Liver Complications in Untreated Treatment-Ineligible versus Treated Treatment-Eligible Patients with Hepatitis B.

BACKGROUND: A substantial number of patients who do not meet treatment criteria for chronic hepatitis B (CHB) later develop adverse outcomes such as cirrhosis and hepatocellular carcinoma (HCC). Our aim was to determine whether current practice guidelines adequately identify CHB patients who will benefit from antiviral therapy. METHODS: We performed a retrospective cohort study comparing the incidence of adverse liver outcomes (cirrhosis and/or HCC) in untreated treatment-ineligible (at baseline and throughout follow-up) versus treated treatment-eligible patients according to standard American Association for the Study of Liver Diseases (AASLD) 2018 guidance (alanine aminotransferase [ALT] >70/50 U/L for men/women plus hepatitis B virus [HBV] DNA >20,000/2,000 IU/mL for HBeAg+/-) and with a sensitivity analyses using a lower threshold (ALT >40 U/L and HBV DNA >2,000 IU/mL). RESULTS: We reviewed records of 5,840 patients from 5 clinics in California and identified 2,987 treatment-naive non-HCC CHB patients. Of those, 271 patients remained untreated treatment-ineligible, 514 patients were treatment-eligible and initiated treatment, with 5-year cumulative adverse liver incidences of 12.5% versus 7.2%, p = 0.074. On multivariable analysis adjusting for age, sex, diabetes, albumin, platelet count, and HBV DNA, compared to treated treatment-eligible patients, untreated treatment-ineligible patients had a significantly higher risk of adverse liver outcomes (adjusted hazard ratio: 2.38, 95% confidence interval 1.03-5.48, p = 0.04) in main analysis by AASLD 2018 criteria but not in sensitivity analysis using the lower treatment threshold (p = 0.09). CONCLUSION: Patients never meeting standard AASLD 2018 criteria for antiviral therapy and never treated had twice the risk of developing cirrhosis and/or HCC when compared to eligible and treated patients.

Natural History and Hepatocellular Carcinoma Risk in Untreated Chronic Hepatitis B Patients With Indeterminate Phase.

BACKGROUND & AIMS: Many patients with chronic hepatitis B (CHB) may not conform to any of the defined phases and hence are classified as indeterminate. We aimed to characterize the baseline prevalence of indeterminate patients and their natural history, phase transition, and hepatocellular carcinoma (HCC) risk. METHODS: This was a retrospective cohort study of 3366 adult untreated noncirrhotic CHB patients seen at 5 US clinics and 7 Taiwanese townships who had at least 1 year of serial laboratory data before enrollment with a mean follow-up period of 12.5 years. Patients' clinical phases were determined at baseline and through serial data during follow-up evaluation, based on the American Association for the Study of Liver Diseases 2018 guidance. RESULTS: At baseline, 1303 (38.7%) patients were in the indeterminate phase. By up to year 10 of follow-up evaluation, 686 patients (52.7%) remained indeterminate, while 283 patients (21.7%) became immune active. Compared with patients who remained inactive, patients who remained indeterminate had a higher 10-year cumulative HCC incidence (4.6% vs 0.5%; P < .0001) and adjusted hazard ratio for HCC of 14.1 (P = .03). Among patients who remained indeterminate, age 45 years and older (adjusted hazard ratio, 18.4; P = .005) was associated independently with HCC development. CONCLUSIONS: Nearly 40% of patients had indeterminate CHB phase. Of these, half remained indeterminate and one-fifth transitioned to the immune active phase. HCC risk in persistently indeterminate CHB was 14 times higher than inactive CHB. Among persistently indeterminate CHB patients, age 45 years and older was associated with an 18 times higher risk for HCC development. Further studies are needed to evaluate the potential benefit of antiviral therapy for indeterminate patients, especially in the older subgroup.

Progression Rates by Age, Sex, Treatment, and Disease Activity by AASLD and EASL Criteria: Data for Precision Medicine.

