Adult Fanconi syndrome progressing to multiple myeloma

Specific impairment of proximal tubular cell proliferation by a monoclonal κ light chain responsible for Fanconi syndrome

BACKGROUND

Fanconi syndrome (FS) is a rare renal disorder featuring proximal tubule dysfunction that may occur following tubular reabsorption of a monoclonal light chain (LC), in patients with multiple myeloma. FS may precede the recognition of multiple myeloma by several years. In most cases, crystalline inclusions of monoclonal κ LCs are observed within the lysosomes of proximal tubular cells (PTCs) and probably participate in their functional alteration.

METHODS

To investigate the mechanism implicated in proximal tubule dysfunction, we compared the effects of κ LC-CHEB obtained from a patient with myeloma-associated FS to those of control κ LC-BON obtained from a patient without evidence of FS, on the viability and proliferation of two different PTC lines.

RESULTS

Our data suggest that the tubular atrophy in myeloma-associated FS does not result from increased apoptosis of PTCs, but from their impaired capacity to proliferate and renew. Indeed, in vitro incubation of cultured PTCs with physiological amounts of the nephrotoxic κ LC-CHEB was sufficient to cause a depression in DNA synthesis and in cell proliferation. This effect was observed neither with control κ LC-BON nor in the absence of κ LC.

CONCLUSIONS

The reduced turnover of PTCs may affect tubular repair and regeneration. In addition, the reduced proliferation of myeloma cells producing the same monoclonal κ LC might explain the frequent association of FS with smoldering multiple myeloma.

Acquired Fanconi syndrome is an indolent disorder in the absence of overt multiple myeloma

Adult-acquired Fanconi syndrome (FS) is a rare complication of monoclonal gammopathy. We retrospectively reviewed 32 patients diagnosed with adult-acquired FS between April 1968 and June 2002 at Mayo Clinic (Rochester, MN). At diagnosis, most patients had monoclonal gammopathy of undetermined significance (MGUS) or smoldering multiple myeloma (SMM), with a median creatinine level of 176.8 μM (2.0 mg/dL; range, 79.56-327.08 μM [0.9-3.7 mg/dL]) and evidence of osteomalacia. During the average 65 months (range, 2-238 months) of follow-up, 5 patients developed end-stage renal disease (ESRD) and only 1 of 14 patients with MGUS transformed to multiple myeloma (MM). Also, 14 deaths occurred, with only 1 from ESRD but 4 from alkylator-related leukemia or myelodysplastic syndrome. Chemotherapy offered little benefit on renal functions of MGUS or SMM patients. In conclusion, FS associated with monoclonal gammopathy does not appear to confer an additional risk of subsequent evolution to MM. ESRD occurs late in the disease process.

Adult Fanconi syndrome secondary to light chain gammopathy. Clinicopathologic heterogeneity and unusual features in 11 patients

Fifty-seven cases of Ig light chain-associated Fanconi syndrome (FS) have been reported so far, mostly as isolated reports. The pioneering work by Maldonado and associates (35), who reviewed the first 17 cases in 1975, led to the unifying concept that patients with FS and Bence Jones proteinuria have a special form of plasma cell dyscrasia characterized by slow progression of the tumor and by prominent crystal formation in proximal tubule cells, in the absence of myeloma casts in the distal tubule. We carefully reappraised these characteristics in a series of 11 patients. Ten renal biopsy specimens were available for electron microscopy, adding to the 15 previously reported cases with ultrastructural studies. Moreover, 10 of the kappa light chains could be entirely or partially sequenced and tested for their resistance to cathepsin B, a lysosomal protease present in proximal tubule cells. Our series showed an unexpected clinicopathologic heterogeneity. Seven patients presented with the typical clinical and pathologic features of FS and low-mass myeloma or monoclonal gammopathy of undetermined significance (MGUS), in keeping with Maldonado et al's description. Crystals in bone marrow cells were detected in patients of this group, only. Three patients who presented with full-blown FS exhibited, however, the characteristic features of myeloma cast nephropathy in the setting of high-mass myeloma. One patient of this group also had numerous crystals in proximal tubule cells. The eleventh patient had complete FS with MGUS, but no crystals in proximal tubule cells even after electron microscopy. Contrasting with the clinicopathologic heterogeneity, genetic and biochemical analyses of the light chains showed a striking homogeneity. First, they all were of the kappa type. Second, 8 of 9 belonged to the V kappa I variability subgroup, which indicates that FS light chains are related by the sequence of their variable regions. Third, the 8 V kappa I light chain sequences most likely originated from only 2 germline genes, LCO2/012 and LCO8/018. Fourth, all 5 LCO2/012-derived sequences presented an unusual hydrophobic or nonpolar residue at position 30. These sequence peculiarities may account for unusual physicochemical properties of the light chains including the resistance of their variable domain V kappa to proteolysis by cathepsin B, observed in 7 of 9 patients in our series, while light chains isolated from patients with myeloma cast nephropathy are completely digested. Resistance of V kappa to proteolysis in FS patients can explain the accumulation of the light chain in the endocytotic compartment of the proximal tubule cells, leading to impairment of proximal tubule functions.

