretion of lysine per mmol creatinine. Additional studies including characterization of appropriate haplotypes should be carried out for a more precise identification of types of cystinuria.

Molecular genetics of cystinuria: mutation analysis of SLC3A1 and evidence for another gene in type I (silent) phenotype

BACKGROUND

Cystinuria is a hereditary disorder that affects luminal transport of cystine and dibasic amino acids in kidney and small intestine. Three subtypes have been defined on the basis of urinary excretion of cystine in obligate heterozygotes. Mutations in the SLC3A1 gene have been associated with the Type I phenotype.

METHODS

We investigated 20 cystinuria patients from Quebec (8 Type I/I, 9 Type I/III and 3 Type II/N) for mutations in SLC3A1. DNA was studied by Southern blotting and by the single strand conformation polymorphism (SSCP) protocol to identify mutations. Expression of mutations in Xenopus oocytes was performed to confirm the effect of missense mutations on cystine uptake.

RESULTS

Six novel mutations (2 large deletions, a 2 bp deletion and 3 single bp substitutions) were identified on the Type I allele. Four missense mutations (T216M, S217R, R270L and I618M) were expressed in vitro; the first three changes significantly decreased uptake.

CONCLUSIONS

Combined with our previous work, we have identified 15/16 mutations in SLC3A1 on Type I alleles in the eight Type I/I patients, but only one SLC3A1 mutation on the nine Type I alleles of the Type I/III patients. Therefore, we propose that the Type I phenotype could be caused by mutations in other, as yet unidentified cystinuria genes.

Cystinuria subtype and the risk of nephrolithiasis

BACKGROUND

Cystinuria patients may be classified into several subgroups based on the urinary phenotype of heterozygotes. However, the relative risk for nephrolithiasis and the prevalence of SLC3A1 mutations in these subgroups are unknown.

METHODS

Urinary cystine excretion, age at onset of nephrolithiasis and nature of SLC3A1 mutations were assessed prospectively in 23 cystinuria patients identified primarily through the Quebec Newborn Screening Program. Probands were classified as to cystinuria subtype on the basis of parental urinary cystine excretion.

RESULTS

For classical Type I/I cystinuria, both parents excrete cystine in the normal range and probands carry two mutations of the SLC3A1 gene in nearly every case. Between ages 1 to 7 years, mean cystine excretion was high (4566 +/- 480 microns cystine/g creatinine) and exceeded the theoretic threshold for solubility on 70% of visits. Four of eight Type I/I patients began forming stones in the first decade. Type I/III patients (N = 12) excreted less cystine (1544 +/- 163 mumol cystine/g creatinine), exceeded the threshold of urinary cystine solubility less frequently (22% of visits) and had no nephrolithiasis in the first decade; one formed a stone at age 16 years. Only one SLC3A1 mutation was identified in this group. Two Type II/N cystinuria children were identified. In these families, the same level of relatively high excretion (> 600 mumol cystine/g creatinine) was noted in two or three generations, but no SLC3A1 mutations were identified.

CONCLUSIONS

Classical recessive Type I/I cystinuria is genetically and phenotypically distinct from the other subtypes (Type I/III and Type II/N) identified in our population.

