[The origins of the Renal Immunopathology Group of the Italian Society of Nephrology].

This article describes the birth and development of the Renal Immunopathology Group of the Italian Society of Nephrology. It collects the stories of nephrologists and pathologists who, since the early Seventies up to the first decade of this century, devoted their professional lives to the study of renal pathology with a strong personal involvement, characterized by enthusiasm, commitment, ability, strong spirit of cooperation, and friendship. All this enabled the Group to: propose the criteria for a standardized histological and immuno-histological examination of renal biopsies and reporting; produce several multicenter studies, whose results were also published in important international journals; to set up a national registry of renal biopsies; to organize a number of courses, some of which were associated with the publication of monographs, on various renal diseases. This article also traces the history of renal pathology in Italy from the second half of the Sixties - when young Italian nephrologists and pathologists from different institutions moved to French laboratories to learn new techniques to apply to renal biopsies - up to the present days. It also shows us how Italian renal pathology has been an essential tool for the development of the nephrological clinical practice and the advancement of scientific research.

Pregnancy-related decidualization of subcutaneous endometriosis occurring in a post-caesarean section scar: Case study and review of the literature.

Endometriosis of surgical scars is a rare complication of caesarean sections (incidence: 0.03-0.4%) and other surgical procedures. As endometriosis could be responsive to hormonal stimulation, decidualization and other secondary changes may occur during pregnancy or progestin therapy, sometimes causing a clinically-evident increase in the size of the endometriotic nodules, which could be mistaken for malignant tumors. To our knowledge, we report the 8th subcutaneous case of a pregnancy-related decidualization occurring in a post-caesarean section scar endometriosis. A 33-year-old woman showed a painless, firm, subcutaneous nodule (size: 1 cm) located near the scar of a caesarean section performed 3 years before. Ultrasound examination revealed a well-delimited, hypoechogenic nodule showing perilesional inflammatory reaction without vascular signals. The nodule was considered a post-surgical granuloma: its size did not increase during 4 years of follow-up. Finally, the nodule was totally excised during a second caesarean section performed at 39 weeks of gestation. Histological examination showed nodules of decidualized stromal cells surrounding rare, small, atrophic endometrial glands. Nuclear atypia and mitoses were absent. On immunohistochemical examination, the epithelial cells were pan-CK(AE1/AE3)+/ER+/PR+/S100-/Calretinin-/Vimentin-, while the stromal cells were pan-CK(AE1/AE3)-/Vimentin+/ER+/PR+/CD10+/S100-/Calretinin-. We reviewed the literature, discussing the main clinic-pathological diagnostic pitfalls and the possible differential diagnoses.

CD44 immunoreactivity in the developing human kidney: a marker of renal progenitor stem cells?

CD44 is a transmembrane adhesion glycoprotein, functioning as a hyaluronan receptor and participating in the uptake and degradation of hyaluronan. Recently, CD44 has been proposed in the adult kidney as a marker of activated glomerular parietal epithelial cells, the putative niche stem cells that, in case of damage to podocytes, might migrate inside the glomerular tuft and undergo transition to podocytes. Here, immunoreactivity for CD44 was tested in 18 human fetuses and newborns with a gestational age ranging from 11 to 39 weeks. CD44 immunoreactivity was observed in all but one developing kidneys, being localized in several renal cell types including intraglomerular, capsular, cortical and medullary interstitial cells and nerve cells. In some cases, CD44 marked scattered cells in nephrogenic subcapsular zone. Our data indicate that CD44 is involved in human nephrogenesis, probably marking a subset of progenitor/stem cells involved in early phases of kidney development and, putatively, in podocyte and/or interstitial cell differentiation.

Mutation analysis of KRAS in primary colorectal cancer and matched metastases by means of highly sensitivity molecular assay.

Mutation analysis of KRAS is needed before starting treatment with anti-EGFR monoclonal antibodies in patients with metastatic colorectal cancer (CRC). In most of the cases, testing is performed on primary tumors, assuming that KRAS mutation status does not change in metastasis although correlation studies gave conflicting results. We evaluated the KRAS status concordance rate between primary tumors and related metastasis using a highly sensitive molecular assay. Forty-five primary tumors and related metastases from patients with CRC (28/45 male-62.2% and 17/45 female-37.8%; mean age 66.4 years) were analyzed by using TheraScreen: KRAS mutational kit. Metastatic samples were collected from lymph nodes (8/45-17.8%) and visceral sites (37/45-82.2%); 23 were synchronous (49%) and 22 were metachronous (51%), obtained after a mean of 30.8 months after the first diagnosis of CRC. Twenty-eight patients had KRAS mutations in both primary CRC and related metastases (62.2%). No differences in type and frequency of mutations were identified, despite different metastatic sites and time of onset of metastatic disease. Our results indicate that the mutation status of KRAS is the same in primary CRC and metastasis, suggesting that in clinical practice, KRAS testing can be performed on both tumor tissues when using a highly sensitive molecular assay.

