BH3 domain-independent apolipoprotein L1 toxicity rescued by BCL2 prosurvival proteins

The potent trypanolytic properties of human apolipoprotein L1 (APOL1) can be neutralized by the trypanosome variant surface antigen gene product known as serum resistance-associated protein. However, two common APOL1 haplotypes present uniquely in individuals of West African ancestry each encode APOL1 variants resistant to serum resistance-associated protein, and each confers substantial resistance to human African sleeping sickness. In contrast to the dominantly inherited anti-trypanosomal activity of APOL1, recessive inheritance of these two trypanoprotective APOL1 alleles predisposes to kidney disease. Proposed mechanisms of APOL1 toxicity have included BH3 domain-dependent autophagy and/or ion channel activity. We probed these potential mechanisms by expressing APOL1 in Xenopus laevis oocytes. APOL1 expression in oocytes increased ion permeability and caused profound morphological deterioration (toxicity). Coexpression of BCL2 family members rescued APOL1-associated oocyte toxicity in the order MCL1 ∼ BCLW > BCLXL ∼ BCL2A1 ≫ BCL2. Deletion of nine nominal core BH3 domain residues abolished APOL1-associated toxicity, but missense substitution of the same residues abolished neither oocyte toxicity nor its rescue by coexpressed MCL1. The APOL1 BH3 domain was similarly dispensable for the ability of APOL1 to rescue intact mice from lethal trypanosome challenge. Replacement of most extracellular Na(+) by K(+) also reduced APOL1-associated oocyte toxicity, allowing demonstration of APOL1-associated increases in Ca(2+) and Cl(-) fluxes and oocyte ion currents, which were similarly reduced by MCL1 coexpression. Thus APOL1 toxicity in Xenopus oocytes is BH3-independent, but can nonetheless be rescued by some BCL2 family proteins.

Mice with altered alpha-actinin-4 expression have distinct morphologic patterns of glomerular disease

Mutations in ACTN4, encoding the actin-binding protein alpha-actinin-4, cause a form of familial focal segmental glomerulosclerosis. We had developed two strains of transgenic mice with distinct alterations in the expression of alpha-actinin-4. One strain carried a human disease-associated mutation in murine Actn4, whereas the other knockout strain did not express alpha-actinin-4 protein. Most adult homozygous Actn4 mutant and knockout mice developed collapsing glomerulopathy. Homozygous Actn4 mutant mice also exhibited actin and alpha-actinin-4-containing electron-dense cytoplasmic structures, that were present but less prominent in heterozygous Actn4 mutant mice and not consistently seen in wild-type or knockout mice. Heterozygous Actn4 mutant mice did not develop glomerulosclerosis, but did exhibit focal glomerular hypertrophy and mild glomerular ultrastructural changes. The ultrastructural abnormalities seen in heterozygous Actn4 mutant mice suggest low-level glomerular damage, which may increase susceptibility to injury caused by genetic or environmental stressors. Our studies show that different genetic defects in the same protein produce a spectrum of glomerular morphologic lesions depending on the specific combination of normal and/or defective alleles.

The glomerular filter: Biologic and genetic complexity

The list of known genes that, when altered, cause proteinuric renal disease continues to increase. Recent mouse and human genetic studies, including that by Hasselbacher et al., are refocusing our attention on glomerular basement membrane components as critical to the barrier to protein filtration.

Familial focal segmental glomerulosclerosis

There is increasing recognition of the importance of genetic factors in the development of focal segmental glomerulosclerosis and related proteinuric disorders. Recently, four genes have been identified which, when defective, cause focal segmental glomerulosclerosis or nephrosis. All of these genes appear to be important in the maintenance of glomerular podocyte function. However, not all cases of familial nephrosis or proteinuria are explained by defects in these genes.

Extracellular calcium elicits a chemokinetic response from monocytes in vitro and in vivo

