Renal iminoglycinuria without intestinal malabsorption of glycine and imino acids

The SLC6A19 gene mutation in a young man with hyperglycinuria and nephrolithiasis: a case report and literature review

BACKGROUND

Hyperglycinuria is a rare disorder, with few reported cases, caused by either a defect in glycine metabolism or a disturbance in renal glycine reabsorption. Genetic findings of hyperglycinuria are rare and have not previously been reported in Chinese young men.

CASE PRESENTATION

A 24-year-old man presented with a compliant of bilateral lumbago for 1 month. Abdominal computed tomography revealed bilateral kidney stones and right upper ureteral dilatation. The 24-h urine analysis showed high urine oxalate levels of 63 mg/day. Analysis of amino acids in urine revealed that his urinary glycine levels were abnormally high (2.38 µmol/mg creatinine). Whole-exome sequencing detected the SLC6A19 variant c.1278 C > T p. (Cys426). Flexible ureteroscopy with holmium laser lithotripsy was conducted twice to remove his bilateral nephrolithiasis. Postoperative stone biochemical composition analysis revealed that the stones were composed of approximately 70% calcium oxalate monohydrate and 30% calcium oxalate dihydrate. The patient was subsequently diagnosed with hyperglycinuria. Three months after the stone surgery, ultrasonography revealed one nodule under the right thyroid lobe during a health checkup. His serum parathyroid hormone (PTH) levels increased to 392.3 pg/mL. Resection of the right parathyroid nodule was performed, and the histopathological examination confirmed right parathyroid adenoma. During the 2-year follow-up period, nephrolithiasis did not relapse, and serum PTH, calcium, and phosphorus levels were normal.

CONCLUSION

The SLC6A19 gene may have been significant in the development of hyperglycinuria in a Chinese young man. Further evaluation for the possibility of a glycine excretion disorder could be considered when encountering nephrolithiasis.

The Multifaceted Roles of Proline in Cell Behavior

Herein, we review the multifaceted roles of proline in cell biology. This peculiar cyclic imino acid is: (i) A main precursor of extracellular collagens (the most abundant human proteins), antimicrobial peptides (involved in innate immunity), salivary proteins (astringency, teeth health) and cornifins (skin permeability); (ii) an energy source for pathogenic bacteria, protozoan parasites, and metastatic cancer cells, which engage in extracellular-protein degradation to invade their host; (iii) an antistress molecule (an osmolyte and chemical chaperone) helpful against various potential harms (UV radiation, drought/salinity, heavy metals, reactive oxygen species); (iv) a neural metabotoxin associated with schizophrenia; (v) a modulator of cell signaling pathways such as the amino acid stress response and extracellular signal-related kinase pathway; (vi) an epigenetic modifier able to promote DNA and histone hypermethylation; (vii) an inducer of proliferation of stem and tumor cells; and (viii) a modulator of cell morphology and migration/invasiveness. We highlight how proline metabolism impacts beneficial tissue regeneration, but also contributes to the progression of devastating pathologies such as fibrosis and metastatic cancer.

Iminoglycinuria and hyperglycinuria are discrete human phenotypes resulting from complex mutations in proline and glycine transporters

Iminoglycinuria (IG) is an autosomal recessive abnormality of renal transport of glycine and the imino acids proline and hydroxyproline, but the specific genetic defect(s) have not been determined. Similarly, although the related disorder hyperglycinuria (HG) without iminoaciduria has been attributed to heterozygosity of a putative defective glycine, proline, and hydroxyproline transporter, confirming the underlying genetic defect(s) has been difficult. Here we applied a candidate gene sequencing approach in 7 families first identified through newborn IG screening programs. Both inheritance and functional studies identified the gene encoding the proton amino acid transporter SLC36A2 (PAT2) as the major gene responsible for IG in these families, and its inheritance was consistent with a classical semidominant pattern in which 2 inherited nonfunctional alleles conferred the IG phenotype, while 1 nonfunctional allele was sufficient to confer the HG phenotype. Mutations in SLC36A2 that retained residual transport activity resulted in the IG phenotype when combined with mutations in the gene encoding the imino acid transporter SLC6A20 (IMINO). Additional mutations were identified in the genes encoding the putative glycine transporter SLC6A18 (XT2) and the neutral amino acid transporter SLC6A19 (B0AT1) in families with either IG or HG, suggesting that mutations in the genes encoding these transporters may also contribute to these phenotypes. In summary, although recognized as apparently simple Mendelian disorders, IG and HG exhibit complex molecular explanations depending on a major gene and accompanying modifier genes.

