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Case Vignette

Clinical History

A 74-year-old male with stable non-albuminuric stage 2-3A CKD, documented from 2011 until Sept 2020, experienced a drop in GFR from a prior baseline of 55-60 ml/min to 35-40 ml/min, with a new baseline creatinine of 140-160 umol/L. His prior stable chronic kidney disease was presumed secondary to nephrosclerosis given his risk factors of type two diabetes, hypertension, dyslipidemia, and coronary artery disease. He underwent broader work up of his CKD as his renal function failed to return to his prior baseline.   Notable findings included a positive serum protein electrophoresis (SPEP) which demonstrated an IgG Kappa monoclonal band quantified at 7.9 g/L, a urinary protein electrophoresis (UPEP) with Kappa light chains quantified at 1.02 g/L, and serum free light chain ratio (SFLCR) of 10.69 with Kappa light chain predominance (189.2 mg/L). Urine albumin to creatinine ratios (ACR) were in the range of 25-30 mg/mmol and protein creatinine ratios (PCR) were 100-200 g/mol. Urine microscopy done at that time was bland and a recent urinalysis showed glycosuria with serum normoglycemia (blood glucose of 8.6 mmol/L). He was noted to be hypouricemic with a serum uric acid level of 192 umol/L. His glucosuria and hypouricemia in the context of stage 3B CKD were suggestive of a partial Fanconi’s syndrome.

He was seen by hematology and underwent a bone marrow biopsy which revealed 7% clonal plasma cells, Kappa-restricted on flow cytometry. As he did not meet criteria for diagnosis of a plasma cell neoplasm, he subsequently underwent a renal biopsy in work up of suspected monoclonal gammopathy of renal significance.   

 

Kidney Biopsy Findings: 

Figure 1: PAS pale intracytoplasmic rhomboid shaped crystals are noted in the proximal tubules on light microscopy, with smaller, morphologically diverse crystals seen both within epithelial cells and intra-luminally (PAS x200; insert x400).

Figure 2: Eosinophilic intracytoplasmic and intraluminal crystals are seen on light microscopy within proximal tubules, with distinct rhomboid shaped crystals amongst the irregular granules (H&E x200; insert x400).

Figure 3: Fuschinophilic intracytoplasmic crystals are seen on light microscopy within proximal tubules. A structurally preserved glomerulus is noted (Masson-Trichrome x400).

Figure 4: Paraffin-embedded and pronase-digested immunofluorescence showing no Lambda staining within proximal tubular epithelial cells (A), compared to Kappa which showed 1-2+ granular staining within tubular resorption droplets and weaker staining of the intracytoplasmic crystalline inclusions (B). Conventional immunofluorescence was negative for both Kappa and Lambda light chains

Figure 5: On electron microscopy (x 4000) multiple rhomboid, rectangular, and needle shaped electron dense crystals are noted within proximal tubule epithelial cells. 

Figure 6: At higher magnification, on electron microscopy (x 12000) multiple electron dense crystals are present within proximal tubule epithelial cells, with rhomboid, rectangular, and needle shaped crystals noted within both membranous inclusions and freely within the cytoplasm. A crystalline lattice is suggested by the horizontal striations present within some crystals at this magnification. 

Figure 7: On electron microscopy (x 15000) an electron dense rhomboid inclusion is noted within the cytoplasm of a podocyte.

Final Diagnosis: Light Chain Proximal Tubulopathy with Crystals, Kappa light chain restricted

Discussion:

The patient’s renal biopsy confirmed his primary nephrologist’s suspicion of an underlying monoclonal gammopathy of renal significance (MGRS), with the diagnosis of Light Chain Proximal Tubulopathy (LCPT).