BACKGROUND & AIMS: Antiviral treatment criteria are based on disease progression risk, and hepatocellular carcinoma (HCC) surveillance recommendations for patients with chronic hepatitis B (CHB) without cirrhosis is based on an annual incidence threshold of 0.2%. However, accurate and precise disease progression estimate data are limited. Thus, we aimed to determine rates of cirrhosis and HCC development stratified by age, sex, treatment status, and disease activity based on the 2018 American Association for the Study of Liver Diseases and 2017 European Association for the Study of the Liver guidelines. METHODS: We analyzed 18,338 patients (8914 treated, 9424 untreated) from 6 centers from the United States and 27 centers from Asia-Pacific countries. The Kaplan-Meier method was used to estimate annual progression rates to cirrhosis or HCC in person-years. RESULTS: The cohort was 63% male, with a mean age of 46.19 years, with baseline cirrhosis of 14.3% and median follow up of 9.60 years. By American Association for the Study of Liver Diseases criteria, depending on age, sex, and disease activity, annual incidence rates ranged from 0.07% to 3.94% for cirrhosis, from 0.04% to 2.19% for HCC in patients without cirrhosis, and from 0.40% to 8.83% for HCC in patients with cirrhosis. Several subgroups of patients without cirrhosis including males younger than 40 years of age and females younger than 50 years of age had annual HCC risk near or exceeding 0.2%. Similar results were found using European Association for the Study of the Liver criteria. CONCLUSION: There is great variability in CHB disease progression rates even among "lower-risk" populations. Future CHB modeling studies, public health planning, and HCC surveillance recommendation should be based on more precise disease progression rates based on sex, age, and disease activity, plus treatment status.

One-year Fibrosis-4 index helps identify minimal HCC risk in non-cirrhotic chronic hepatitis B patients with antiviral treatment.

BACKGROUND AND AIMS: Fibrosis-4 (FIB-4) index is a HCC predictor in chronic hepatitis B (CHB) patients. However, little is known about whether FIB-4 helps identify non-cirrhotic CHB patients with minimal HCC risk after prolonged nucleos(t)ide analogue (NA) therapy. METHODS: A total of 1936 ethnically diverse, non-cirrhotic CHB patients were enrolled in this retrospective multi-national study. All patients received prolonged NA treatment, including entecavir and tenofovir disoproxil fumarate. We explored whether FIB-4 cutoff of 1.30, a marker indicative of mild fibrosis severity, could stratify HCC risks in these patients. RESULTS: A total of 48 patients developed HCC after a mean follow-up of 6.98 years. FIB-4 level at 1 year after treatment (1-year FIB-4) was shown to be associated with HCC development and was superior to pre-treatment FIB-4 value. When patients were stratified by 1-year FIB-4 of 1.30, the high FIB-4 group was at an increased HCC risk compared to the low FIB-4 group, with a hazard ratio of 4.87 (95% confidence interval: 2.48-9.55). Multivariable analysis showed that sex and 1-year FIB-4 were independent predictors, with none of the 314 female patients with low 1-year FIB-4 developing HCC. Finally, 1-year FIB-4 of 1.30 consistently stratified HCC risks in patients with low PAGE-B score, a score composed of baseline age, sex and platelet count, and the annual incidence rate of HCC was 0.11% in those with PAGE-B < 10 + 1-year FIB-4 < 1.30. CONCLUSIONS: In non-cirrhotic CHB patients receiving prolonged NA therapy, 1-year FIB-4 < 1.30 is useful for identifying those with minimal HCC risk by combining with female sex or low PAGE-B score.

Transition rates to cirrhosis and liver cancer by age, gender, disease and treatment status in Asian chronic hepatitis B patients.

BACKGROUND: Increasing hepatitis-related mortality has reignited interest to fulfill the World Health Organization's goal of viral hepatitis elimination by 2030. However, economic barriers have enabled only 28% of countries to implement countermeasures. Given the high disease burden among Asians, we aimed to present age, sex, disease activity and treatment-specific annual progression rates among Asian chronic hepatitis B (CHB) patients to inform health economic modeling efforts and cost-effective public health interventions. METHODS: We analyzed 18,056 CHB patients from 36 centers across the U.S. and seven countries/regions of Asia Pacific (9530 treated; 8526 untreated). We used Kaplan-Meier methods to estimate annual incidence of cirrhosis and hepatocellular carcinoma (HCC). Active disease was defined by meeting the APASL treatment guideline criteria. RESULTS: Over a median follow-up of 8.55 years, there were 1178 incidences of cirrhosis and 1212 incidences of HCC (297 without cirrhosis, 915 with cirrhosis). Among the 8526 untreated patients (7977 inactive, 549 active), the annual cirrhosis and HCC incidence ranged from 0.26% to 1.30% and 0.04% to 3.80% in inactive patients, and 0.55 to 4.05% and 0.19 to 6.03% in active patients, respectively. Of the 9530 treated patients, the annual HCC rates ranged 0.03-1.57% among noncirrhotic males and 2.57-6.93% among cirrhotic males, with lower rates for females. Generally, transition rates increased with age, male sex, the presence of fibrosis/cirrhosis, and active disease and/or antiviral treatment. CONCLUSION: Using data from a large and diverse real-world cohort of Asian CHB patients, the study provided detailed annual transition rates to inform practice, research and public health planning.

