Monoclonal gammopathies causing renal lesions may be caused by an underlying hematologic disease which, in turn, may be frankly malignant conditions like symptomatic MM or a B-cell lymphoma or premalignant states like smoldering myeloma, MGUS and MBL (the B-cell counterpart of MGUS) [3]. Renal pathology in association with all premalignant hematologic diseases is now considered under the umbrella of MGRS [2, 3]. While MGRS in association with MGUS has been known for some time now, MGRS caused by nephrotoxic paraproteins released by premalignant small B-cell clones has only recently gained attention [3].
CAN is a rare paraprotein crystal-induced renal disease caused by glomerular intracapillary and renal arteriolar occlusion by crystals, the other subtypes of paraprotein crystal-induced nephropathies include - crystalline variant of light chain proximal tubulopathy (intracellular light chain crystals precipitating within the proximal tubular epithelial cells and often associated with Fanconi syndrome) [2, 4], monoclonal crystalloid glomerulopathy (intracellular light chain crystals within individual glomerular cell compartments like endothelial cells or podocytes) [5,6,7] and crystal-storing histiocytosis (light chain crystal-bearing histiocytes found either in interstitium, mesangium or glomerular capillary loops), as highlighted in Fig. 3 [8, 9]. Although there have been case reports of crystalglobulinemia since 1930s [10], it is only recently that a specific renal lesion in association with these extracellular circulating crystals has been delineated [1]. CAN has been described mostly in association with MM and MGUS [1, 11]. Our report describes a rare case of CAN associated with MBL (with CLL phenotype) presenting as MGRS in a patient with acute renal failure leading to hemodialysis. A B-cell clone under 5 K/uL falls under the MBL category as per latest WHO guidelines [12], and our patient’s clonal B-cell count was quite low, at 0.19 K/uL. Two large case series [13, 14] which have studied the spectrum of renal lesions associated with CLL/MBL have described various MGRS entities such as cryoglobulinemic glomerulonephritis (GN), immunotactoid GN, fibrillary GN, amyloidosis and light chain cast nephropathy, but CAN developing as a result of CLL/MBL has not been described previously. A 2014 series of 20 cases of non-Hodgkin lymphoma with biopsy-proven renal involvement describes a single case of CLL having light microscopy appearance similar to ours with intracapillary plugging by PAS-positive pseudothrombi which stained for IgM and kappa by IF but on EM, showed only granular electron-dense deposits occluding the capillary loops but without any substructure [15].
Histologically, the renal biopsy in CAN shows glomeruli with large intracapillary eosinophilic pseudothrombi with or without glomerular hypercellularity - while endocapillary hypercellularity has been variably described, mesangial hypercellularity (present in our case) has not been described in previous case reports [1, 11]. Segmental capillary wall double contours were described in one case [16]. The double contours seen in the glomeruli by light microscopy (and corroborated by subendothelial widening seen in some capillary loops on EM) were likely the result of endothelial injury from the patient’s severe hypertension or direct injury from the intraluminal occluding crystals, or both. Although classic features of thrombotic microangiopathy (TMA) such as fibrin thrombi or mucoid intimal thickening were not seen within the glomeruli or vasculature sampled (only one artery being present in the biopsy), the possibility of a monoclonal gammopathy-associated TMA cannot be completely excluded [17]. Frozen section immunofluorescence (IF) can be challenging to interpret in cases of CAN (as was in our case) and, where available, paraffin IF with pronase digestion, has been recommended as a more sensitive IF technique in such cases [18]. However, immunogold EM (IEM), can also serve as an alternative and equally (if not more) sensitive technique for confirming monoclonality of crystalline deposits by actually localizing kappa or lambda light chains within the crystals [8, 19, 20]. In our case, IEM proved indispensable in confirming the kappa restriction of the light chain intraluminal crystals. IEM for immunoglobulins was not attempted as it not standardized in the electron microscopy lab where the IEM procedure was done, and it has not been well described in literature [19, 20]. Patients with CAN (particularly those with cryocrystalglobulinemia) may develop symptoms like rash, arthralgia and peripheral neuropathy as usually seen in patients with cryoglobulinemic and rheumatic diseases [1], and even renal arterial thrombosis in rare instances [11]. Our patient had arthralgia but none of the other symptoms. Usually, kappa predominates as the restricted light chain that is overproduced in monoclonal crystal-related nephropathies [1, 11], although occasional lambda-predominant cases have been reported [16].
The histologic differential diagnoses for our case included – TMA associated with various etiologies like hemolytic-uremic syndrome, complement abnormality, malignant hypertension, autoimmune diseases (including antiphospholipid antibody syndrome) or solid malignancies, type 1 cryoglobulinemic glomerulonephritis secondary to kappa monoclonal disease or staphylococcal infection-related glomerulonephritis (SAGN) with cryoglobulinemic features. However, in none of these conditions are actual monoclonal crystals with lattice substructure found within capillary loops on EM as was seen in our case [21, 22]. Also in our case, the patient had no recent history of infection or pre-existing diabetes to support a diagnosis of SAGN.
The exact pathologic/biochemical basis of crystallization of monoclonal proteins and the reason for kappa predominance are not well understood. However researchers have suggested possible abnormal N-glycosylation of the light chain portion of monoclonal paraproteins, aided by stasis in microvasculature as the reason for crystal formation [23], while unusual hydrophobic residues at the variable (V) domain of the kappa light chain at position 30 or unusual amino acid substitutions in the V region have been cited as a reason why the kappa crystals are resistant to normal lysosomal degradation in the B- or plasma cell- microenvironment [24].
In conclusion, we describe a rare case of CAN in a patient with MBL (CLL subtype) and renal failure and without systemic symptoms, organomegaly, bone pain or lymphadenopathy. Our case highlights the importance of being aware of rare entities like CAN and also stresses the fact that even a small B-cell clone as in MBL should not be ignored [25] as it can sometimes lead to selective kidney dysfunction as MGRS. Our case also shows how IEM can be of immense help in clinching a diagnosis when standard immunofluorescence is equivocal or even false-negative [20]. Lastly, the case stresses the importance of doing repeat SPEP testing (when initial results are negative) and free light chain assay as a mandatory workup in any patient with suspicion of a monoclonal gammopathy, with the aim of initiating proper treatment and/or follow-up at the earliest [1].
