VP1 messenger RNA levels in urine for diagnosing BK virus nephropathy?

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Ding and coworkers (1) explored polyomavirus BK VP1 mRNA levels in urine to predict BK virus nephropathy in renal allograft recipients. The authors performed a thorough analysis of 8 patients with and 28 patients without histologic evidence of the disease. However, several points should be mentioned. 1. The authors state that “decoy cells” correspond to renal tubular cells with intranuclear viral inclusion bodies. It is well documented that decoy cells stem from at least two different sources (i.e., renal tubular cells and urothelial cells from the ureters or the bladder). In fact, Figure 9 of the reference quoted by Ding et al. demonstrates this point unambiguously (2). Thus, shedding of decoy cells can be observed without renal involvement (3–5). 2. Our work (2) is incorrectly quoted by stating that “decoy cells are absent in the urine of 72% of patients with BK virus nephritis”. On the contrary, we have repeatedly indicated (2–5) that shedding of decoy cells is highly sensitive, but not specific, for BK virus nephropathy for the reasons outlined above. Urine cytology is a simple screening tool of high negative and low positive predictive value and should be complemented by adjunct diagnostic measures such as the search for BK virus DNA in plasma (3–5). In fact, increasing BK virus loads in plasma of 4 or more log10/mL are highly specific (>95%) and more than 95% sensitive for histologically confirmed BK virus nephropathy [(5) and H.H. Hirsch, unpublished data]. 3. The relevance of VP1 mRNA quantification is dependent on the purity of the RNA preparation before reverse-transcription to cDNA. Importantly, contamination by the encoding BK virus genomic DNA could yield falsely high results. Technically, this could be avoided by selecting PCR primers for sequences upstream and downstream of introns that are removed from the mRNA by splicing. However, the BK virus VP1 gene lacks introns as noted (1). To estimate the degree of viral genomic VP1 DNA contaminating the VP1 cDNA preparation, the authors should quantify VP1 signals after omitting reverse transcription or after prior DNase digestion. 4. Finally, as indicated above, an important conceptual drawback stems from the fact that polyomavirus replication efficiently occurs in the transitional urothelial cell layer, which results in shedding of urothelially derived decoy cells in high numbers even in the absence of BK virus nephropathy (2–5). Thus, VP1 mRNA expression cannot be attributed to the originating cell type (e.g., urothelia or tubulus cells) with certainty, and the interpretation of VP1 mRNA quantification may be limited. Possibly, tubulus epithelial- and urothelial-specific cell transcripts might be required for normalization. Thus, the elegant rationale to use late gene expression as a marker for viral replication as suggested for cytomegalovirus some years ago may be faced with numerous obstacles, particularly if performed on urinary cells and not on allograft biopsy specimens. Hans H. Hirsch3