The molecular basis of cystinuria: the role of the rBAT gene

The cDNAs of mammalian amino acid transporters already identified could be grouped into four families. One of these protein families is composed of the protein rBAT and the heavy chain of the cell surface antigen 4F2 (4F2hc). The cRNAs of rBAT and 4F2hc induce amino acid transport activity via systems b(0,+) -like and y(+)L -like inXenopus oocytes respectively. Surprisingly, neither rBAT nor 4F2hc is very hydrophobic, and they seem to be unable to form a pore in the plasma membrane. This prompted the hypothesis that rBAT and 4F2hc are subunits or modulators of the corresponding amino acid transporters. The association of rBAT with a light subunit of ~40kDa has been suggested, and such an association has been demonstrated for 4F2hc.The b(0,+)-like system expressed in oocytes by rBAT cRNA transports L-cystine, L-dibasic and L-neutral amino acids with high-affinity. This transport system shows exchange of amino acids through the plasma membrane ofXenopus oocytes, suggesting a tertiary active transport mechanism. The rBAT gene is mainly expressed in the outer stripe of the outer medulla of the kidney and in the mucosa of the small intestine. The protein localizes to the microvilli of the proximal straight tubules (S3 segment) of the nephron and the mucosa of the small intestine. All this suggested the participation of rBAT in a high-affinity reabsorption system of cystine and dibasic amino acids in kidney and intestine, and indicated rBAT (named SLC3A1 in Gene Data Bank) as a good candidate gene for cystinuria. This is an inherited aminoaciduria due to defective renal and intestinal reabsorption of cystine and dibasic amino acids. The poor solubility of cystine causes the formation of renal cystine calculi. Mutational analysis of the rBAT gene of patients with cystinuria is revealing a growing number (~20) of cystinuria-specific mutations, including missense, nonsense, deletions and insertions. Mutations M467T (substitution of methionine 467 residue for threonine) and R270X (stop codon at arginine residue 270) represent approximately half of the cystinuric chromosomes where mutations have been found. Mutation M467T reduces transport activity of rBAT in oocytes. All this demonstrates that mutations in the rBAT gene cause cystinuria.Three types of cystinuria (types, I, II and III) have been described on the basis of the genetic, biochemical and clinical manifestations of the disease. Type I cystinuria has a complete recessive inheritance; type I heterozygotes are totally silent. In contrast, type II and III heterozygotes show, respectively, high or moderate hyperaminoaciduria of cystine and dibasic amino acids. Type III homozygotes show moderate, if any, alteration of intestinal absorption of cystine and dibasic amino acids; type II homozygotes clearly show defective intestinal absorption of these amino acids. To date, all the rBAT cystinuria-specific mutations we have found are associated with type I cystinuria (~70% of the chromosomes studied) but not to types II or III. This strongly suggests genetic heterogeneity for cystinuria. Genetic linkage analysis with markers of the genomic region of rBAT in chromosome 2 (G band 2p16.3) and intragenic markers of rBAT have demonstrated genetic heterogeneity for cystinuria; the rBAT gene is linked to type I cystinuria, but not to type III. Biochemical, genetic and clinical studies are needed to identify the additional cystinuria genes; a low-affinity cystine reabsortion system and the putative light subunit of rBAT are additional candidate genes for cystinuria.

Molecular genetics of cystinuria in French Canadians: identification of four novel mutations in type I patients

Cystinuria, a hereditary disorder of cystine and dibasic amino acid reabsorption, has been classified into three subtypes on the basis of urinary excretion in obligate heterozygous parents. Thirteen cystinuric patients, identified primarily through the Quebec newborn urinary screening program, were investigated by phenotypic classification and by mutational analysis of the D2H (rBAT) gene. Mutations were identified on 7 of 25 alleles; all of these 7 mutant alleles were associated with Type I cystinuria. Four of the mutations (a large deletion, a 5'splice site mutation, a 2 bp deletion, and a nonsense mutation) have not been previously reported. These findings suggest that abnormalities in the D2H gene may account for only one subtype (Type I) of cystinuria, and that this subtype can be caused by a wide variety of population-specific mutations.

Is the broad range amino acid transporter which is induced by a renal microvillar cDNA clone the cystinuria gene?

On expression cloning in Xenopus oocytes, the renal cDNA expressed functionally in both the rat and human proximal tubule yields an amino acid transporter with properties generally consistent with representation of the cystinuria gene.

Prospective analysis and classification of patients with cystinuria identified in a newborn screening program

Patients who inherit mutant cystinuria genes excrete high concentrations of cystine, ornithine, arginine, and lysine in the urine. At least three variants of cystinuria can be distinguished in heterozygotes. To determine whether certain combinations of mutant genes are more disadvantageous than others, we analyzed amino acid excretion in families of 17 probands with cystinuria identified by the Quebec neonatal screening program. Parents of the probands were classified into the three known phenotypes by calculating the sum of cystine, ornithine, arginine, and lysine excretion. Although parents of type I/I homozygotes excreted amounts of cystine in the normal range, their offspring excreted significantly greater amounts of urinary cystine than did children who have type I/III genetic compounds. This observation suggests that types I and III cystinuria mutations might involve two distinct genetic loci. Children with type I/I homozygous cystinuria often excrete cystine at levels greater than the theoretic solubility limit and may be at greatest risk for nephrolithiasis. We outline an approach to monitoring children with cystinuria who come to medical attention before formation of cystine stones.

