Arginine transport through system y(+)L in cultured human fibroblasts: normal phenotype of cells from LPI subjects

In lysinuric protein intolerance (LPI), impaired transport of cationic amino acids in kidney and intestine is due to mutations of the SLC7A7 gene. To assess the functional consequences of the LPI defect in nonepithelial cells, we have characterized cationic amino acid (CAA) transport in human fibroblasts obtained from LPI patients and a normal subject. In both cell types the bidirectional fluxes of arginine are due to the additive contributions of two Na(+)-independent, transstimulated transport systems. One of these mechanisms, inhibited by N-ethylmaleimide (NEM) and sensitive to the membrane potential, is identifiable with system y(+). The NEM- and potential-insensitive component, suppressed by L-leucine only in the presence of Na(+), is mostly due to the activity of system y(+)L. The inward and outward activities of the two systems are comparable in control and LPI fibroblasts. Both cell types express SLC7A1 (CAT1) and SLC7A2 (CAT2B and CAT2A) as well as SLC7A6 (y+LAT2) and SLC7A7 (y+LAT1). We conclude that LPI fibroblasts exhibit normal CAA transport through system y(+)L, probably referable to the activity of SLC7A6/y+LAT2.

Cystathionine beta-synthase mutations in homocystinuria

The major cause of homocystinuria is mutation of the gene encoding the enzyme cystathionine beta-synthase (CBS). Deficiency of CBS activity results in elevated levels of homocysteine as well as methionine in plasma and urine and decreased levels of cystathionine and cysteine. Ninety-two different disease-associated mutations have been identified in the CBS gene in 310 examined homocystinuric alleles in more than a dozen laboratories around the world. Most of these mutations are missense, and the vast majority of these are private mutations. The two most frequently encountered of these mutations are the pyridoxine-responsive I278T and the pyridoxine-nonresponsive G307S. Mutations due to deaminations of methylcytosines represent 53% of all point substitutions in the coding region of the CBS gene.

Smith-Lemli-Opitz syndrome: evidence of T93M as a common mutation of delta7-sterol reductase in Italy and report of three novel mutations

The Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive disorder of cholesterol biosynthesis characterised by facial dysmorphisms, mental retardation and multiple congenital anomalies. SLOS is caused by mutations of the human Delta7-sterol reductase (DHCR7) gene and, so far, 19 different mutations have been described. Among these, mutations impairing the activity of the C-terminus appear to be the most severe. Here we report the mutational analysis of the DHCR7 gene in nine Italian SLOS patients. The T93M mutation, previously reported in one patient, results the most frequent one (7/18 alleles) in our survey. Furthermore, we identified three novel mutations, two missense mutations (N407Y and E448K), and a 33 bp deletion spanning part of exon 5 and the donor splice site of intron 5.

SLC7A8, a gene mapping within the lysinuric protein intolerance critical region, encodes a new member of the glycoprotein-associated amino acid transporter family

Using a bioinformatic approach, we have identified a new transcript, SLC7A8, mapping to 14q11.2, within the lysinuric protein intolerance (LPI) critical region. This gene is highly expressed in skeletal muscle, intestine, kidney, and placenta and encodes a predicted protein of 535 amino acids, homologous to the amino acid permease CD98 light chain and cationic amino acid transporters. RNA in situ hybridization data on mouse embryos confirm the expression in kidney and intestine and, interestingly, reveal that SLC7A8 is also highly expressed in eye, in retinal pigmented epithelium, and in tooth buds at day 16.5 of gestation. Mutational analysis excluded any direct involvement of the SLC7A8 gene product in LPI disease. The homology data and the expression pattern are in agreement with the hypothesis that SLC7A8 represents a novel light chain interacting with the 4F2 heavy chain in the multimeric complex mediating neutral and/or cationic amino acid transport and cystine/glutamate exchange.

Feasibility of prenatal diagnosis of lysinuric protein intolerance by linkage analysis: a case report

Lysinuric protein intolerance (LPI) is a rare autosomal recessive defect of cationic amino acid transport (CAA), relatively common in Finland and Italy. After weaning, LPI patients present poor feeding, vomiting and failure to thrive. A severe pulmonary complication and episodes of metabolic imbalance may lead to death. Prenatal diagnosis has not been available due to lack of either biochemical or molecular markers to be used in the fetal period. The LPI locus has recently been assigned to chromosome 14q12, very close to the T-cell receptor alpha-chain (TCRA) locus. We carried out a prenatal diagnosis for LPI by linkage analysis in one LPI Italian family after CVS. For the haplotype analysis 11 DNA markers from the LPI critical region were used (D14S742, D14S50, D14S283, five TCRA intragenic polymorphic sites, D14S990, MYH7 and D14S80). It was concluded that the haplotype analysis indicated that the fetus was healthy as he had inherited the two wild alleles of the LPI locus. After birth, the clearances of CAA were measured and found to be in the normal range, thus confirming the result of the prenatal diagnosis. The prenatal diagnosis of LPI can now be offered to families affected by LPI.

