Abstract:
The object of the present invention is to provide a method for estimating the glomerular renal function in a convenient and non-invasive manner. As a result of intensive studies to achieve the above object, the present inventors found that there is a high correlation between the urinary megalin excretion rate and the estimated glomerular filtration rate (eGFR) in renal disease patients, and the glomerular filtration rate (GFR) can be estimated with high probability in a non-invasive manner by measuring the megalin level in the urine. This has led to the completion of the present invention.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage application of PCT/JP2014/053871, filed Feb. 19, 2014, which claims priority from Japanese application JP 2013-030206, filed Feb. 19, 2013. 
     The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-WEB and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Aug. 17, 2015, is named sequence.txt and is 56 KB. 
     TECHNICAL FIELD 
     The present invention relates to a method for estimating a glomerular filtration rate (GFR) from a measured value of megalin in urine. 
     BACKGROUND ART 
     The number of end-stage kidney disease (ESKD) patients in need of dialysis or transplantation has been increasing all over the world. During the 10 years from 1990 to 2000, the number of ESKD patients increased from 430,000 to 1,065,000. In 2008, the number reached at least about 1,650,000. In 2011, the number of patients receiving maintenance dialysis reached approximately 300,000 in Japan. This means that the number of patients per million people is 2,126, which is the second-highest such figure in the world. Chronic kidney diseases (CKDs) are preliminary forms of ESKDs that have been increasing all over the world. The number of CKD patients in the U.S. in 2000 was estimated to account for 13.07% (25,610,000 individuals) of the adult population. The number of CKD patients in Japan in 2005 accounted for 12.9% (13,300,000 individuals) of the adult population. The number of CKD patients and patients with preliminary forms of CKDs is said to have reached as high as 20,000,000. In addition, CKDs are risk factors for cardiovascular diseases (CVDs). In Western countries, the number of CKD patients who die due to CVDs is greater than the number of CKD patients who start dialysis. Also in Japan, CKDs are risk factors for CVDs. Meanwhile, if each CKD patient could receive adequate treatment in accordance with his/her own disease state, it would be possible to significantly reduce the number of patients who start dialysis or die due to heart diseases. For such purpose, adequate evaluation methods are necessary. In the existing test methods, the glomerular filtration rate (GFR) can be used for accurate renal function assessment. The term “glomerular filtration rate” refers to the volume of serum filtered through all glomeruli of the kidneys per unit of time. A decline in the renal function due to renal failure or the like is associated with a decline in the filtration capacity of glomeruli. Therefore, the degree of deterioration of the renal function can be confirmed by determining GFR. In order to accurately determine GFR, inulin clearance measurement with the use of inulin is recommended. However, since inulin clearance measurement is complex, creatinine clearance based on the creatinine level and estimated glomerular filtration rate (eGFR) are used in clinical practice. In order to obtain the creatinine clearance or eGFR, it is necessary to obtain the serum creatinine level. Serum creatinine testing is invasive and thus imposes a significant burden on patients. That is, if GFR could be estimated in a non-invasive manner, it would be beneficial to patients. 
     Megalin in urine is a substance observed in association with renal diseases. Convenient renal disorder tests involving measurement of urinary megalin have been disclosed (Patent Documents 1 and 2). 
     Megalin also known as Glycoprotein 330 (gp330) or Low Density Lipoprotein (LDL)-receptor relate protein 2 (LRP2) is a glycoprotein having a molecular weight of about 600 kDa, which is expressed in renal proximal tubular epithelial cells (Non-patent Documents 1 and 2). 
     As a result of cell culture experiments using renal proximal tubular epithelial cells, the presence of two types of megalin, membrane-bound full length megalin and soluble-form megalin (fragment containing the extracellular domain) lacking the intracellular domain, is known (Non-patent Document 3). A method for measuring urinary full-length human megalin, the extracellular domain thereof, and the intracellular domain thereof has also been reported (Patent Document 3). 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: WO2002/037099 
         Patent Document 2: WO2010/126055 
         Patent Document 3: WO2010/126043 
       