Recruitment of macrophages to sites of cell death is critical for induction of an immunologic response. Calcium concentrations in extracellular fluids vary markedly, and are particularly high at sites of injury or infection. We hypothesized that extracellular calcium participates in modulating the immune response, perhaps acting via the seven-transmembrane calcium-sensing receptor (CaR) on mature monocytes/macrophages. We observed a dose-dependent increase in monocyte chemotaxis in response to extracellular calcium or the selective allosteric CaR activator NPS R-467. In contrast, monocytes derived from mice deficient in CaR lacked the normal chemotactic response to a calcium gradient. Notably, CaR activation of monocytes bearing the receptor synergistically augmented the transmigration response of monocytes to the chemokine MCP-1 in association with increased cell-surface expression of its cognate receptor, CCR2. Conversely, stimulation of monocytes with MCP-1 or SDF-1alpha reciprocally increased CaR expression, suggesting a dual-enhancing interaction of Ca(2+) with chemokines in recruiting inflammatory cells. Subcutaneous administration in mice of Ca(2+), MCP-1, or (more potently) the combination of Ca(2+) and MCP-1, elicited an inflammatory infiltrate consisting of monocytes/macrophages. Thus extracellular calcium functions as an ionic chemokinetic agent capable of modulating the innate immune response in vivo and in vitro by direct and indirect actions on monocytic cells. Calcium deposition may be both consequence and cause of chronic inflammatory changes at sites of injury, infection, and atherosclerosis.

Mutations in ACTN4, encoding alpha-actinin-4, cause familial focal segmental glomerulosclerosis

Focal and segmental glomerulosclerosis (FSGS) is a common, non-specific renal lesion. Although it is often secondary to other disorders, including HIV infection, obesity, hypertension and diabetes, FSGS also appears as an isolated, idiopathic condition. FSGS is characterized by increased urinary protein excretion and decreasing kidney function. Often, renal insufficiency in affected patients progresses to end-stage renal failure, a highly morbid state requiring either dialysis therapy or kidney transplantation. Here we present evidence implicating mutations in the gene encoding alpha-actinin-4 (ACTN4; ref. 2), an actin-filament crosslinking protein, as the cause of disease in three families with an autosomal dominant form of FSGS. In vitro, mutant alpha-actinin-4 binds filamentous actin (F-actin) more strongly than does wild-type alpha-actinin-4. Regulation of the actin cytoskeleton of glomerular podocytes may be altered in this group of patients. Our results have implications for understanding the role of the cytoskeleton in the pathophysiology of kidney disease and may lead to a better understanding of the genetic basis of susceptibility to kidney damage.

Successful pregnancy outcome in a woman with a gain-of-function mutation of the calcium-sensing receptor. A case report

BACKGROUND

Gain-of-function mutations of the calcium-sensing receptor gene have recently been identified as a cause of familial hypercalciuric hypocalcemia. There have been no earlier reported cases of pregnancy among patients with this disorder.

CASE

A 26-year-old woman, gravida 1, para 0, was diagnosed at age 18 as being a heterozygous carrier of a mutation in the calcium-sensing receptor gene. Stable maternal hypocalcemia was achieved during pregnancy with high-dose calcium and 1,25-dihydroxyvitamin D3 therapy. Prenatal diagnosis was accomplished via amniocentesis at 16 weeks' gestation. The patient underwent cesarean delivery at 35 5/7 weeks' gestation after developing the HELLP syndrome.

CONCLUSION

Patients with mutations of the calcium-sensing receptor may have a successful pregnancy outcome. This abnormality may be transmitted to the fetus via an autosomal dominant pattern.

A locus for inherited focal segmental glomerulosclerosis maps to chromosome 19q13

We performed a genome-wide linkage analysis search for a genetic locus responsible for kidney dysfunction in a large family. This inherited condition, characterized by proteinuria, progressive renal insufficiency, and focal segmental glomerulosclerosis, follows autosomal dominant inheritance. We show with a high degree of certainty (maximum 2-point lod score 12.28) that the gene responsible for this condition is located on chromosome 19q13.

Gitelman's syndrome (Bartter's variant) maps to the thiazide-sensitive cotransporter gene locus on chromosome 16q13 in a large kindred

A defect in distal renal tubular sodium chloride handling is thought to be responsible for the clinical phenotype of Gitelman's syndrome, a variant of Bartter's syndrome. To study the possible involvement of the renal thiazide-sensitive NaCl cotransporter gene in the syndrome, a linkage analysis study in the largest reported kindred with the syndrome was performed. A human homolog of rat thiazide-sensitive cotransporter was cloned and mapped to chromosome 16q13 by fluorescent in situ hybridization. All 17 family members in two generations were genotyped at loci in this region. There were no recombinants observed between the Gitelman's syndrome phenotype and inheritance of D16S408 alleles, yielding a lod score of 3.88 at Q = 0. By contrast, recombinants were observed between Gitelman's syndrome and the flanking markers D16S419 and D16S400, localizing the responsible gene in this family to a 15 centimorgan region on chromosome 16q. These genetic data, together with current understanding of the molecular physiology of the thiazide-sensitive cotransporter, are strong evidence that the latter is defective in this kindred with Gitelman's syndrome.

