Correspondence

Nephrolithiasis, Osteoporosis, and Mutations in the Type 2a Sodium–Phosphate Cotransporter

N Engl J Med 2003; 348:264-265January 16, 2003

Article

To the Editor:

In the report by Prié et al. (Sept. 26 issue)1 indicating that inactivating mutations in the sodium–phosphate cotransporter gene, NPT2a, are associated with renal phosphate wasting and bone demineralization, the data on expression do not support a dominant negative effect of the V147M mutation. As the authors themselves acknowledge, coinjection of 10 ng of wild-type RNA and 10 ng of mutant (V147M) RNA yielded a phosphate-induced current “similar to that in oocytes expressing 10 ng of wild-type NPT2a RNA alone.” The comparison of this current with that produced by injection of 20 ng of wild-type RNA is largely irrelevant. Furthermore, the V147M mutation predicts an amino acid substitution in a transmembrane domain, a less likely site for interaction with other proteins.

In contrast, the data on the A48F mutation do support a dominant negative mechanism. This mutation is in the NH2 terminal portion of the protein, thought to reside inside the cell,2 and could plausibly alter protein–protein interactions. The serum phosphorus concentration in the patient with this mutation was distinctly lower than those in the other 19 patients in the study. Although heterozygous for the A48F mutation, this patient has many of the phenotypic features of mice homozygous for the disrupted NPT2a gene,3 including renal calcification.4 However, it is important to note that heterozygous mutant mice (with one disrupted NPT2a allele) have neither hypercalciuria3 nor nephrocalcinosis.

Steven J. Scheinman, M.D.
SUNY Upstate Medical University, Syracuse, NY 13210
scheinms@upstate.edu

Harriet S. Tenenhouse, Ph.D.
Montreal Children's Hospital Research Institute, Montreal, QC H3Z 2Z3, Canada

4 References

1. 1

   Prie D, Huart V, Bakouh N, et al. Nephrolithiasis and osteoporosis associated with hypophosphatemia caused by mutations in the type 2a sodium-phosphate cotransporter. N Engl J Med 2002;347:983-991
   Full Text | Web of Science | Medline

2. 2

   Lambert G, Traebert M, Hernando N, Biber J, Murer H. Studies on the topology of the renal type II NaPi-cotransporter. Pflugers Arch 1999;437:972-978
   CrossRef | Web of Science | Medline

3. 3

   Beck L, Karaplis AC, Amizuka N, Hewson AS, Ozawa H, Tenenhouse HS. Targeted inactivation of Npt2 in mice leads to severe renal phosphate wasting, hypercalciuria, and skeletal abnormalities. Proc Natl Acad Sci U S A 1998;95:5372-5377
   CrossRef | Web of Science | Medline

4. 4

   Chau H, Tenenhouse HS. Renal phosphate wasting and hypercalciuria in Npt2 knockout mice are associated with nephrocalcinosis. J Am Soc Nephrol 2001;12:751A, abstract-751A, abstract
   

Author/Editor Response

Drs. Scheinman and Tenenhouse question the possibility that the V147M mutation has a dominant negative effect on phosphate transport. As shown in Figure 3 of our article, phosphate-induced current increases with increasing doses of NPT2a RNA and reaches a plateau at approximately 30 ng of RNA. Thus, in the absence of a dominant negative effect, the current measured in oocytes coexpressing 10 ng of wild-type RNA and 10 ng of mutant RNA should approximately equal the sum of the currents measured in oocytes expressing either wild-type or mutant NPT2a RNA alone. We found that phosphate-induced current in oocytes coinjected with wild-type RNA and either V147M or A48F mutant RNA (10 ng of each) was significantly lower than the sum of these two values (P<0.005 for both comparisons). These results demonstrate a dominant negative effect. The current produced by 20 ng of wild-type RNA was measured (Figure 4 of the article) to demonstrate that the reduction in phosphate-induced current in coinjected oocytes could not be explained by a saturation of the cellular pathways required to increase the current.

Mutations in transmembrane domains leading to a dominant negative effect have been reported for various proteins, including ion channels and seven transmembrane receptors.1-3 These effects may be attributable to modifications in the secondary structure of the protein resulting from amino acid substitution. In addition, involvement of transmembrane domains in protein dimerization has been reported.4 Therefore, changes in the ability of NPT2a to form multimers or participate in other macromolecular complexes could explain the dominant negative effect that we observed for the V147M mutation.

Dominique Prié, M.D., Ph.D.
Gérard Friedlander, M.D., Ph.D.
Hôpital Bichat, 75018 Paris, France
dprie@bichat.inserm.fr

Caroline Silve, M.D., Ph.D.
INSERM Unité 426, 75018 Paris, France

4 References

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   Zuberi SM, Eunson LH, Spauschus A, et al. A novel mutation in the human voltage-gated potassium channel gene (Kv1.1) associates with episodic ataxia type 1 and sometimes with partial epilepsy. Brain 1999;122:817-825
   CrossRef | Web of Science | Medline

3. 3

   Oury C, Toth-Zsamboki E, Van Geet C, et al. A natural dominant negative P2X1 receptor due to deletion of a single amino acid residue. J Biol Chem 2000;275:22611-22614
   CrossRef | Web of Science | Medline

4. 4

   Fafournoux P, Noel J, Pouyssegur J. Evidence that Na+/H+ exchanger isoforms NHE1 and NHE3 exist as stable dimers in membranes with a high degree of specificity for homodimers. J Biol Chem 1994;269:2589-2596
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Citing Articles (2)

Citing Articles

1. 1

   F. Jakob. (2007) Metabolische Knochenerkrankungen. Der Internist 48:10, 1101-1118
   CrossRef

2. 2

   Giovanni Gambaro, Giuseppe Vezzoli, Giorgio Casari, Luca Rampoldi, Angela D’Angelo, Loris Borghi. (2004) Genetics of hypercalciuria and calcium nephrolithiasis: From the rare monogenic to the common polygenic forms. American Journal of Kidney Diseases 44:6, 963-986
   CrossRef