    
     Non-Patent Documents 
     
         
         Non-Patent Document 1: Christensen E. I., Willnow T. E. (1999), J. Am. Soc. Nephrol. 10, 2224-2236 
         Non-Patent Document 2: Zheng G, McCluskey R. T. et al. (1994), J. Histochem. Cytochem. 42, 531-542 
         Non-Patent Document 3: Flavia F. J., Julie R. I. et al. (1998), Kidney. International. 53, 358-366 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     The object of the present invention is to provide a method for estimating GFR without the use of creatinine serum. 
     Means for Solving the Problem 
     As a result of intensive studies to achieve the above object, the present inventors found that there is a high correlation between the urinary megalin excretion rate and the estimated glomerular filtration rate (eGFR) in renal disease patients, and the glomerular filtration rate (GFR) can be estimated with high accuracy in a non-invasive manner by measuring the urinary megalin level. This has led to the completion of the present invention. 
     Specifically, the present invention is as follows. 
     [1] A method for estimating a glomerular filtration rate from a measured value of megalin in urine. 
     [2] The method for estimating a glomerular filtration rate according to [1], wherein the measured value of megalin in urine is associated with the glomerular filtration rate for estimation of the glomerular filtration rate from the measured value of megalin in urine based on the association.
 
[3] The method for estimating a glomerular filtration rate according to [1] or [2], wherein the measured value of megalin in urine is associated with the glomerular filtration rate by regression analysis.
 
[4] The method for estimating a glomerular filtration rate according to any one of [1] to [3], wherein the measured value of megalin in urine is a urinary megalin excretion rate corrected with a urinary creatinine concentration.
 
[5] The method for estimating a glomerular filtration rate according to any one of [1] to [4], wherein the glomerular filtration rate is obtained based on inulin clearance.
 
[6] The method for estimating a glomerular filtration rate according to any one of [1] to [4], wherein the glomerular filtration rate is obtained based on creatinine clearance.
 
[7] The method for estimating a glomerular filtration rate according to any one of [1] to [4], wherein the glomerular filtration rate is obtained from an estimated glomerular filtration rate.
 
[8] The method for estimating a glomerular filtration rate according to any one of [1] to [7], wherein megalin in urine is an extracellular domain fragment of urinary megalin.
 
