Browsing by Author "Konrad, Martin"

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    Clinical utility of simultaneous quantitation of 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D by LC-MS/MS involving derivatization with DMEQ-TAD
    (The Endocrine Society, 2014-07) Kaufmann, Martin; Gallagher, J. Christopher; Peacock, Munro; Schlingmann, Karl-Peter; Konrad, Martin; DeLuca, Hector F.; Sigueiro, Rita; Lopez, Borja; Mourino, Antonio; Maestro, Miguel; St-Arnaud, René; Finkelstein, Joel S.; Cooper, Donald P.; Jones, Glenville; Department of Medicine, IU School of Medicine
    CONTEXT: The discovery of hypercalcemic diseases due to loss-of-function mutations in 25-hydroxyvitamin D-24-hydroxylase has placed a new demand for sensitive and precise assays for 24,25-dihydroxyvitamin D [24,25-(OH)2D]. OBJECTIVE: We describe a novel liquid chromatography and tandem mass spectrometry-based method involving derivatization with DMEQ-TAD {4-[2-(6,7-dimethoxy-4-methyl-3,4-dihydroquinoxalinyl)ethyl]-1,2,4-triazoline-3,5-dione} to simultaneously assay multiple vitamin D metabolites including 25-hydroxyvitamin D (25-OH-D) and 24,25-(OH)2D using 100 μL of serum with a 5-minute run time. DESIGN: The assay uses a newly synthesized internal standard d6-24,25-(OH)2D3 enabling the quantitation of 24,25-(OH)2D3 as well as the determination of the ratio of 25-OH-D3 to 24,25-(OH)2D3, a physiologically useful parameter. SETTING: We report data on more than 1000 normal and disease samples involving vitamin D deficiency or hypercalcemia in addition to studies involving knockout mouse models. RESULTS: The assay showed good correlation with samples from quality assurance schemes for 25-OH-D (25-OH-D2 and 25-OH-D3) determination (-2% to -5% bias) and exhibited low inter- and intraassay coefficients of variation (4%-7%) and lower limits of quantitation of 0.25-0.45 nmol/L. In clinical studies, we found a strong correlation between serum levels of 25-OH-D3 and 24,25-(OH)2D3 (r(2) = 0.80) in subjects over a broad range of 25-OH-D3 values and a marked lack of production of 24,25-(OH)2D3 below 25 nmol/L of 25-OH-D. The ratio of 25-OH-D3 to 24,25-(OH)2D3, which remained less than 25 in vitamin D-sufficient subjects (serum 25-OH-D < 50 nmol/L) but was greatly elevated (80-100) in patients with idiopathic infantile hypercalcemia. CONCLUSIONS: The new method showed good utility in clinical settings involving vitamin D deficiency; supplementation with vitamin D and idiopathic infantile hypercalcemia, as well as in animal models with ablation of selected cytochrome P450-containing enzymes involved in vitamin D metabolism.
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    Mutations in SLC34A3/NPT2c are associated with kidney stones and nephrocalcinosis
    (American Society of Nephrology, 2014-10) Dasgupta, Debayan; Wee, Mark J.; Reyes, Monica; Li, Yuwen; Simm, Peter J.; Sharma, Amita; Schlingmann, Karl-Peter; Janer, Marco; Biggin, Andrew; Lazier, Joanna; Gessner, Michaela; Chrysis, Dionisios; Tuchman, Shamir; Baluarte, H. Jorge; Levine, Michael A.; Tiosano, Dov; Insogna, Karl; Hanley, David A.; Carpenter, Thomas O.; Ichikawa, Shoji; Hoppe, Bernd; Konrad, Martin; Sävendahl, Lars; Munns, Craig F.; Lee, Hang; Jüppner, Harald; Bergwitz, Clemens; Department of Medicine, IU School of Medicine
    Compound heterozygous and homozygous (comp/hom) mutations in solute carrier family 34, member 3 (SLC34A3), the gene encoding the sodium (Na(+))-dependent phosphate cotransporter 2c (NPT2c), cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a disorder characterized by renal phosphate wasting resulting in hypophosphatemia, correspondingly elevated 1,25(OH)2 vitamin D levels, hypercalciuria, and rickets/osteomalacia. Similar, albeit less severe, biochemical changes are observed in heterozygous (het) carriers and indistinguishable from those changes encountered in idiopathic hypercalciuria (IH). Here, we report a review of clinical and laboratory records of 133 individuals from 27 kindreds, including 5 previously unreported HHRH kindreds and two cases with IH, in which known and novel SLC34A3 mutations (c.1357delTTC [p.F453del]; c.G1369A [p.G457S]; c.367delC) were identified. Individuals with mutations affecting both SLC34A3 alleles had a significantly increased risk of kidney stone formation or medullary nephrocalcinosis, namely 46% compared with 6% observed in healthy family members carrying only the wild-type SLC34A3 allele (P=0.005) or 5.64% in the general population (P<0.001). Renal calcifications were also more frequent in het carriers (16%; P=0.003 compared with the general population) and were more likely to occur in comp/hom and het individuals with decreased serum phosphate (odds ratio [OR], 0.75, 95% confidence interval [95% CI], 0.59 to 0.96; P=0.02), decreased tubular reabsorption of phosphate (OR, 0.41; 95% CI, 0.23 to 0.72; P=0.002), and increased serum 1,25(OH)2 vitamin D (OR, 1.22; 95% CI, 1.05 to 1.41; P=0.008). Additional studies are needed to determine whether these biochemical parameters are independent of genotype and can guide therapy to prevent nephrocalcinosis, nephrolithiasis, and potentially, CKD.