LCPT is a result of monotypic light chain reabsorption within proximal tubular epithelial cells. LCPT can be of both a crystalline and non-crystalline variant. In the crystalline variant, as in our case, light chain crystals are seen within proximal tubular epithelial cells – as aggregates in the cytoplasm, or within lysosomes. ¹ The crystalline variant of light chain proximal tubulopathy has been shown in multiple case series to be associated with Kappa light chain deposition¹, which correlates with our patient’s dysproteinemia. The association between Kappa light chain dysproteinemias and LCPT is suspected to be due to innate physicochemical properties of a subgroup of Kappa light chains, most often belonging to the V Kappa 1 (or more rarely V Kappa 3) variability subgroups, which resist proteolysis, and promote crystal formation at the site of their physiologic reabsorption – the proximal tubule.^2,3,4  Importantly, overproduction of free light chains is a necessary pre-condition for the development of LCPT, as intact immunoglobulins are not freely filtered through the glomerulus. ⁵ One of the initial manifestations is overflow tubular proteinuria as the resorptive capacity of the proximal convoluted tubular cells is exceeded.³ Monoclonal, but not polyclonal, free light chains have been shown to lead to apoptosis of proximal tubular cells through the generation of reactive oxygen species, and in so doing they contribute to the creation of an inflammatory milieu that facilitates accelerated tubulointerstitial fibrosis.⁵

Histopathologic diagnosis involves characteristic findings on light microscopy, immunofluorescence (IF), and electron microscopy (EM). On light microscopy, crystal inclusions are typically noted within proximal tubular epithelial cells and intra-luminally. Crystals are most apparent on PAS and trichrome stains (Figures 1 and 3).⁶ Conventional immunofluorescence on frozen tissue is often non-contributory as antigen extraction is required to obtain the expected Kappa restricted staining. Performance of immunofluorescence on pronase-digested paraffin (IF-P) sections can expose the Kappa light chain epitopes to Kappa anti-sera. Pronase digestion helps expose the crystals sequestered within lysosomal membranes, and may help expose epitopes bound within the crystalline lattices. Stokes et al. demonstrated a sensitivity of 97% with IF-P as compared to 35% with frozen IF.³ Recognizing the importance of IF-P in the diagnosis of LCPT, Nasr et al. have listed suspected LCPT as an indication to perform IF-P in their recent review.⁷ As the process is not 100% sensitive, there are reported IF-P negative cases as well.^3,8 On EM, proximal tubular epithelial cells are noted to contain electron dense intra-cytoplasmic crystalline inclusions, often rod, rhomboid, needle, or irregularly shaped. The intracellular crystals are often contained within lysosomal membranes. Associated tubular injury is evident on EM as well.⁶ Our case included all the previously reported histopathologic features of crystalline LCPT, with the addition of occasional crystal inclusions within the cytoplasm of structurally preserved podocytes. This phenomenon has been previously reported³, and is not entirely unexpected as podocytes are epithelial cells as well.

LCPT is a rare MGRS-associated disease. In a relatively recent case series conducted at Columbia University in 2016, Stokes et al. demonstrated light chain proximal tubulopathy in 54 of 1078 renal biopsies done in patients with concomitant dysproteinemias, of which 46 represented isolated cases of LCPT. The remaining 8 cases showed concomitant findings including 4 patients with light chain cast nephropathy, two patients with amyloid, one patient with monoclonal immunoglobulin deposition disease, and one patient with collapsing glomerulopathy. ³

As LCPT is a rare MGRS associated disease, the true incidence of which remains unclear, optimal therapies have not been established with randomized control trials. In Stokes et al.’s case series, patients underwent clone targeted therapy with chemotherapy and, in some cases, hematopoietic stem cell transplant if they carried a concomitant diagnosis of malignant hematologic disease. This approach to therapy is in line with the current recommendations from the International Kidney and Monoclonal Gammopathy Research Group. In their cohort of patients, they found that predictors of final eGFR in crystalline LCPT patients included both age and initial eGFR, highlighting the importance of early diagnosis. However, they found that LCPT can follow an indolent disease course – of 7 patients who did not receive any therapy, 5/7 had no progression, and 2/7 developed ESRD. However, in the combined chemotherapy and stem cell transplant group, none of the 10 patients exhibited progressive renal disease. In the chemotherapy group 4/12 progressed to ESRD, which is likely an indication of sub-optimal clone directed therapy, as the pathogenic clone was not eliminated.³ Importantly, recurrence of LCPT has been reported in renal allografts in the presence of a persistent dysproteinemia.^9,10,11

 

Bibliography:

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