"Physiological" renal regenerating medicine in VLBW preterm infants: could a dream come true?

An emerging hypothesis from the recent literature explain how specific adverse factors related with growth retardation as well as of low birth weight (LBW) might influence renal development during fetal life and then the insurgence of hypertension and renal disease in adulthood. In this article, after introducing a brief overview of human nephrogenesis, the most important factors influencing nephron number at birth will be reviewed, focusing on the "in utero" experiences that lead to an increased risk of developing hypertension and/or kidney disease in adult. Since nephrogenesis in preterm human newborns does not stop at birth, but it continues for 4-6 weeks postnatally, a better knowledge of the mechanisms able to accelerate nephrogenesis in the perinatal period, could represent a powerful tool in the hands of neonatologists. We suggest to define this approach to a possible therapy of a deficient nephrogenesis at birth "physiological renal regenerating medicine". Our goal in preterm infants, especially VLBW, could be to prolong the nephrogenesis not only for 6 weeks after birth but until 36 weeks of post conceptual age, allowing newborn kidneys to restore their nephron endowment, escaping susceptibility to hypertension and to renal disease later in life.

Epidermal growth factor receptor gene analysis with a highly sensitive molecular assay in routine cytologic specimens of lung adenocarcinoma.

Epidermal growth factor receptor (EGFR) gene mutational analysis is critical for guiding the treatment of lung adenocarcinoma. In everyday clinical practice, EGFR testing is frequently centralized in referral laboratories that may receive paucicellular cytologic specimens, often fixed in various ways. We conducted a search for EGFR mutations in 108 cytologic samples of lung adenocarcinoma from different hospitals using the TheraScreen EGFR29 kit. These samples included 80 (74.1%) fine-needle aspirations, 13 (12%) pleural/ascitic fluids, 13 (12%) bronchial washings, and 2 bronchial brushings. The samples were fixed in ethanol (n = 79), Duboscq-Brasil (n = 18) or formalin (n = 10); 1 was unfixed. Ninety-two (85.2%) were amplified, 16 (14.8%) were not. Mutations were detected in 22 (23.9%) of 92 amplified samples, 9 containing less than 200 cancer cells, and 4 with less than 50% cancer cells. DNA was amplified in 12 of 18 Duboscq-Brasil-fixed samples. These findings indicate that cytologic specimens are adequate for EGFR testing when a highly sensitive assay is used, even if they are paucicellular or not optimally fixed.

FAN1 mutations cause karyomegalic interstitial nephritis, linking chronic kidney failure to defective DNA damage repair.

Chronic kidney disease (CKD) represents a major health burden. Its central feature of renal fibrosis is not well understood. By exome sequencing, we identified mutations in FAN1 as a cause of karyomegalic interstitial nephritis (KIN), a disorder that serves as a model for renal fibrosis. Renal histology in KIN is indistinguishable from that of nephronophthisis, except for the presence of karyomegaly. The FAN1 protein has nuclease activity and acts in DNA interstrand cross-link (ICL) repair within the Fanconi anemia DNA damage response (DDR) pathway. We show that cells from individuals with FAN1 mutations have sensitivity to the ICL-inducing agent mitomycin C but do not exhibit chromosome breakage or cell cycle arrest after diepoxybutane treatment, unlike cells from individuals with Fanconi anemia. We complemented ICL sensitivity with wild-type FAN1 but not with cDNA having mutations found in individuals with KIN. Depletion of fan1 in zebrafish caused increased DDR, apoptosis and kidney cysts. Our findings implicate susceptibility to environmental genotoxins and inadequate DNA repair as novel mechanisms contributing to renal fibrosis and CKD.

Extra-osseous Ewing sarcoma of the thyroid gland mimicking lymphoma recurrence: a case report.

Extra-osseous Ewing sarcomas/peripheral primitive neuroectodermal tumors (EOES/pPNETs) are high-grade malignant tumors found in various organs, such as the lung, skin, intestine, kidney and female genital tract; however, to the best of our knowledge, only two cases have previously been identified in the thyroid gland. We describe a case of primary EOES/PNET of the thyroid gland in a 66-year-old man with a previous history of large B cell lymphoma. During a routine follow-up examination, the patient underwent an ultrasound cervical scan showing a solid nodule of the left thyroid lobe. The fine-needle aspiration biopsy of the nodule suggested a neuroendocrine tumor. Histological and immunohistochemical examination of the surgical specimen supported a diagnosis of EOES/PNET, which was further confirmed by the demonstration of EWSR1 gene translocation by means of fluorescent in situ hybridization and by the detection of glycogen particles and neurosecretory granules by means of electron microscopy. Total body computed tomography and magnetic resonance imaging excluded the involvement of other sites, and therefore a diagnosis of primary EOES/PNET of the thyroid gland was made.This paper also discusses the main differential diagnoses, including lymphoma recurrence, other small round cell tumors (primary or metastatic), and a thyroid localization of an EWS/PNET from another organ.

Expression of WT1 during normal human kidney development.

Wilms Tumor 1 (WT1) is a zinc finger protein, expressed by human podocytes in the adult kidney, which plays a relevant role in different phases of nephrogenesis in experimental animals. Since no data are available for specific role in the human fetal kidney, this study aimed at investigating the expression of WT1 during the different phases of nephrogenesis. To this end, the expression of WT1 was evaluated in the kidneys, from four human fetuses and two newborns. WT1 immunoreactivity was detected in all the examined kidneys, but not in the kidneys of the newborn at term. Immunostaining for WT1 was observed in podocytes of the glomeruli and in the subcapsular regions, in areas of active glomerulogenesis. The extent and the intensity of immunoreactivity for WT1 changed from one case to the next according to the different gestational age. This study confirms in human kidney the relevant role played by WT1 during nephrogenesis. Its expression pattern suggests a main role in the regulation of the process of Mesenchimal-Epithelial-Transition and in the development and maturation of podocytes. Further studies are needed to verify the correlation between the expression pattern of WT1 and that of other genes products involved in nephrogenesis, in order to better understand their relationship at protein level.

MUC1 in mesenchymal-to-epithelial transition during human nephrogenesis: changing the fate of renal progenitor/stem cells?

BACKGROUND: The development of the human kidney is a complex process requiring interactions between epithelial and mesenchymal cells. The condensed cap mesenchyme is hypothesized to generate a population of stem/progenitor cells that undergo mesenchymal-epithelial transition (MET) originating nephrons. Few immunohistochemical markers are available for detecting cap mesenchymal cells in the early phases of MET. METHODS: The expression of MUC1 was evaluated in the kidneys, of 4 human foetuses and 2 newborns. RESULTS: MUC1 immunoreactivity was detected in all the examined kidneys in the cap mesenchyme and in the renal vesicles. Immunostaining for MUC1 in cap mesenchymal cells changed from one nodule to the next: some mesenchymal nodules were negative, some showed MUC1 reactivity in scattered cells, whereas in others, positive cells revealed the presence of a roundish developing epithelial structure. CONCLUSIONS: Our data clearly indicates, for the first time to the best of our knowledge, immunohistochemical evidence of MUC1 expression during human kidney development. We focused on MUC1 reactivity in the cap mesenchyme. On the basis of these preliminary data, we speculate that MUC1 may be involved in human nephrogenesis and may play a relevant role in MET from the cap mesenchyme to the renal vesicle, changing the fate of renal stem/progenitor cells.

Fluorescence in situ hybridisation in the cytological diagnosis of pancreatobiliary tumours.

AIMS: To assess the sensitivity, specificity, positive and negative predictive values of fluorescence in situ hybridisation (FISH) and conventional cytology in identifying bile duct stricture malignancies. METHODS: Brushing samples were collected from 64 patients by means of endoscopic retrograde cholangiopancreatography, and assessed cytologically and by means of a multi-probe FISH set. The cytological diagnoses were: positive, negative and suspicious, whereas criteria for FISH positivity were: more than five polysomic cells or more than 10 trisomic cells for chromosomes 3 or 7. RESULTS: Forty-eight of the 64 patients showed histological or clinical signs of malignancy. The sensitivity of cytology was high (77%) if suspicious cases were considered positive, but was significantly lower than that of FISH if suspicious cases were considered negative (58% versus 90%; p < 0.05). The specificity of cytology was 81% (positive and suspicious) or 100% (negative and suspicious), and the specificity of FISH was 94% (p = 1). FISH yielded one false negative result (isolated chromosome 7 trisomy). FISH allowed a definite diagnosis of 9/12 cytologically inconclusive cases. CONCLUSIONS: Our findings suggest using FISH in the case of bile duct strictures cytologically negative or inconclusive; a FISH diagnosis of malignancy should only be made in the presence of polysomic pattern.

Serological evidence of vertical transmission of JC and BK polyomaviruses in humans.

Vertical transmission of JC virus and BK virus has been investigated by few authors, with conflicting results. We performed a combined serological and genomic study of 19 unselected pregnant women and their newborns. Blood and urine samples were collected during each gestational trimester from the pregnant women. Umbilical cord blood, peripheral blood, urine and nasopharyngeal secretion samples were taken from newborns at delivery and after 1 week and 1 month of life. Polyomavirus DNA was detected by nested PCR. Polyomavirus IgG-, IgM- and IgA-specific antibodies were measured in maternal and newborn serum samples using a virus-like-particle-based ELISA method. BKV and JCV DNA were detected in urine from 4 (21 %) and 5 (26 %) women, respectively. BKV and JCV seroprevalences in the pregnant women were 84 % and 42 %, respectively. Using a rise in the IgG level or the transient appearance of an IgA or IgM response as evidence of infection in the newborn, we detected BKV and JCV infections in four (21 %) and three (16 %) newborns, respectively. Three infants had serological evidence of infection with both BKV and JCV. In two of the four possible BKV-infected newborns, the mothers seroconverted during pregnancy, while another mother was viruric and IgA seropositive. The mother of one of the three possible JCV-infected newborns was viruric and IgA seropositive; another mother was viruric. These results suggest JC virus and BK virus can be transmitted from mother to newborn during pregnancy or soon after birth.

Kidney involvement in a patient affected by placental site trophoblastic tumor.

We report a 42-year-old woman who presents a few days after a spontaneous incomplete abortion at the ninth week of pregnancy with hypertension and nephrotic syndrome. Curettage findings and increased values for the ß subunit of human chorionic gonadotrophin were suspicious for a trophoblastic disease. A uterine placental site trophoblastic tumor was diagnosed 2 months later after hysterectomy and treated successfully using chemotherapy. Kidney biopsy showed features consistent with an unusual form of thrombotic microangiopathy characterized by the presence of large thrombus-like structures occluding the capillary lumina and smaller aggregates in the mesangium and along glomerular basement membranes. These deposits were positive for immunoglobulin M, C4, C1q, κ and λ light chains, and fibrinogen. Electron microscopy showed fibrin deposits located primarily in the subendothelial space. The differential diagnosis of this presentation included pre-eclamptic nephropathy, Waldenström disease, lupus anticoagulant glomerulonephritis, systemic lupus erythematosus, and cryoglobulinemic glomerulonephritis. We review the pathogenic mechanisms involved in this case.

Renal outcome and monoclonal immunoglobulin deposition disease in 289 old patients with blood cell dyscrasias: a single center experience.

Monoclonal components (MC) formed by chains/fragments of intact/truncated globulin components produced in different lymphoproliferative diseases are responsible for monoclonal immunoglobulin deposition disease (MIDD) and consequent tissue damage by organized (amyloid fibrils) or non-organized (amorphous) deposits. The kidneys are the most commonly affected organs in MIDD, and renal failure represents an important adverse factor for prognosis. The renal outcome and the role of renal pathology in diagnosing MIDD was evaluated in 289 elderly patients with multiple myeloma (MM, n=115) and monoclonal gammopathy (MGUS, n=174). Renal impairment was the only significant risk factor for patient death, while significant risk factors for renal impairment were diabetes (HR 3.65, 95% CI: 2.08-6.41), light chain (LC) proteinuria (HR 2.18; 95% CI: 1.10-4.32) and type of MC (p=0.0019). Renal pathology documented MIDD in 12/30 cases (40%): six cases of AL-amyloidosis, two of LC disease, one of heavy chain disease and three of cast nephropathy, as well as four cases of glomerulonephritis, eight of arteriolosclerosis and six of normal picture. Main conclusions are that diabetes, sharing common glomerular damage with LC disease, is the strongest risk factor for progression of renal disease, and glomerular proteinuria or heavy LC proteinuria should raise a strong suspicion index of MIDD and prompt pathology assessment to reach the correct diagnosis.