     This description includes all or part of the contents as disclosed in the description and/or drawings of Japanese Patent Application No. 2013-030206, which is a priority document of the present application. 
     Advantageous Effects of Invention 
     According to the method for estimating the glomerular filtration rate (GFR) based on the measured value of megalin in urine of the present invention, GFR, which in the past has been measured by invasive tests or obtained as eGFR, can be estimated in a non-invasive manner. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the relationship between the urinary megalin excretion rate and eGFR in renal disease patients (Example 2 and the Comparative Example). 
         FIG. 2  shows the relationship between α1-microglobulin and eGFR in renal disease patients (the Comparative Example). 
         FIG. 3  shows the relationship between β2-microglobulin and eGFR in renal disease patients (the Comparative Example). 
         FIG. 4  shows the relationship between N-acetyl-β-D-glucosaminidase and eGFR in renal disease patients (the Comparative Example). 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     According to the method of the present invention, the glomerular filtration rate (GFR) is estimated from the measured value of megalin in urine of a subject with high accuracy. 
     Full-length human megalin comprises an amino acid sequence of amino acids 26 to 4655 of the amino acid sequence as shown in SEQ ID NO: 2. SEQ ID NO: 1 shows the nucleotide sequence of human megalin. The sequence of amino acids 1 to 25 of the amino acid sequence as shown in SEQ ID NO: 2 is a signal peptide sequence. The amino acid sequence of human megalin is disclosed under Accession No: NP_004516 (RefSeq protein ID: 126012573) in the National Center for Biotechnology Information (NCBI). Human megalin is a single-pass transmembrane glycoprotein composed of the following three domains: an extracellular domain; a transmembrane domain; and an intracellular domain. A domain consisting of amino acids 26 to 4424 of the amino acid sequence as shown in SEQ ID NO: 2 is referred to as a human megalin extracellular domain. In addition, a domain consisting of amino acids 4447 to 4655 of the amino acid sequence as shown in SEQ ID NO: 2 is referred to as a human megalin intracellular domain. Similarly, a domain consisting of amino acids 4425 to 4446 of the amino acid sequence as shown in SEQ ID NO: 2 is referred to as a human megalin transmembrane domain. The term “human megalin extracellular domain fragment” refers to a fragment comprising the whole or part of an extracellular domain, which is the whole or part of a domain consisting of amino acids 26 to 4424 of the amino acid sequence as shown in SEQ ID NO: 2, and lacks an intracellular domain. An example of a human megalin extracellular domain fragment lacking an intracellular domain is a fragment consisting of an amino acid sequence of amino acids 26 to 4361 of the amino acid sequence as shown in SEQ ID NO: 2. Such fragment is a remnant extracellular domain fragment generated during a process of preparation of a primary cleavage product consisting of amino acids 4362 to 4655 of the amino acid sequence as shown in SEQ ID NO: 2. A further example of a human megalin extracellular domain fragment lacking an intracellular domain is a fragment consisting of an amino acid sequence of amino acids 4362 to 4437 of the amino acid sequence as shown in SEQ ID NO: 2. Such fragment is a remnant extracellular domain fragment generated during a process of preparation of a secondary cleavage product consisting of amino acids 4438 to 4655 of the amino acid sequence as shown in SEQ ID NO: 2. 
     In the present invention, the measurement of megalin excreted in the urine is performed. Megalin subjected to measurement may be full-length megalin, the extracellular domain thereof, or the intracellular domain thereof, and it is preferably the extracellular domain. Urine specified herein as a specimen can be obtained from any subject. Any method for collecting urine may be employed herein. Morning urine or spot urine is preferably used. In addition, the amount of urine required for the method of the present invention ranges from about 10 μL to 200 μL. The test method of the present invention may be performed together with other general urine tests. 
     Urine used as a specimen can be processed by adding a treatment liquid to collected urine, followed by mixing. The treatment liquid may be any liquid as long as it allows adjustment of urine pH, masking of urine sediment, and/or solubilization of megalin. However, a preferable example is a solution obtained by adding a chelating agent, a surfactant, and the like to a buffer solution. A buffer solution or a chelating agent may be any known product while a surfactant to be used is preferably a non-ionic surfactant. An example of a treatment liquid is a solution containing 2M Tris-HCl (pH 8.0), 0.2M EDTA, and 10% (vol./vol.) Triton X-100. A urine sample solution can be obtained by adding 10 μl of such treatment liquid to 90 μL of a urine specimen, followed by mixing. 
     Various methods can be used for detecting megalin from a urine sample solution. An example of a method for detecting megalin is an immunological technique. An immunological technique can be performed by immunostaining methods (including a fluorescent antibody technique, an enzyme antibody method, a heavy metal-labeled antibody method, and a radioisotope-labeled antibody method), methods using a combination of separation by electrophoresis and detection using fluorescence, enzyme, and radioisotope etc., (including Western blot method and fluorescence two-dimensional electrophoresis), an enzyme-linked immunosorbent assay (ELISA) method, a dot blotting method, a latex agglutination method (LA: Latex Agglutination-Turbidimetric Immunoassay), immunochromatography method, or the like. Preferably, ELISA methods or LA methods are employed. Among ELISA methods, a sandwich method is preferably employed from a quantitative viewpoint. In the case of a sandwich method, a urine sample solution is added to a microtiter plate to which an anti-megalin antibody has been immobilized for antigen-antibody reaction to take place, an enzyme-labeled anti-megalin antibody is further added for an antigen-antibody reaction to take place, washing is performed, the resultant is reacted with an enzyme substrate for color development, absorbance is measured, and thus urinary megalin is detected and the concentration of urinary megalin can be calculated from the measured value. Moreover, an antigen-antibody reaction is performed using a fluorescence-labeled anti-megalin antibody, and then fluorescence can be measured. 
     An anti-megalin antibody that is used in an immunological technique may be an antibody with which human megalin can be recognized and detected. An anti-megalin antibody that is used in the present invention may be a known antibody or an antibody that could be developed in the future. Examples of an anti-megalin antibody include, but are not particularly limited to: a monoclonal antibody, a polyclonal antibody, a chimeric antibody, a humanized antibody, and fragments thereof with binding activity. In addition, a megalin domain to be recognized is mot limited. Thus, either an antibody that recognizes an extracellular domain or an antibody that recognizes an intracellular domain can be used. These antibodies may be labeled with enzymes or fluorescent dyes. In addition, 2 or more types of anti-megalin antibody may be used. Two (2) or more types of anti-megalin antibody are used for the above sandwich method, and are preferably antibodies that recognize epitopes differing from each other. For example, as an anti-megalin antibody, an antibody against a megalin extracellular domain fragment can be used. Specifically, two or more types of anti-megalin antibodies that recognize different epitopes selected from among epitopes located in a domain (LBD1) consisting of amino acids 26 to 314 in the extracellular domain of the amino acid the amino acid sequence as shown in SEQ ID NO: 2 can be used. 
     According to the present invention, the urinary megalin level means the measured value of megalin in urine. It may be the urinary megalin concentration or a value obtained by correcting the urinary megalin concentration with the values of urine components that are stably excreted in the urine (urine component values). 
     A particularly preferable urine component is urinary creatinine. It is preferable to correct the urinary megalin concentration with the urinary creatinine concentration. It is thought that the urinary creatinine concentration is substantially constant for each individual regardless of disease because creatinine production depends on muscle amount. When testing urine components, it is common to use a technique for correcting the amount of a urine component of interest based on the amount per 1 g of creatinine in order to avoid the influence of urine dilution or concentration. This allows comparison of urine components per unit gram of creatinine. The value obtained by correcting the urinary megalin concentration with the urinary creatinine concentration is referred to as the urinary megalin excretion rate (MEG/Cre), and it can be calculated with the following equation.
 