Expression and characterization of inactivating and activating mutations in the human Ca2+o-sensing receptor

Nearly 30 mutations have been identified to date in the coding region of the extracellular calcium-sensing receptor (CaR) that are associated with inherited human hypo- and hypercalcemic disorders. To understand the mechanisms by which the mutations alter the function of the receptor may help to discern the structure-function relationships in terms of ligand-binding and G protein coupling. In the present studies, we transiently expressed eight known CaR mutations in HEK293 cells. The effects of the mutations on extracellular calcium- and gadolinium-elicited increases in the cytosolic calcium concentration were then examined. Seven inactivating mutations, which cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, show a reduced functional activity of the receptor because they may 1) reduce its affinity for agonists; 2) prevent conversion of the receptor from a putatively immature, high mannose form into the fully glycosylated and biologically active form of the CaR, in addition to lowering its affinity for agonists; or 3) fail to couple the receptor to and/or activate its respective G protein(s). Conversely, one activating mutation, which causes a form of autosomal dominant hypocalcemia, appears to increase the affinity of the receptor for its agonists.

Three inherited disorders of calcium sensing

Three distinct disorders of calcium homeostasis can result from mutations in the gene encoding the human calcium-sensing receptors (CASR; MIM 145980). One form of autosomal dominant familial hypocalciuric hypercalcemia results from the heterozygous state of inactivating mutations in the CASR gene. Neonatal severe hyperparathyroidism results from homozygosity for inactivating mutations in the CASR gene. The severe phenotype demonstrates the fundamental role the calcium-sensing receptor plays in parathyroid function. Activating mutations can lead to autosomal dominant hypocalcemia. The role of the calcium-sensing receptor in the kidney, brain, and other organs in health and disease awaits clarification.

A mouse model of human familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

Mice lacking the calcium-sensing receptor (Casr) were created to examine the receptor's role in calcium homeostasis and to elucidate the mechanism by which inherited human Casr gene defects cause diseases. Casr+/- mice, analogous to humans with familial hypocalciuric hypercalcemia, had benign and modest elevations of serum calcium, magnesium and parathyroid hormone levels as well as hypocalciuria. In contrast, Casr-/- mice, like humans with neonatal severe hyperparathyroidism, had markedly elevated serum calcium and parathyroid hormone levels, parathyroid hyperplasia, bone abnormalities, retarded growth and premature death. Our findings suggest that Casr mutations cause these human disorders by reducing the number of functional receptor molecules on the cell surface.

Mutational analysis of the extracellular Ca(2+)-sensing receptor gene in human parathyroid tumors

Despite recent progress, such as the identification of PRAD1/cyclin D1 as a parathyroid oncogene, it is likely that many genes involved in the molecular pathogenesis of parathyroid tumors remain unknown. Individuals heterozygous for inherited mutations in the extracellular Ca(2+)-sensing receptor gene that reduce its biological activity exhibit a disorder termed familial hypocalciuric hypercalcemia or familial benign hypercalcemia, which is characterized by reduced responsiveness of parathyroid and kidney to calcium and by PTH-dependent hypercalcemia. Those who are homozygous for such mutations present with neonatal severe hyperparathyroidism and have marked parathyroid hypercellularity. Thus, the Ca(2+)-sensing receptor gene is a candidate parathyroid tumor suppressor gene, with inactivating mutations plausibly explaining set-point abnormalities in the regulation of both parathyroid cellular proliferation and PTH secretion by extracellular Ca2+ similar to those seen in hyperparathyroidism. Using a ribonuclease A protection assay that has detected multiple mutations in the Ca(2+)-sensing receptor gene in familial hypocalciuric hypercalcemia and covers more than 90% of its coding region, we sought somatic mutations in this gene in a total of 44 human parathyroid tumors (23 adenomas, 4 carcinomas, 5 primary hyperplasias, and 12 secondary hyperplasias). No such mutations were detected in these 44 tumors. Thus, our studies suggest that somatic mutation of the Ca(2+)-sensing receptor gene does not commonly contribute to the pathogenesis of sporadic parathyroid tumors. As such, PTH set-point dysfunction in parathyroid tumors may well be secondary to other clonal proliferative defects and/or mutations in other components of the extracellular Ca(2+)-sensing pathway.