Mosaicism for the full mutation and a microdeletion involving the CGG repeat and flanking sequences in the FMR1 gene in eight fragile X patients

The molecular mechanism of the fragile X syndrome is based on the expansion of an unstable CGG repeat in the 5' untranslated region of the FMR1 gene in most patients. This expansion is associated with an abnormal DNA methylation leading to the absence of production of FMR1 protein (FMRP). Such expansion apparently predisposes the repeat and flanking regions to further instability that may lead to mosaic conditions with a full mutation and a premutation or, rarely, with normal or reduced alleles that can sometimes be transcriptionally active. In this study we describe eight unrelated fragile X patients who are mosaic for both a full mutation and an allele of normal (four cases) or reduced size (four cases). Sequencing analysis of the deletion breakpoints in 6 patients demonstrated an internal deletion confined to the CGG repeat in four of them, which represents the most likely explanation for the regression of the full mutation to a normal sized allele. In two patients with a reduced allele, the deletion encompassed the entire CGG repeat and part of the flanking regions. Analysis of FMRP by Western blot was performed in one of the mosaics with a normal sized allele and in three of those with a reduced allele. In the first patient's lymphocytes FMRP was detected, whereas in the three other patients the deletion is likely to impair transcription as no FMRP was present in their lymphocytes.

SLC7A7, encoding a putative permease-related protein, is mutated in patients with lysinuric protein intolerance

Lysinuric protein intolerance (LPI, MIM 222700) is an autosomal recessive multisystem disorder found mainly in Finland and Italy. On a normal diet, LPI patients present poor feeding, vomiting, diarrhoea, episodes of hyperammoniaemic coma and failure to thrive. Hepatosplenomegaly, osteoporosis and a life-threatening pulmonary involvement (alveolar proteinosis) are also seen. LPI is caused by defective cationic amino acid (CAA) transport at the basolateral membrane of epithelial cells in kidney and intestine. Metabolic derangement is characterized by increased renal excretion of CAA, reduced CAA absorption from intestine and orotic aciduria. The gene causing LPI was assigned using linkage analysis to chromosome 14q11.2 near the T-cell receptor alpha/delta chains locus, and a critical region has been defined. We have identified two new transcripts (SLC7A8 and SLC7A7) homologous to amino acid transporters, highly expressed in kidney and mapping in the LPI critical region. Mutational analysis of both transcripts revealed that SLC7A7 (for solute carrier family 7, member 7) is mutated in LPI. In five Italian patients, we found either an insertion or deletion in the coding sequence, which provides evidence of a causative role of SLC7A7 in LPI. Furthermore, we detected a splice acceptor change resulting in a frameshift and premature translation termination in four unrelated Finnish patients. This mutation may represent the founder LPI allele in Finland.

Mild phenotype associated with an interstitial deletion of the long arm of chromosome 1

We report on a 21 month old child referred to us because of facial dysmorphism and psychomotor retardation. The patient's phenotype was characterised by a wide and receding forehead, broad nasal bridge, redundant retronuchal skin, low set and poorly shaped ears, micrognathia, and small hands and feet. High resolution R and G banding karyotype analysis of peripheral blood lymphocytes showed an interstitial deletion of the long arm of chromosome 1 spanning bands q22 to q24. The cytogenetic results were confirmed by molecular analysis. The phenotype observed in our patient was relatively milder than those reported in other patients with an interstitial deletion of chromosome 1q.

The C677T mutation of the 5,10-methylenetetrahydrofolate reductase gene is a moderate risk factor for spina bifida in Italy

OBJECTIVE

To estimate the risk for spina bifida associated with the common mutation C677T of the MTHFR gene in a country with a relatively low prevalence of NTDs.

DESIGN

Case-control study.

SUBJECTS

CASES

203 living patients affected with spina bifida (173 myelomeningocele and 30 lipomeningocele); controls: 583 subjects (306 young adults and 277 unselected newborns) from northern and central-southern Italy.

SETTING

CASES

three spina bifida centres; young adult controls: DNA banks; newborn controls: regional neonatal screening centres.

