Glomerular visceral epithelial cells (i.e., podocytes) are an essential component of the tripartite glomerular filtration barrier. Healthy podocytes are terminally differentiated cells with limited replicative capacity; however, inappropriate cell cycle reentry can be induced in podocytes by various injurious stimuli. In this issue of the JCI, Yamaguchi et al. report on a somatic mosaic gain-of-function mutation in the phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic α subunit (p110α, encoded by PIK3CA). The study reveals that activating mutations of p110α can drive podocyte proliferation in PIK3CA-related overgrowth syndrome (PROS). They also showed that selective, small-molecule inhibitors of p110 may be useful for the treatment of proliferative glomerulonephritis.

Membranoproliferative glomerulonephritis (MPGN) is a rare glomerular disease characterized by mesangial hypercellularity and thickening of the glomerular basement membrane (GBM). MPGN can be idiopathic or associated with malignancy, systemic immune complex disorders and chronic infections. It has rarely been associated with solid organ tumors, such as lung, gastric, breast or prostate cancer. We report a patient with MPGN and coexisting colorectal carcinoma. A 48-year-old man presented with anemia, loss of weight, hypertension, and nephrotic syndrome. The renal biopsy findings were compatible with type 1 MPGN. The antineutrophilic cytoplasmic antibodies, antinuclear antibodies, anti-GBM, serologic markers of hepatitis B and hepatitis C and tumor markers were negative. After ruling out the secondary causes of MPGN, the patient was treated with pulse doses of methylprednisolone and a single dose of cyclophosphamide. However, due to the worsening anemia and rectal bleeding, a colonoscopy was performed, which established a diagnosis of adenocarcinoma of the descending colon. The patient was treated with left hemicolectomy and oral corticosteroids. Within a year after the cancer treatment, the patient experienced a complete resolution of the proteinuria and improvement of the kidney function. Although rare, MPGN can be associated with hematologic malignancies and solid organ tumors. The most common causes of secondary MPGN should be ruled out before starting specific treatment. In our patient, cancer treatment has led to a subsequent remission of the nephrotic syndrome, which indicated that this association was not coincidental but rather causal. In patients with a tumor and concomitant glomerulopathy which is suspected to be paraneoplastic in etiology, the treatment of the underlying malignancy should be prioritized.

Membranoproliferative glomerulonephritis (MPGN) is no longer a disease but a pattern of injury in various diseases. Characterized by electron-dense deposits, mesangial proliferation, and duplication of the glomerular basement membrane, MPGN was previously classified by findings seen by electron microscopy. However, recognizing complement dysfunction in relation to cases with the MPGN pattern of injury substantially changed our view of its pathogenesis. A new classification, including immune complex-mediated and complement-mediated MPGN, has become preferable and has been adopted by international guidelines. Despite these advancements, accurate diagnosis of MPGN remains a clinical challenge, given the pathological and clinical similarities between immune complex-mediated and complement-mediated MPGN. Additional testing, such as molecular and genetic testing, is often necessary. Here, we will summarize our current understanding of the MPGN pattern of injury from a pathology perspective as an introductory article in the following chapters.

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In 1952, X-linked agammaglobulinemia (XLA) was discovered as a rare inherited disorder. It markedly compromises the ability of the body to combat infectious microorganisms. Membranoproliferative glomerulonephritis (MPGN) Type I is characterized by subendothelial immune complex deposits. Patients with XLA can rarely develop immune-complex-induced diseases. Here, we report a case of MPGN Type I in a 12-year-old male patient with a past and family history of XLA. The patient presented with fever, productive cough, vomiting, and lower limb edema. Clinical and radiological examinations established a diagnosis of bronchopneumonia. The laboratory findings revealed proteinuria and hematuria, and a renal biopsy was performed. The histological examination of this biopsy revealed mesangial hypercellularity and thickened basement membranes. Immunofluorescence studies showed mesangiocapillary staining for Complement 3 and Immunoglobulin (Ig) G and, to a lesser extent, for IgA, IgM, and Complement 1q. Ultrastructural studies revealed partly thick, double-contoured glomerular basement membranes, glomerular endothelial cells with swollen cell bodies, and podocytes with effaced foot processes. Small subendothelial and mesangial eosinophilic deposits were identified. The diagnosis of MPGN type I was established. The patient was started on prednisolone. To the best of our knowledge, this is a rare case of MPGN Type I in a patient with XLA. The pathogenetic mechanisms underlying the development of MPGN Type I were not apparent in our patient. However, residual humoral immunity may play a role in the development of MPGN.

