A translocation causing increased alpha-klotho level results in hypophosphatemic rickets and hyperparathyroidism

Abstract

Phosphate homeostasis is central to diverse physiologic processes including energy homeostasis, formation of lipid bilayers, and bone formation. Reduced phosphate levels due to excessive renal loss cause hypophosphatemic rickets, a disease characterized by prominent bone defects; conversely, hyperphosphatemia, a major complication of renal failure, is accompanied by parathyroid hyperplasia, hyperparathyroidism, and osteodystrophy. Here, we define a syndrome featuring both hypophosphatemic rickets and hyperparathyroidism due to parathyroid hyperplasia as well as other skeletal abnormalities. We show that this disease is due to a de novo translocation with a breakpoint adjacent to alpha-Klotho, which encodes a beta-glucuronidase, and is implicated in aging and regulation of FGF signaling. Plasma alpha-Klotho levels and beta-glucuronidase activity are markedly increased in the affected patient; unexpectedly, the circulating FGF23 level is also markedly elevated. These findings suggest that the elevated alpha-Klotho level mimics aspects of the normal response to hyperphosphatemia and implicate alpha-Klotho in the selective regulation of phosphate levels and in the regulation of parathyroid mass and function; they also have implications for the pathogenesis and treatment of renal osteodystrophy in patients with kidney failure.

Fig. 1.

Parathyroid hyperplasia in index case. (A) A parathyroid gland removed at the initial surgery revealed enlargement of the gland with preservation of normal trabecular architecture consistent with hyperplasia. (Original magnification, ×10). The other three glands showed similar histology. (B) Hyperplastic parathyroid remnant partially removed at the second surgery; findings again were consistent with parathyroid hyperplasia. (Hematoxylin–eosin stain.)

Fig. 2.

A 9;13 translocation in the index case with hypophosphatemic rickets. Representative metaphase chromosomes are shown. The patient has a balanced translocation with breakpoints at 9q21.13 and 13q13.1.

Fig. 3.

Mapping breakpoints by FISH. (A) Mapping the chromosome 13q13.1 breakpoint. The relative position of YAC and BAC clones on chromosome 13 used in FISH experiments are shown in the schematic at the left, and the location of the inferred translocation breakpoint is indicated by the jagged line. The upper metaphase chromosome spread shows representative results of FISH using probes on opposite sides of the breakpoint. The hybridizing chromosomes are enlarged at the right, with a schematic diagram showing the signals coming from the wild-type chromosome 13 (gray) and its two derivatives. The lower spread shows the result with the spanning BAC (in red) and a distal probe, revealing three hybridization signals from the spanning BAC. (B) Mapping the chromosome 9q21.13 breakpoint. A representative metaphase spread in which the spanning chromosome 9 BAC and a distal marker have been hybridized. Three signals from the spanning BAC are detected.

Fig. 4.

Confirmation and refinement of translocation breakpoints. (A) A map of a segment of normal chromosome 13 spanning the inferred translocation breakpointis shown as a solid bar, and the locations of known cleavage sites for XbaI (X), HindIII (H), and EcoRI (R) are indicated. The sizes in kilobases of restriction endonuclease cleavage fragments predicted to result from Southern blotting after digestion with each enzyme and hybridization with the indicated probe are shown. (B) The inferred structure of the derivative chromosome is shown below, with the segment from chromosome 9 shown as a white box, along with the sizes of the observed restriction endonuclease cleavage fragment. (C–E) The results of Southern blotting of two control subjects (+/+) and the case (+/der) after digestion with indicated enzymes are shown. EcoRI and HindIII both yield heterozygous fragments that confirm the translocation; the normal fragments produced by XbaI delimit the proximal boundary of the translocation breakpoint. (F) Location of translocation breakpoint on chromosome 13. At a larger scale, the breakpoint is seen to be ≈50 kb proximal to the 5′ end of Klotho and 430 kb distal to the 3′ end of APRIN.

Fig. 5.

Increased α-Klotho levels and activity in patient with hypophosphatemic rickets. (A Upper) Western blot of immunoprecipitants from serum from nine controls and the t9;13 patient for α-Klotho protein. For comparison, immunoprecipitants from 100, 10, and 1 pmol/liter recombinant human secreted α-Klotho were loaded for quantification of protein amounts. (Lower) Western blot of same sera probed with antibody to IgG heavy chain. (B) β-Glucuronidase activity in plasma of the t9;13 patient and eight age- and sex-matched controls. Levels from three independent samples from the case are compared with levels from eight age- and sex-matched controls.