MAIN OUTCOME MEASURES

Prevalence of the C677T genotypes in cases and controls by place of birth; odds ratios for spina bifida and estimated attributable fraction.

RESULTS

The prevalence of T/T, T/C, and C/C genotype was 16.6%, 53.7%, and 29.7% in controls and 25.6%, 43.8%, and 30.6% in cases, respectively. We found no differences between type of defect or place of birth. The odds ratio for spina bifida associated with the T/T genotype v C/C plus T/C was 1.73 (95% CI 1.15, 2.59) and the corresponding attributable fraction was 10.8%. No increased risk was found for heterozygous patients (OR=0.79, 95% CI 0.53-1.18).

CONCLUSION

This study, as well as the meta-analysis we updated, shows that homozygosity for the MTHFR C677T mutation is a moderate risk factor in Europe, and even in Italy where there is a relatively low prevalence of spina bifida. The estimated attributable fraction associated with this risk factor explains only a small proportion of cases preventable by periconceptional folic acid supplementation. Thus, other genes involved in folate-homocysteine metabolism, their interaction, and the interaction between genetic and environmental factors should be investigated further.

Genetic homogeneity of lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is an autosomal recessive disorder in which transport of the cationic amino acids lysine, arginine and ornithine is defective at the basolateral membrane of the epithelial cells in the intestine and renal tubules. LPI is unusually common in Finland, but patients have been described on all continents. Linkage analysis in Finnish LPI families recently assigned the LPI gene locus to a 10 cM interval between markers D14S72 and MYH7 on the long arm of chromosome 14. In the present study linkage analysis of LPI families from six different non-Finnish populations strongly suggests genetic homogeneity in LPI. Peak lod scores were obtained at the chromosomal area between D14S72 and MYH7 with the same markers as in the Finnish families. The non-Finnish families showed no linkage disequilibrium except in an Italian family cluster, whereas strong allelic association in the Finnish families implies that LPI in Finland is caused by a founder mutation.

The gene encoding a cationic amino acid transporter (SLC7A4) maps to the region deleted in the velocardiofacial syndrome

By screening an expressed sequence tag database, we identified a novel human gene, SLC7A4, encoding a solute carrier family 7 [cationic amino acid (CAA) CAT-4 transporter, y+ system] member 4. The SLC7A4 cDNA is 2325 nt long and includes 78, 1911, and 336 nt in the 5' noncoding, coding, and 3'-noncoding regions, respectively. SLC7A4 displays high homology with SLC7A1 and SLC7A2, two previously known CAA transporters. By chromosomal in situ hybridization and YAC identification, SLC7A4 was mapped to 22q11.2, the commonly deleted region of the velocardiofacial syndrome (VCFS, Shprintzen syndrome). In a patient affected by VCFS, deletion of SLC7A4 was demonstrated by chromosomal FISH. By Northern analysis, an abundant transcript was detected in brain, testis, and placenta. Microinjection of SLC7A4 mRNA into Xenopus laevis oocytes demonstrates a significant stimulation of CAA transport.

Clinical aspects of cystathionine beta-synthase deficiency: how wide is the spectrum? The Italian Collaborative Study Group on Homocystinuria

Homocystinuria due to cystathionine beta-synthase (CBS) deficiency is an autosomal recessive disease of sulphur amino acid metabolism. Major clinical manifestations include disorders of the eye, the skeleton, the central nervous system and the vascular system. A wide clinical spectrum of the disease has been reported. We discuss the role of genetic factors (e.g. different mutations of the CBS gene and a variable genetic background) and the importance of environmental factors (e.g. diet, vitamins, perinatal factors and drugs) in explaining the phenotypic variability observed in homocystinuria.

CONCLUSION

Homocystinuria represents a good model to explain the clinical differences frequently observed among patients affected by monogenic diseases.

Combined immunodeficiency phenotype associated with inappropriate spontaneous and activation-induced apoptosis

Programmed death of T cells has been proposed as one of the mechanisms by which HIV induces a decline in the number and functions of T cells in advanced AIDS. In this study we report on a patient affected by a congenital form of combined immunodeficiency presenting as a profound T cell activation deficiency. Subsequently, a gradual loss of T cells occurred, eventually resulting in a classical form of severe combined immunodeficiency (SCID). In this patient a sizeable fraction of apoptotic cells was documented in the first phase of the disease by either propidium iodide staining or DNA fragmentation analysis. The presence of anergic T cells of maternal origin and engrafted in the child was excluded by analysis of DNA polymorphic regions. At 4 years of age the patient died of disseminated interstitial pneumopathy, while still awaiting an HLA-matched bone marrow transplantation. On the occasion of a new pregnancy in the mother, the prenatal immunological evaluation of the female fetus revealed a T B+ SCID phenotype. This is the first observation of a primary immunodeficiency associated with inappropriate apoptosis.