Acquired Fanconi's syndrome associated with monoclonal gammopathies

Franconi's syndrome is a rare complication of the monoclonal gammopathies characterized by diffuse failure in reabsorption at the level of the proximal renal tubule resulting in glycosuria, generalized aminoaciduria, and hypophosphatemia. Current evidence suggests monoclonal light chains that are incompletely digested in renal tubule lysosomes cause renal injury. The light-chain fragments serve as a nidus for crystal formation, and the crystals interfere with a broad range of apical membrane transporters. Franconi's syndrome is predominantly associated with monoclonal kappa in the urine, but rare instances of FS caused by lambda light chains have been described. Overt hematologic malignancies such as multiple myeloma, Waldenström's macroglobulinemia, or other lymphoproliferative disorders occur in one third of patients. Clinical manifestations include slowly progressive renal failure and bone pain caused by osteomalacia. The osteomalacia is caused by chronic hypophosphatemia and may be exacerbated by secondary hyperparathyroidism and renal tubular acidosis. Treatment consists of supplementation with phosphorus, calcium, and vitamin D. The osteomalacia is often completely reversible with mineral supplementation. Patients with an associated symptomatic malignancy or rapidly progressive renal failure may also benefit from chemotherapy. The prognosis is good in the absence of overt malignant disease.

Osteomalacia associated with adult Fanconi's syndrome: clinical and diagnostic features

OBJECTIVE

Osteomalacia associated with adult acquired Fanconi's syndrome is thought to result from hypophosphataemia and relative 1,25-dihydroxyvitamin D deficiency. We have followed the clinical and diagnostic features of patients with osteomalacia associated with adult Fanconi's syndrome, with particular emphasis on their responses to treatment with calcium, phosphate and vitamin D.

DESIGN

Retrospective Mayo Clinic case-note review from 1975 to 1994 and prospective follow-up study, combined with literature review.

PATIENTS

Eleven patients (7 male, 4 female) were identified who satisfied criteria for diagnosis of osteomalacia and adult Fanconi's syndrome. Twenty-five additional patients were identified in a literature review from 1954 to the present.

METHODS

Clinical history and physical examination, serum and urine bone and mineral parameter, X-ray radiography and iliac crest bone histomorphometry.

RESULTS

All patients presented with typical symptoms of osteomalacia, including lower extremity or low back bone pain, and all had fractures, pseudofractures, and/or bone demineralization on X-ray radiography. Osteomalacia and Fanconi's syndrome were diagnosed concurrently in 10 patients, whereas osteomalacia preceded diagnosis of Fanconi's syndrome by 5 years in one patient. Pre-treatment bone biopsies in 9 of the 11 patients demonstrated increased osteoid surface, volume and width. In the one patient labelled with tetracycline prior to biopsy, mineralization lag time was prolonged at 111 days (normal 19.2 +/- 1.0 days). Hypophosphataemia, inappropriately low 1,25-dihydroxyvitamin D levels, renal insufficiency, and chronic acidosis due to bicarbonate leak and uraemia, contributed to the osteomalacia in these patients. Secondary hyperparathyroidism was present in two patients. Eight of the 11 patients with osteomalacia associated with Fanconi's syndrome had monoclonal disorders, including multiple myeloma or lymphoma, many of them manifest by light-chain proteinuria. Over a mean patient follow-up period of 46 months (range 1-239 months), patients responded symptomatically to calcium, phosphate, and vitamin D replacement typically within 1-6 months. In 8 patients in whom follow-up data were available, post-treatment serum phosphate and 1,25-dihydroxyvitamin D levels improved in the setting of stable mild renal insufficiency; only one patient developed end-stage renal failure after 20 years, suggesting that these patients do not invariably progress rapidly to renal failure.

CONCLUSIONS

Regardless of the underlying cause, osteomalacia associated with adult acquired Fanconi's syndrome appears to respond well to calcium, phosphate, and vitamin D replacement. These patients do not appear to necessarily require 1,25-dihydroxyvitamin D replacement.