UNASSIGNED: IgM-dominant immune complex-mediated glomerulonephritis (IgM-dominant ICMGN) is a rare renal entity, characterized by a membranoproliferative pattern by light microscopy, dominant IgM staining by immunofluorescent staining, and subendothelial deposits by electron microscopy. This study was to investigate if some of IgM-ICMGN were associated with autoimmune disorders induced by hydralazine.
UNASSIGNED: Seven IgM-dominant ICMGN cases were identified over 8 years. Their pathologic phenotypes and clinical scenarios were analyzed in detail.
UNASSIGNED: Patients\' ages ranged from 47 to 87 years old with 5 women and two men. Six of seven patients had drug-induced autoimmune phenomenon (hydralazine-induced positive ANCA and ANA). All of them had renal dysfunction and some proteinuria. Most pathologic features showed a membranoproliferative pattern of glomerulonephritis with dominant IgM deposits at subendothelial spaces. IgM nephropathy (a variant of focal segmental glomerulosclerosis), chronic thrombotic microangiopathy, and cryoglobulinemic glomerulopathy were ruled out in the cases.
UNASSIGNED: The hydralazine-induced autoimmune phenomenon can be seen in IgM-dominant ICMGN, which should be classified as a subtype of membranoproliferative glomerulonephritis.

BACKGROUND: Membranoproliferative glomerulonephritis (MPGN) can be divided into immune-complex MPGN (IC-MPGN) and C3 glomerulopathy (C3G), which includes dense deposit disease (DDD) and C3 glomerulonephritis (C3GN). These conditions result from abnormalities in different complement pathways and may lead to different prognoses. However, there are limited studies describing the respective clinical courses.
METHODS: In this study, Japanese pediatric patients diagnosed with MPGN based on kidney biopsies conducted between February 2002 and December 2022 were reclassified as having IC-MPGN or C3G (DDD or C3GN). We retrospectively analyzed the clinical characteristics and outcomes of these patients.
RESULTS: Out of 25 patients with MPGN, three (12.0%) were diagnosed with DDD, 20 (80.0%) with C3GN, and two (8.0%) with IC-MPGN. There were 13 (65.0%) patients and one (33.3%) patient in remission after treatment for C3GN and DDD, respectively, and no patients with IC-MPGN achieved remission. The median follow-up period was 5.3 (2.5-8.9) years, and none of the patients in either group progressed to an estimated glomerular filtration rate < 15 ml/min/1.73 m2. Patients with C3GN presenting mild to moderate proteinuria (n = 8) received a renin-angiotensin system inhibitor (RAS-I) alone, and these patients exhibited a significant decrease in the urinary protein creatinine ratio and a notable increase in serum C3 levels at the last follow-up.
CONCLUSIONS: Most patients with MPGN were diagnosed with C3GN. The remission rate for C3GN was high, and no patients developed kidney failure during the approximately 5-year follow-up. Additionally, patients with C3GN with mild to moderate proteinuria had good outcomes with RAS-I alone, but continued vigilance is necessary to determine long-term prognosis.

Little is known about what constitutes the dense deposits of dense deposit disease (DDD), apart from components of the complement pathway. This study presents the novel finding that large accumulations of apolipoprotein E are present in the deposits of DDD, as revealed by mass spectroscopy and confirmed by both confocal microscopy and immunohistochemistry. The findings suggest a new modality for diagnosis of DDD and introduce potential new mechanisms for understanding DDD pathophysiology.

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