BK virus sequences in specimens from aborted fetuses.

Given the conflicting results of the few published studies, the aim of this retrospective molecular-based study of 10 aborted fetuses that underwent complete autopsy and 10 placentas was carried out to determine whether BK polyomavirus (BKV) can be transmitted transplacentally. The interruption of pregnancy was due to a miscarriage (five cases) or a prenatal diagnosis of severe intrauterine malformations (five cases). Samples from the brain, heart, lung, thymus, liver, and kidney were taken from each fetus, and two samples were obtained from all of the placentas. The presence of BKV was investigated by means of PCR using primers specific for the transcription control region (TCR) and viral capsidic protein 1 (VP1) and DNA extracted from formalin-fixed, paraffin-embedded tissue. BKV genome was detected in 22 of 60 samples (36.6%) from seven fetuses (70%), regardless of the cause of abortion: VP1 was amplified in 12 samples (54%), TCR in seven (32%), and both in three (14%). VP1 was also detected in one placental sample. BKV sequences were most frequently detected in heart and lung (five cases), but sequence analyses of TCR and VP1 revealed a high degree of genomic variability among the samples taken from different organs and the placenta. These results indicate that BKV can cross the placenta during pregnancy and become latent in fetal organs other than the kidney and brain (previously considered the main targets of BKV latency). This may happen in early pregnancy and does not seem to be associated with an increased risk of abortion.

Marked interindividual variability in renal maturation of preterm infants: lessons from autopsy.

The kidney of low birthweight preterm infants is characterized by a reduced number of mature nephrons at birth. The aim of the present study was to determine whether, in preterms, active glomerulogenesis occurs in the postnatal period and whether it may compensate the reduced number of nephrons developed during the intrauterine life. Kidney samples were obtained at autopsy from 8 human fetuses, 12 premature infants, and 3 term newborns. Glomerulogenesis, as measured by radial glomerular count (RGC), was markedly decreased in all preterm infants as compared with term newborns. A marked interindividual variability was detected in the level of glomerulogenesis, which, in the vast majority of cases, did neither correlate with the gestational age at birth nor with birthweight. Active glomerulogenesis, as demonstrated by the presence of S-shaped bodies in the subcapsular region, was present in all preterm infants in the perinatal period, but it ceased in a preterm surviving for 3 months. Our data show that active glomerulogenesis continues even after birth for a short period, although it is not able to compensate a marked oligonephronia at birth. As a consequence, the incomplete nephrogenesis typical of all extremely low birthweight preterm infants possibly results in a persistent oligonephronia which should likelihood represent a major risk factors of progressive renal disease in adulthood.

Thymosin beta-10 expression in developing human kidney.

Thymosin beta-10 (Tß10) is a member of beta-thymosins (Tßs), a family of low molecular mass peptides, which play essential roles in many cellular functions, including apoptosis, cell proliferation, cell migration, and endocytosis. The report that the Tß10 gene is expressed at high levels in embryonic human brain as well in human kidney induced us to study Tß10 reactivity in the preterm kidney in order to verify, at tissue level, the expression of this peptide during renal embryogenesis. To this end, we analyzed, using immunocytochemistry, the expression of Tß10 in samples of human kidney obtained, at autopsy, from 8 fetuses, 12 preterm infants, ranging from 25 to 36 weeks of gestation and 3 at term newborns. Tß10 immunoreactivity was detected in 20 out of 22 kidneys examined, and was mainly localized in proximal and distal tubular structures, in the cytoplasm and occasionally in the nuclei of ductal cells. In 11 cases, we also detected a focal and mild reactivity for the peptide in glomeruli. In 13 kidneys, we also observed immunostaining for Tß10 inside the "comma-shaped bodies" and the "S-shaped bodies" during active glomerulogenesis. Our data show, for the first time, the expression of Tß10 in the human kidney during the initial phases of its physiological development, mainly restricted in the proximal and the distal tubuli. Further studies are needed in order to better characterize the role of Tß10 in kidney embryogenesis.

The reliability of pre-transplant donor renal biopsies (PTDB) in predicting the kidney state. A comparative single-centre study on 154 untransplanted kidneys.