Association Between Fatty Liver and Cirrhosis, Hepatocellular Carcinoma, and Hepatitis B Surface Antigen Seroclearance in Chronic Hepatitis B.

BACKGROUND: Chronic hepatitis B (CHB) and fatty liver (FL) are common, natural history data on concurrent FL and CHB (FL-CHB) are limited. This study aimed to evaluate the effect of FL on cirrhosis, hepatocellular carcinoma (HCC), and hepatitis B surface antigen (HBsAg) seroclearance incidence in CHB patients. METHODS: In a retrospective cohort study of 6786 adult CHB patients, we used propensity score matching (PSM) to balance the FL-CHB and non-FL CHB groups. Kaplan-Meier methods were used to compare cumulative cirrhosis, HCC, and HBsAg seroclearance rates between subgroups. RESULTS: Before PSM, compared to non-FL CHB, FL-CHB patients had lower 10-year cumulative rates of cirrhosis, HCC, and a higher HBsAg seroclearance rate. Similar results were found in the matched FL-CHB and non-FL CHB patients, as well as in the antiviral-treated PSM cohort. Cox proportional hazards model indicated FL to remain significantly and strongly associated with lower risk of cirrhosis and HCC (hazard ratio [HR], 0.19 [95% confidence interval {CI}, .12-.33], P < .001 and HR, 0.21 [95% CI, .09-.51], P = .001, respectively) in antiviral-treated patients but not in untreated patients. CONCLUSIONS: FL was significantly associated with lower cirrhosis and HCC risk and higher HBsAg seroclearance. Further studies are needed to confirm our funding and investigate the mechanisms underlying the impact of FL on CHB.

Fully human anti-CD39 antibody potently inhibits ATPase activity in cancer cells via uncompetitive allosteric mechanism.

The extracellular ATP/adenosine axis in the tumor microenvironment (TME) has emerged as an important immune-regulatory pathway. Nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), otherwise known as CD39, is highly expressed in the TME, both on infiltrating immune cells and tumor cells across a broad set of cancer indications. CD39 processes pro-inflammatory extracellular ATP to ADP and AMP, which is then processed by Ecto-5'-nucleotidase/CD73 to immunosuppressive adenosine. Directly inhibiting the enzymatic function of CD39 via an antibody has the potential to unleash an immune-mediated anti-tumor response via two mechanisms: 1) increasing the availability of immunostimulatory extracellular ATP released by damaged and/or dying cells, and 2) reducing the generation and accumulation of suppressive adenosine within the TME. Tizona Therapeutics has engineered a novel first-in-class fully human anti-CD39 antibody, TTX-030, that directly inhibits CD39 ATPase enzymatic function with sub-nanomolar potency. Further characterization of the mechanism of inhibition by TTX-030 using CD39+ human melanoma cell line SK-MEL-28 revealed an uncompetitive allosteric mechanism (α < 1). The uncompetitive mechanism of action enables TTX-030 to inhibit CD39 at the elevated ATP concentrations reported in the TME. Maximal inhibition of cellular CD39 ATPase velocity was 85%, which compares favorably to results reported for antibody inhibitors to other enzyme targets. The allosteric mechanism of TTX-030 was confirmed via mapping the epitope to a region of CD39 distant from its active site, which suggests possible models for how potent inhibition is achieved. In summary, TTX-030 is a potent allosteric inhibitor of CD39 ATPase activity that is currently being evaluated in clinical trials for cancer therapy.