Urolithiasis in children: current medical management

The mechanism of stone formation in the urinary tract is reviewed. Diet, urinary tract infection and metabolic disorders account for the different epidemiological patterns of stone formation. The diagnosis and management of renal tract calculi are discussed. Calcium stones are associated with hypercalciuria, urine acidification defects, the use of furosemide in premature babies, hypercalcaemia, hyperoxaluria, hyperuricosuria, an alkaline urine and hypocitraturia. Uric acid stones occur in acid urine, from increased purine synthesis with lympho- or myeloproliferative disorders or from several inborn errors of purine metabolism which can also cause xanthine or dihydroxyadenine stones. Cystinuria, inherited as an autosomal recessive disorder is best treated with a low sodium diet, a fluid intake exceeding 40 ml/kg per day maintaining urine pH between 7.5 and 8 and, if necessary, with oral penicillamine. Oxalate stones occur in relation to diet, bowel disease and primary inherited defects in oxalate metabolism. Urinary tract infection causing struvite and carbonate apatite formation is the commonest cause of stones in Europe.

Newborn urine screening experience with over one million infants in the Quebec Network of Genetic Medicine

We screened urine for chemical individuality in over 1 million newborn infants, by various chromatographic (thin-layer), chemical and spectrophotometric methods, 12 procedures in all. The programme is part of the Quebec Network of Genetic Medicine. Voluntary urine screening began in 1971 and has evolved with changes in choice of tests and times of sample collection. Urine samples were collected on filter paper at either 5, 14 or 21 days after birth; results were best with the 21-day test. Compliance is over 94% with the latter and over 98% with requests for repeat samples. Screening is centralized in one laboratory; follow-up diagnosis, counselling and management are done at four regional centres. Incidence of phenotypes ranged from 1:4300 live births (for expressed cystinuria alleles) to 1 per million (for hyperargininaemia). Over 20 inherited Mendelian disorders were identified. 30 patients required aggressive medical management. We show how this programme can be used for neuroblastoma screening.

Heterozygous cystinuria and urinary lithiasis

Cystinuria is a recessively inherited transport disorder, with at least three mutant alleles (I, II, and III) demonstrable. I/I, II/II, and III/III homozygotes and I/II, I/III, and II/III compound heterozygotes (cystinuric patients) have high urinary concentrations of cystine, lysine, arginine, and ornithine and frequently form cystine stones. +/I heterozygotes (nondetectable) are phenotypically normal, whereas +/II and +/III heterozygotes (detectable) show variable increases in urinary cystine and lysine concentration and at times increases in urinary arginine levels. The objectives of the present study were to determine the frequency of +/II heterozygotes among stone-forming and nonstone-forming individuals from the same region of Brazil and to evaluate the possible relationship between heterozygous cystinuria and urinary lithiasis. When urine samples from 5,150 individuals (5,000 nonstone-forming individuals and 150 stone-forming individuals) were screened by the qualitative cyanide-nitroprusside cystine test, by thin-layer amino acid chromatography, and by quantitative amino acid determination by ion-exchange chromatography, 32 +/II or +/III heterozygotes (26 nonstone-forming and six stone-forming individuals) were detected. The frequency of detectable heterozygotes among the stone-forming individuals (1:25) was significantly higher than that among nonstone-forming individuals (1:104), which provides additional evidence that heterozygosity for +/II and +/III cystinuria is a risk factor in the formation of urinary stones. No significant difference was detected in urinary cystine concentration or in terms of the various characteristics of urolithiasis when stone-forming heterozygotes were compared to nonstone-forming heterozygotes. These data suggest that the tendency towards stone-forming among heterozygotes is probably owing to a complex and multifactorial mechanism.