Mutations in the human Ca(2+)-sensing-receptor gene that cause familial hypocalciuric hypercalcemia

We report five novel mutations in the human Ca(2+)-sensing-receptor gene that cause familial hypocalciuric hypercalcemia (FHH) or neonatal severe hyperparathyroidism. Each gene defect is a missense mutation (228Arg-->Gln, 139Thr-->Met, 144Gly-->Glu, 63Arg-->Met, and 67Arg-->Cys) that encodes a nonconservative amino acid alteration. These mutations are each predicted to be in the Ca(2+)-sensing receptor's large extracellular domain. In three families with FHH linked to the Ca(2+)-sensing-receptor gene on chromosome 3 and in unrelated individuals probands with FHH, mutations were not detected in protein-coding sequences. On the basis of these data and previous analyses, we suggest that there are a wide range of mutations that cause FHH. Mutations that perturb the structure and function of the extracellular or transmembrane domains of the receptor and those that affect noncoding sequences of the Ca(2+)-sensing-receptor gene can cause FHH.

Autosomal dominant hypocalcaemia caused by a Ca(2+)-sensing receptor gene mutation

Defects in the human Ca(2+)-sensing receptor gene have recently been shown to cause familial hypocalciuric hypercalcaemia and neonatal severe hyperparathyroidism. We now demonstrate that a missense mutation (Glu128Ala) in this gene causes familial hypocalcaemia in affected members of one family. Xenopus oocytes expressing the mutant receptor exhibit a larger increase in inositol 1,4,5-triphosphate in response to Ca2+ than oocytes expressing the wild-type receptor. We conclude that this extracellular domain mutation increases the receptor's activity at low Ca2+ concentrations, causing hypocalcaemia in patients heterozygous for such a mutation.

Familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Effects of mutant gene dosage on phenotype

Neonatal severe hyperparathyroidism is a rare life-threatening disorder characterized by very high serum calcium concentrations (> 15 mg/dl). Many cases have occurred in families with familial hypocalciuric hypercalcemia, a benign condition transmitted as a dominant trait. Among several hypothesized relationships between the two syndromes is the suggestion that neonatal severe hyperparathyroidism is the homozygous form of familial hypocalciuric hypercalcemia. To test this hypothesis, we refined the map location of the gene responsible for familial hypocalciuric hypercalcemia on chromosome 3q. Analyses in 11 families defined marker loci closely linked to the gene responsible for familial hypocalciuric hypercalcemia. These loci were then analyzed in four families with parental consanguinity and offspring with neonatal severe hyperparathyroidism. Each individual who was homozygous for loci that are closely linked to the gene responsible for familial hypocalciuric hypercalcemia had neonatal severe hyperparathyroidism. The calculated odds of linkage between these disorders of > 350,000:1 (lod score = 5.56). We conclude that dosage of the gene defect accounts for these widely disparate clinical phenotypes; a single defective allele causes familial hypocalciuric hypercalcemia, while two defective alleles causes neonatal severe hyperparathyroidism.

Mutations in the human Ca(2+)-sensing receptor gene cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

We demonstrate that mutations in the human Ca(2+)-sensing receptor gene cause familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT), two inherited conditions characterized by altered calcium homeostasis. The Ca(2+)-sensing receptor belongs to the superfamily of seven membrane-spanning G protein-coupled receptors. Three nonconservative missense mutations are reported: two occur in the extracellular N-terminal domain of the receptor; the third occurs in the final intracellular loop. One mutated receptor identified in FHH individuals was expressed in X. laevis oocytes. The expressed wild-type receptor elicited large inward currents in response to perfused polyvalent cations; a markedly attenuated response was observed with the mutated protein. We conclude that the mammalian Ca(2+)-sensing receptor "sets" the extracellular Ca2+ level and is defective in individuals with FHH and NSHPT.

Homozygosity mapping of the gene for alkaptonuria to chromosome 3q2

Alkaptonuria, the first human disorder recognized by Garrod as an inborn error of metabolism, is a rare recessive condition that darkens urine and causes a debilitating arthritis termed ochronosis. We have studied two families with consanguineous parents and four affected children in order to map the gene responsible for alkaptonuria. Coinheritance of either neonatal severe hyperparathyroidism or sucrase-isomaltase deficiency and alkaptonuria provided a candidate location for the mutated genes on chromosome 3. Homozygosity mapping with polymorphic loci identified a 16 centiMorgan region on chromosome 3q2 that contains the alkaptonuria gene. Analysis of two additional nonconsanguineous families supports linkage of alkaptonuria to this single locus (combined lod score = 4.3, theta = 0).