A novel apolipoprotein E mutation (p.Arg150Cys) in a Chinese patient with lipoprotein glomerulopathy

Abstract

To the Editor: Lipoprotein glomerulopathy (LPG) is an uncommon inherited renal disease characterized by lipoprotein thrombi in the markedly dilated capillary lumina of affected glomeruli, high plasma concentrations of apolipoprotein E (apoE), and proteinuria.[1] It mainly affects people of Japanese and Chinese origin, and approximately 117 cases have been reported to date.[2,3] Several genetic studies have demonstrated that APOE gene mutations may lead to the development of LPG;[4-6] however, conflicting published evidence has found that some patients with APOE variants were unaffected.[7] Thus, further studies are needed to examine the role of APOE mutations in the pathogenesis of LPG. This study investigated a novel point mutation in the APOE gene in a family with one LPG patient and an asymptomatic carrier of the same APOE variant. This study was conducted in accordance with the declaration of Helsinki and was approved by the Ethics Committee of Shenzhen University. Written informed consent was obtained from all participants. A 21-year-old Chinese woman was in good health until December 2016, when during a 3-month-antenatal examination, it was determined that she had moderate proteinuria. Throughout her pregnancy, she had moderate to severe proteinuria (up to 5.2 g/day). Four months after giving birth to a healthy baby girl, she was admitted to the hospital. She had no known family history of renal disease or lipidosis. The patient was 162 cm tall and weighed 55 kg. Her blood pressure was 115/77 mmHg. She did not have pedal edema. Her urinary protein excretion was 4.0 g/day, her glomerular filtration rate was 145 mL/min, and her urine sediment contained blood cells (5–8 red blood cells/high-power field). Twenty-four-hour urine protein was 2.4 g. Other laboratory examinations revealed white blood cell count: 5.08 × 109/L, hemoglobin: 119 g/L, platelet count: 146 × 109/L, albumin: 38 g/L, blood urea nitrogen: 4.2 mmol/L, serum creatinine: 45.8 μmol/L, uric acid: 525.6 μmol/L, total cholesterol (TC): 8.53 mmol/L, low-density lipoprotein cholesterol (LDL-C): 6.44 mmol/L, high-density lipoprotein cholesterol: 1.34 mmol/L, triglycerides (TG): 1.56 mmol/L, and fasting blood glucose: 4.46 mmol/L. Hepatitis B surface antigen, anti-hepatitis C antibody, and antinuclear antibody tests were all negative. Complement levels were normal. An abdominal ultrasound showed that her renal shape and size were normal, but marked splenomegaly was observed. A percutaneous renal biopsy was performed for definitive diagnosis. By light microscopy, a total of 21 glomeruli were evaluated, 2 of which showed global sclerosis. Glomerular volume was slightly increased and was distorted by markedly dilated diffuse capillaries containing pale-staining thrombi in a layered structure, and mesangial cells and stroma showed mild to moderate proliferation. A few of the glomeruli had small synechiae between the glomerular capillaries and Bowman capsules [Figure 1A]. Staining with oil red O showed a number of lipid droplets in the glomerular capillary lumina [Figure 1B]. Foam cells were not present in either the glomeruli or the tubulointerstitium. There was interstitial edema with focal inflammatory cellular infiltration, mild and limited interstitial fibrosis, and tubular atrophy. Routine fluorescent microscopy showed trace to 1+ linear deposits of immunoglobulin M (not shown). Other immunoglobulins, C3, C4, and C1q were not detected. Strong, segmental staining for both apoE [Figure 1D] and apoB (not shown) was observed in the capillary lumina. One glomerulus was examined by electron microscopy. Almost all of the dilated capillary lumina were occluded by numerous lipid granules and lamellate vacuoles [Figure 1C].Figure 1: Glomerular findings from renal biopsy specimens in a 21-year-old Chinese LPG patient (II-1, proband). Light microscopy showed that the capillary lumina were dilated by amorphous lipoprotein thrombi (A; periodic Schiff-methenamine staining, × 400), and massive pale-stained deposits were observed in the capillary lumen (B; Oil-red-O staining, × 400). Electron micrograph showed Lamellate thrombi occupy the capillary lumen (C, × 4000). Segmental immunofluorescent stainings of apoE were present in the glomeruli (D, × 400). (E) Nucleotide sequence of a segment of exon 4 in the APOE gene for both alleles of the proband (patient II-1). The lower panel shows the allele containing a nucleotide C to T point mutation at nucleotide 208 in codon 150 of the gene. (F) Family tree of patients with LPG. LPG: Lipoprotein glomerulopathy; apoE: apolipoprotein E.In this study, exon 4 of the APOE gene was analyzed in 4 separate individuals by DNA sequencing. Genomic DNA was extracted from blood using the DNA Blood Magen Kit (Magen, Guangzhou, China) and 1 mL of the extraction was used as a template for PCR amplification. Exon 4 of the APOE gene was amplified using PCR 2xTaq Master Mix (Vazyme, Nanjing, China). The primers for exon 4 were as follows: