Our study suggests that the urinary NAG/Cr may represent a useful functional and structural marker in ADPKD patients. A single measurement of urinary NAG was well correlated with concurrent estimated GFR and TKV. In addition, when compared with other urinary biomarkers, urinary NAG levels showed a better association with renal functional and structural markers.
In fact, NAG has several advantages for use as a novel biomarker. First, it is relatively stable with little diurnal variation . Second, it is a sensitive marker for tubular damage. Subtle alterations in the epithelial cells in the brush border of the proximal tubules result in shedding of NAG into the urine, and the amount of shed enzyme can be directly correlated to tubular injury . Third, NAG can be easily measured and quantified by spectrophotometric assay . However, NAG, a sensitive renal tubular injury marker, has not been thoroughly investigated in ADPKD patients.
Meanwhile, several factors have been suggested to contribute to hypertension and renal failure in ADPKD. First, Chapman et al. reported that decreased renal blood flow (or increased vascular resistance) is an independent predictor of renal function decline . Another study demonstrated morphological evidence of vascular malformation using angiograms. The authors suggested that neo-vascularization, which may be caused by cystic compression of adjacent tissue, is a key factor for renal progression . Some investigators have suggested that intrarenal activation of the renin-angiotensin system, which is likely the result of renal ischemia, may play an important role in disease progression [16, 17]. Eckardt et al. demonstrated increased erythropoietin formation in interstitial cells juxtaposed with proximal tubular cysts in ADPKD patients . Results from these studies point to the importance of ischemic insult and subsequent vascular remodeling in the disease progression of ADPKD.
When we performed immunohistochemical staining of NAG, staining was strongest along the cyst-lining epithelial cells and the proximal tubular epithelial cells. In particular, the cells that were compressed by nearby cysts were more strongly stained than normal control cells, suggesting that ischemic insult by cyst growth to the adjacent tissue may be the key factor in the production of NAG. Moreover, most of the enlarged cysts do not have any connection with the original tubules, and therefore NAG production from the cyst-lining epithelial cells is less likely to contribute to the urinary NAG level. Increased urinary NAG may be better explained by NAG production from the nearby compressed tubules. Therefore, urinary NAG may represent ongoing ischemic insult in ADPKD patients.
In our study, urinary NAG/Cr was correlated with current TKV. We measured TKV by modified ellipsoid method from CT images. Most previous studies measured TKV by computer-based volumetry using magnetic resonance imaging (MRI). We used CT images in our patients because it is a more useful tool to diagnose renal stones or obstructive uropathy and because it is less expensive than MRI. The modified ellipsoid method was developed to measure kidney volumes from ultrasonographic images. It underestimates TKV when compared with MRI volumetry . However, it is easy, economical, and useful to evaluate renal enlargement in serial measurements. Although the modified ellipsoid method has not been used for CT images, our results are comparable to that from the CRISP study (R = −0.54, P < 0.001 vs. R = −0.37, P < 0.001). The correlation between urinary NAG/Cr and TKV was weaker than that with the estimated GFR. The reasons for this remain to be further elucidated, but one possible explanation is that cysts are small in the early stages and therefore inadequate to produce ischemic insults to the adjacent tissue.
Previously, Meijer et al. investigated the role of NAG in their cross-sectional study with 102 ADPKD patients and 102 age- and sex-matched control subjects . The authors concluded that NGAL was the most useful marker for ADPKD disease progression. However, their results should be interpreted cautiously because they included relatively small number of patients and mainly Caucasian. In addition, because we included patients with relatively conserved renal function (IDMS-traceable MDRD GFR 85.0 ± 24.7 vs. 68 ± 27 mL/min/1.73 m2) and smaller TKVs (1014 [211–5324] vs. 1500 [900–2200] mL) than Meijer’s population, the urinary NAG/Cr may be the better marker to represent early disease progression in ADPKD patients. Moreover, we demonstrated the possible link between cyst growth and pronounced NAG production from compressed tubules. Therefore, urinary NAG/Cr may be more useful surrogate marker for renal function that shows underlying pathogenesis of ADPKD disease progression.
Unfortunately, our study failed to prove the role of urinary NAG/Cr in the prediction of renal function deterioration in 1 year. In our study, both single and repeated measurements of urinary NAG/Cr were not able to predict a decline in the estimated GFR. However, the patients with higher baseline NAG/Cr (≥ 4.95 IU/g) showed relatively higher annual percentage decrement in estimated GFR (−22.9% vs. -20.7%, P = 0.3). In addition, the persistently high NAG/Cr (Group H-H ≥ 4.95 IU/g) group showed higher annual percentage decrement in estimated GFR compared to the other groups (Group H-H vs. Group V vs. Group L-L, -23.0% vs. -20.8% vs. -21.2%, P = 0.77). Our findings are in the same context with the results from the recent 3-year prospective study demonstrated by CR Parikh et al. . They showed both urinary Il-18 and NGAL levels were elevated in ADPKD but did not correlate with worsening in kidney function or increase in TKV. Since the natural course of ADPKD is indolent in the early stage, 1 year of follow-up seems too short to evaluate the renal function decline in our cohort. Moreover, the effect of baseline estimated GFR is too strong to see the independent effect of urinary NAG/Cr upon the subsequent renal function decline. Therefore, we assume that longer period of follow-up time is necessary to evaluate the rate of renal function decline.
This is the first 1-year prospective study demonstrating the usefulness of urinary NAG/Cr as a surrogate marker for renal function in ADPKD. To the best of our knowledge, our study evaluated the largest ADPKD cohort to date for a urinary biomarker study. However, our study has a few limitations. This is a single-center study that included only Korean patients. Second, we excluded patients in advanced CKD stages. Therefore, further large-scale studies should be conducted to extend our results to other populations. Third, we measured TKV using a modified ellipsoid method from CT images. Although our results are comparable with CRISP results, validation is needed to using this method in CT volumetry. Finally, long-term follow-up data should be analyzed later to determine whether urinary NAG is useful in the prediction of renal function decline.