A 68-bp insertion found in a homocystinuric patient is a common variant and is skipped by alternative splicing of the cystathionine beta-synthase mRNA

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Fragile X founder chromosomes in Italy: a few initial events and possible explanation for their heterogeneity

A total of 137 fragile X and 235 control chromosomes from various regions of Italy were haplotyped by analyzing two neighbouring marker microsatellites, FRAXAC1 and DXS548. The number of CGG repeats at the 5' end of the FMR1 gene was also assessed in 141 control chromosomes and correlated with their haplotypes. Significant linkage disequilibrium between some "major" haplotypes and fragile X was observed, while other "minor" haplotypes may have originated by subsequent mutation at the marker microsatellite loci and/or recombination between them. Recent evidence suggests that the initial mechanism leading to CGG instability might consist of rare (10 (-6/-7)) CGG repeat slippage events and/or loss of a stabilizing AGG via A-to-C transversion. Also, the apparently high variety of fragile X chromosomes may be partly due to the relatively high mutation rate (10 (-4/-5)) of the microsatellite markers used in haplotyping. Our fragile X sample also showed a higher than expected heterozygosity when compared to the control sample and we suggest that this might be explained by the chance occurrence of the few founding events on different chromosomes, irrespective of their actual frequency in the population. Alternatively, a local mechanism could enhance the microsatellite mutation rate only on fragile X chromosomes, or fragile X mutations might occur more frequently on certain background haplotypes.

Homocysteine response to methionine challenge in four obligate heterozygotes for homocystinuria and relationship with cystathionine beta-synthase mutations

Fasting and post-methionine load plasma total homocysteine concentrations were investigated in the parents of two homocystinuric patients. Three genetic mutations in the cystathionine beta-synthase gene were found. In the patient of family 1, a frequent Caucasian mutation. T833C, was found on one allele, while the mutation on the other allele has not yet been defined. In the patient of family 2, a mutation C569T, recently described by Sperandeo and colleagues, was found on one allele, while a novel mutation, G346A, was characterized on the other allele. The frequent gene mutation T833C was detected in a heterozygous mother who, surprisingly, exhibited strictly normal fasting and post-methionine load homocysteinaemia. In contrast, in the other family, we found a novel mutation (G346A) in the mother located near Lys 119, the putative binding site of phosphopyridoxal phosphate. This mother exhibited increased fasting and post-methionine load homocysteinaemia. These observations could explain the conflicting results reported for vascular pathologies in parents of homocystinuric patients and direct the search for genetic mutations in these vascular pathologies.

Variable penetrance of hypogonadism in a sibship with Kallmann syndrome due to a deletion of the KAL gene

We report on the clinical and molecular characterization of 3 sibs with X-linked ichthyosis and variable expression of Kallmann syndrome. One of the affected brothers had mild hyposmia and showed normal pubertal progression. However, we demonstrated the same partial deletion of the X-linked Kallmann gene, sparing the first exon in the mildly affected patient as well as in one of his severely affected brothers.

The molecular basis of homocystinuria due to cystathionine beta-synthase deficiency in Italian families, and report of four novel mutations

Four new mutations in the cystathionine beta-synthase (CBS) gene have been identified in Italian patients with homocystinuria. The first mutation is a G-to-A transition at base 374 in exon 3, causing an arginine-to-glutamic acid substitution at position 125 of the protein (R125Q). This mutation has been found in homozygosity in a patient partially responsive to pyridoxine treatment. The second mutation is a C-to-T transition at base 770 in exon 7, causing a threonine-to-methionine substitution at amino acid 257 of the protein (T257M). This mutation has been observed in homozygosity in a patient nonresponsive to the cofactor treatment. The third mutation, found in heterozygosity in a patient responsive to pyridoxine treatment, is an insertion of 68 bp in exon 8 at base 844, which introduces a premature termination codon. The fourth mutation is C-to-T transition in exon 2 at base 262, causing a proline-to-serine substitution at position 88 of the protein (P88S). This mutation is carried on a single allele in three affected sisters responsive to the cofactor treatment. In addition, six previously reported mutations (A114V, E131D, P145L, I278T, G307S, and A1224-2C) have been tested in 14 independent Italian families. Mutations A114V and I278T are carried by three and by seven independent alleles, respectively. The other four mutations--including G307S and A1224-2C, common among northern European patients--have not been detected.