Urinary megalin excretion rate (pmol/gCre)=100×Urinary megalin concentration (pM)/Urinary creatinine concentration (mg/dL)  &lt;Equation&gt;MEG/Cre:
 
     According to the method of the present invention, the measured value of megalin in urine is associated with the glomerular filtration rate to estimate the glomerular filtration rate based on the measured value of megalin in urine. 
     In order to accurately determine the glomerular filtration rate (GFR), creatinine clearance can be used, as well as inulin clearance. Further, the estimated glomerular filtration rate (eGFR) can be used as the glomerular filtration rate. 
     The estimated glomerular filtration rate (eGFR) can be obtained using the following predictive equation disclosed in March 2008 in the “Japanese Equation for Estimating GFR” project launched by the Japanese Society of Nephrology based on the serum creatinine level and age. eGFR (mL/min/1.73 m 2 )=194×Cr −1.094 ×Age −0.287  (where “Cr” means the serum creatinine level and “Age” means the subject&#39;s age) (If the subject is a female, the solution is further multiplied by 0.739.) 
     There is a positive correlation between eGFR obtained as a result of estimation by the above equation and the urinary megalin excretion rate expressed as “pmol/gCre” above. For example, when the urinary megalin excretion rate is 0-100 pmol/gCre and eGFR is 0-150 mL/min/1.73 m 2 , R 2  (where R represents a correlation coefficient) is 0.3 or more and preferably 0.4 or more. As the urinary megalin excretion rate increases, eGFR also increases. The term “eGFR” represents the estimated value of GFR. Since there is a high correlation between the urinary megalin excretion rate and eGFR, it is possible to estimate GFR from the urinary megalin excretion rate with high accuracy. GFR estimated by the method of the present invention is highly likely to approximate actual GFR. In this regard, GFR can be estimated with high probability by the method of the present invention. 
     It is possible to associate the measured value of megalin in urine with the glomerular filtration rate by, for example, obtaining an equation showing the relationship between the measured value of megalin in urine and eGFR through regression analysis. eGFR can be obtained from the urinary megalin excretion rate based on the obtained regression equation. GFR can be estimated based on eGFR. The present invention also encompasses a method for determining the measured value of megalin in urine and eGFR or GFR for a plurality of subjects and obtaining a regression equation representing the relationship between the measured value of megalin and eGFR or GFR through regression analysis so as to obtain a calculation formula that is used for estimating eGFR or GFR from the measured value of megalin in urine. 
     The above equation for obtaining eGFR is intended to be used for Japanese people. eGFR, however, can be estimated based on the serum creatinine level and age regardless of race. That is, GFR can be estimated from the urinary megalin excretion rate regardless of race. 
     It is also possible to associate the measured value of megalin in urine with the glomerular filtration rate determined with the use of inulin clearance or creatinine clearance so as to estimate the glomerular filtration rate from the measured value of megalin in urine based on the obtained association. 
     A decrease in GFR estimated from the measured value of megalin in urine means a decrease in the renal function. Thus, it is possible to select a therapy and the like for a patient with decreased renal function based on GFR estimated by the method of the present invention. 
     EXAMPLES 
     The present invention is described in detail with reference to examples of the present invention. However, the present invention is not limited thereto and various applications of the present invention are feasible within the technical idea of the present invention. 
     Example 1 
     Determination of the Urinary Megalin Excretion Rate 
     The rate of excretion of a fragment (the extracellular domain of megalin) containing human megalin extracellular domain in the urine was determined using a monoclonal antibody (anti-megalin extracellular domain monoclonal antibody) against the human megalin extracellular domain. The anti-megalin extracellular domain monoclonal antibody is a mouse monoclonal antibody that recognizes an epitope that is present in the domain (LBD1) consisting of amino acids 26 and 314 of the amino acid sequence as shown in SEQ ID NO: 2. Determination and evaluation were carried out using an anti-human megalin LBD1 monoclonal antibody A and an anti-human megalin LBD1 monoclonal antibody B recognizing two different epitopes in LBD1. The human megalin extracellular domain-containing fragment in the urine was measured using a microtiter plate onto which an anti-human megalin LBD1 monoclonal antibody A had been immobilized, and an ALP (alkaline phosphatase)-labeled anti-human megalin LBD1 monoclonal antibody B. 
     First, 90 μL of urine and 10 μL of a solution comprising 2 M Tris-HCl, 0.2 M ethylenediamine-N,N,N′,N′-tetraacetic acid (hereafter abbreviated as “EDTA”), 10% (vol./vol.) polyethylene glycol mono-p-isooctylphenyl ether (hereinafter, polyethylene glycol mono-p-isooctylphenyl ether is referred to as Triton X-100) (pH 8.0) were mixed. The mixed solution (100 μL) was added to wells of the microtiter plate onto which an anti-human megalin LBD1 monoclonal antibody A had been immobilized (FluoroNunc (Trademark) Module F16 Black-Maxisorp (Trademark) Surface plate, Nalge Nunc International), and then left to stand at 37° C. for 1 hour. Subsequently, the urine sample solution that had been added to wells was removed by decantation. TBS-T was added at 200 μL/well to the wells of the microtiter plate, and then removed by decantation, followed by washing. The washing step was performed 3 times in total. Subsequently, an ALP-labeled anti-human megalin LBD1 monoclonal antibody B (0.5 ng/mL) solution was added at 100 μL/well. The ALP-labeled anti-human megalin LBD1 monoclonal antibody B was prepared with a diluent for a labeled antibody. The resultant was left to stand at 37° C. for 1 hour, and then the ALP-labeled antibody solution that had been added to wells was removed by decantation. TBS-T was added at 200 μL/well to the wells of the microtiter plate, and then removed by decantation, followed by washing. The washing step was performed 4 times in total. Subsequently, an assay buffer was added at 200 μL/well to the wells of the microtiter plate, and then removed by decantation, followed by washing. The washing step was performed 2 times in total. Next, CDP-Star (registered trademark) Chemiluminescent Substrate for Alkaline Phosphatase Ready-to-Use (0.4 mM) with Emerald-II (trademark) Enhancer (ELISA-Light (trademark) System: Applied Biosystems) was added as a substrate solution for ALP enzymatic reaction to wells at 100 μL/well, and then left to stand at 37° C. for 30 minutes while protecting the solution from light. Immediately after the procedure, the cumulative emission intensity of the wells was measured for 1 second. The measured values were used as indicators for evaluation of the measurement of urinary megalin. Microplate Luminometer Centro LB960 and MicroWin2000 software (Berthold) were used for measuring chemiluminescence intensity. As the reference sample for the calibration curve, native human megalin extracted from the kidney was used. The urinary megalin excretion rate was calculated by the following equation by correcting the urinary megalin concentration with the urinary creatinine concentration.
 