BACKGROUND: Pre-transplant donor biopsy (PTDB) is a common practice in marginal donors, taking for granted that it represents the whole kidney state, but its reliability has not yet been thoroughly investigated. This prompted us to carry out a comparative study on a needle biopsy group (NBG) and a wedge biopsy group (WBG) and their corresponding untransplanted kidneys. METHODS: One hundred and fifty-four biopsies and matched kidneys were scored for four morphologic indexes, i.e. tubular atrophy, interstitial fibrosis, vascular damage and global glomerulosclerosis. Categorical indexes were statistically evaluated for concordance with the k index, and the percentage of sclerotic glomeruli with correlation and linear regression analysis. RESULTS: Agreement between biopsies and kidneys was similar in both NBG and WBG with high scores for vascular damage (k 0.74 and 0.75) and intermediate ones for tubular atrophy (k 0.54 and 0.50). Agreement as to fibrosis and glomerular sclerosis was intermediate in the WBG (k 0.56 and 0.55) and poor in the NBG (k 0. 34 and 0.18). Vascular damage was underscored and glomerulosclerosis overscored in both groups, whereas interstitial fibrosis was underscored in the NBG and overscored in WBG. The agreement for the total score, i.e. the sum of the single indexes, was high in the NBG (k 0.73) and intermediate in WBG (k 0.57). Agreement for glomerulosclerosis and total score rose consistently in both groups along with the increasing number of biopsy glomeruli. There was an agreement as to biopsy and kidney evaluation for fitness for transplantation in 85% of NBG and 81% of WBG. CONCLUSIONS: PTDB supplies reliable data on the actual kidney state, with better results for needle biopsy. Although the biopsy size plays a role, samples with over 10 glomeruli suffice for clinical purposes. Vascular damage is the most faithful single parameter, whereas global glomerulosclerosis estimation requires some caution.

Beta-2-microglobulin is an independent predictor of progression in asymptomatic multiple myeloma.

BACKGROUND: Although serum beta-2 microglobulin (B2M) represents a key variable for symptomatic multiple myeloma (MM) prognostication, its role in predicting the risk of progression of asymptomatic MM to symptomatic disease has not been explored. METHODS: This study was bases on a consecutive series of 148 patients with asymptomatic MM and explored the cumulative probability of progression to symptomatic MM as the primary endpoint. RESULTS: In univariate analysis, a serum B2M level >2.5 mg/L was associated with an increased probability of disease progression (5-year risk, 64.5%; P < .001) along with serum monoclonal component (sMC) (P < .001), urinary monoclonal component (uMC) (P < .001), and bone marrow plasma cells (BMPCs) (P < .001). In multivariate analysis, serum B2M was selected as an independent predictor of progression (hazard ratio, 3.30; P = .002). Serum B2M was combined with sMC, uMC, and BMPC to create a risk-stratification model based on 4 groups with different risk of progression: very low (5-year risk, 0%), low-intermediate (5-year risk, 19.6%), high-intermediate (5-year risk, 60.7%), and high (5-year risk, 80.7%). The model that included serum B2M along with sMC, uMC, and BMPC was able to predict disease progression better than the model that was based on sMC, uMC, and BMPC without serum B2M (C statistics, 0.760 vs 0.726). CONCLUSIONS: The current results indicated that 1) serum B2M is an independent predictor of asymptomatic MM progression, and 2) serum B2M adds prognostic information when combined with the most widely used prognosticators of asymptomatic MM progression.

Genomic mutations of viral protein 1 and BK virus nephropathy in kidney transplant recipients.

Genomic variability in the viral protein 1 region of BK polyomavirus (BKV) may change the ability of the virus to replicate. The significance of such changes was studied in clinical samples taken from kidney transplant patients with and without BKV nephropathy. A 94 base-pair fragment of viral protein 1 was amplified from 68 urine, 28 blood, and 12 renal biopsy samples from eight patients with BKV nephropathy, and from 100 urine samples, 17 blood and three renal biopsy samples from 41 of 218 controls. The DNA was sequenced and the amino acid changes were predicted by the Expert Protein Analysis System program (ExPASy, Swiss Institute of Bioinformatics, Geneva, Switzerland). Single base-pair mutations were detected more frequently in the samples from the BKV nephropathy patients than in the controls, and this was the only statistically significant finding of the study (P < 0.05), thus suggesting a greater genetic instability in BKV nephropathy associated strains. The amino acid changes were distributed at random in both BKV nephropathy patients and controls. However, one aspartic acid-to-asparagine substitution at residue 75 was detected in all samples of the one patient with BKV-associated nephropathy, who developed disease progression confirmed by histology, and not in any of the other patient or control samples. Whether this specific amino acid change plays a role in disease deserves further study.