Iminoglycinuria and hyperglycinuria are discrete human phenotypes resulting from complex mutations in proline and glycine transporters

Iminoglycinuria (IG) is an autosomal recessive abnormality of renal transport of glycine and the imino acids proline and hydroxyproline, but the specific genetic defect(s) have not been determined. Similarly, although the related disorder hyperglycinuria (HG) without iminoaciduria has been attributed to heterozygosity of a putative defective glycine, proline, and hydroxyproline transporter, confirming the underlying genetic defect(s) has been difficult. Here we applied a candidate gene sequencing approach in 7 families first identified through newborn IG screening programs. Both inheritance and functional studies identified the gene encoding the proton amino acid transporter SLC36A2 (PAT2) as the major gene responsible for IG in these families, and its inheritance was consistent with a classical semidominant pattern in which 2 inherited nonfunctional alleles conferred the IG phenotype, while 1 nonfunctional allele was sufficient to confer the HG phenotype. Mutations in SLC36A2 that retained residual transport activity resulted in the IG phenotype when combined with mutations in the gene encoding the imino acid transporter SLC6A20 (IMINO). Additional mutations were identified in the genes encoding the putative glycine transporter SLC6A18 (XT2) and the neutral amino acid transporter SLC6A19 (B0AT1) in families with either IG or HG, suggesting that mutations in the genes encoding these transporters may also contribute to these phenotypes. In summary, although recognized as apparently simple Mendelian disorders, IG and HG exhibit complex molecular explanations depending on a major gene and accompanying modifier genes.

The transport and metabolism of L-proline-14C in the rat in vivo

The physiologic disposition, metabolic fate, and renal clearance of intravenously injected 14C-L-proline was determined in the rat. The disappearance of radioactivity from plasma occurred with a biphasic curve, the initial high levels reaching a nadir about 30 min after injection with subsequent increasing amounts of radioactivity. Examination of the 14C components in plasma revealed that 14C-proline disappeared rapidly during the first 30 min. At this time, the labeling of circulating plasma proteins ensued and continued to increase during the following 45 min of observations. Plasma glucose became labeled 10 min after injection and, thereafter, increased its 14C content. The extensive labeling of plasma proteins and glucose accounted for the increasing 14C found in plasma 30 min after injection. The course of radioactive labeling of brain, kidney, diaphragm, and liver was assessed. The greatest number of cpm/mg of tissue was found in the kidney. Determination of the distribution ratio, the ratio of cpm/ml intracellular nonprotein 14C to that in plasma in kidney revealed a peak of 3.9 within 15 min, a value comparable to that found in vitro. Twelve percent of the administered radioactivity was excreted as 14CO2 within 180 min. The oxidation was inhibited by known transport and metabolic inhibitors, the greatest effect observed with hydroxyproline, followed in order by thioproline, 3,4-dihydroproline, and glycine. The fractional urinary excretion of proline, Cproline/Cinulin, was determined and found to be 1% or less. This was increased by inhibitors, the greatest effect due to hydroxyproline followed in order by dehydroproline and glycine, a result similar to the observed extent of inhibition of proline oxidation to 14C O2. The physiologic disposition of proline was not altered by ligation of the renal vasculature.