Urinary megalin excretion rate (pmol/g)=100×Urinary megalin concentration (pM)/Urinary creatinine concentration (mg/dL)  &lt;Equation&gt;MEG/Cre:
 
     Example 2 
     Relationship Between the Urinary Megalin Excretion Rate and the Estimated Glomerular Filtration Rate (eGFR) 
     The megalin concentrations in 45 cases of renal disease patients were determined by the method specified in Example 1 to obtain the urinary megalin excretion rates. In addition, eGFR was obtained using the following predictive equation disclosed in March 2008 in the “Japanese Equation for Estimating GFR” project launched by the Japanese Society of Nephrology. 
     eGFR (mL/min/1.73 m 2 )=194×Cr −1.094 ×Age −0.287  (where “Cr” means the serum creatinine level and “Age” means the subject&#39;s age) (If the subject is a female, the solution is further multiplied by 0.739.) 
       FIG. 1  shows the relationship between the urinary megalin excretion rate and eGFR.  FIG. 1  revealed that there is a high correlation between the urinary megalin excretion rate and eGFR (R 2 =0.4266). That is, eGFR can be estimated from the measured value of megalin in urine. 
     Comparative Example 
     Relationship Between Other Renal Disease Markers and eGFR 
     The urinary megalin excretion rates in 45 cases of renal disease patients were determined by the method specified in Example 1. In addition, eGFR was obtained using the predictive equation disclosed in March 2008 in the “Japanese Equation for Estimating GFR” project launched by the Japanese Society of Nephrology. 
     Further, measurement was performed for al-microglobulin (AMG), β2-microglobulin (BMG), and N-acetyl-β-D-glucosaminidase (NAG), which are proximal tubular defect markers, as well as urinary megalin. Since these AMG, BMG, and NAG are urine markers, they are corrected with the urinary creatinine level. Measurement of AMG, BMG, and NAG was performed using commercially available kits. 
       FIGS. 2, 3, and 4  show the relationships between eGFR and AMG, eGFR and BMG, and eGFR and NAG, respectively. 
     The results of Example 2 and the Comparative Example revealed that the highest correlation was observed between eGFR and urinary megalin from among four different urine markers, including urinary megalin. That is, although urinary megalin is a proximal tubular defect marker, it is a marker capable of estimating eGFR with high accuracy. 
     INDUSTRIAL APPLICABILITY 
     As explained above, with the use of the present invention, it is possible to estimate GFR with high accuracy by measuring megalin in urine. In addition, since urine is used as a sample in the test method of the present invention, the test can be performed in a non-invasive manner, unlike testing with the conventional methods. This allows reduction of the physical burdens imposed on patients, which is advantageous. 
     All publications, patents, and patent applications cited herein are incorporated herein by reference in their entirety.