Real-World Effectiveness From the Asia Pacific Rim Liver Consortium for HBV Risk Score for the Prediction of Hepatocellular Carcinoma in Chronic Hepatitis B Patients Treated With Oral Antiviral Therapy.

BACKGROUND: Patients on oral antiviral (OAV) therapy remain at hepatocellular carcinoma (HCC) risk. Risk prediction tools distinguishing treated patients with residual HCC risk are limited. The aim of this study was to develop an accurate, precise, simple-to-use HCC risk score using routine clinical variables among a treated Asian cohort. METHODS: Adult Asian chronic hepatitis B (CHB) patients on OAV were recruited from 25 centers in the United States and the Asia-Pacific region. Excluded persons were coinfected with hepatitis C, D, or human immunodeficiency virus, had HCC before or within 1 year of study entry, or their follow-up was <1 year. Patients were randomized to derivation and validation cohorts on a 2:1 ratio. Statistically significant predictors from multivariate modeling formed the Real-world Effectiveness from the Asia Pacific Rim Liver Consortium for HBV (REAL-B) score. RESULTS: A total of 8048 patients were randomized to the derivation (n = 5365) or validation group (n = 2683). The REAL-B model included 7 variables (male gender, age, alcohol use, diabetes, baseline cirrhosis, platelet count, and alpha fetoprotein), and scores were categorized as follows: 0-3 low risk, 4-7 moderate risk, and 8-13 high risk. Area under receiver operating characteristics were >0.80 for HCC risk at 3, 5, and 10 years, and these were significantly higher than other risk models (p < .001). CONCLUSIONS: The REAL-B score provides 3 distinct risk categories for HCC development in Asian CHB patients on OAV guiding HCC surveillance strategy.

Tenofovir Versus Entecavir for Hepatocellular Carcinoma Prevention in an International Consortium of Chronic Hepatitis B.

INTRODUCTION: It is unclear whether entecavir (ETV) and tenofovir disoproxil fumarate (TDF) differ in their effectiveness for preventing hepatocellular carcinoma (HCC) in patients with chronic hepatitis B (CHB). METHODS: This retrospective cohort study analyzed an international consortium that encompassed 19 centers from 6 countries or regions composed of previously untreated CHB patients then treated with either ETV or TDF monotherapy. Those who developed HCC before antiviral treatment or within 1 year of therapy were excluded. The association between antiviral regimen and HCC risk was evaluated using competing-risk survival regression. We also applied propensity score matching (PSM) to 1:1 balance the 2 treatment cohorts. A total of 5,537 patients were eligible (n = 4,837 received ETV and n = 700 received TDF) and observed for HCC occurrence until December 23, 2018. Before PSM, the TDF cohort was significantly younger and had generally less advanced diseases. RESULTS: In the unadjusted analysis, TDF was associated with a lower risk of HCC (subdistribution hazard ratio [SHR], 0.45; 95% confidence interval [CI], 0.26-0.79; P = 0.005). The multivariable analysis, however, found that the association between TDF and HCC no longer existed (SHR, 0.81; 95% CI, 0.42-1.56; P = 0.52) after adjustment for age, sex, country, albumin, platelet, α-fetoprotein, cirrhosis, and diabetes mellitus. Furthermore, the PSM analysis (n = 1,040) found no between-cohort differences in HCC incidences (P = 0.51) and no association between regimens (TDF or ETV) and HCC risk in the multivariable-adjusted analysis (adjusted SHR, 0.89; 95% CI, 0.41-1.92; P = 0.77). DISCUSSION: TDF and ETV did not significantly differ in the prevention of HCC in patients with CHB.

Targeting CD39 in Cancer Reveals an Extracellular ATP- and Inflammasome-Driven Tumor Immunity.

We explored the mechanism of action of CD39 antibodies that inhibit ectoenzyme CD39 conversion of extracellular ATP (eATP) to AMP and thus potentially augment eATP-P2-mediated proinflammatory responses. Using syngeneic and humanized tumor models, we contrast the potency and mechanism of anti-CD39 mAbs with other agents targeting the adenosinergic pathway. We demonstrate the critical importance of an eATP-P2X7-ASC-NALP3-inflammasome-IL18 pathway in the antitumor activity mediated by CD39 enzyme blockade, rather than simply reducing adenosine as mechanism of action. Efficacy of anti-CD39 activity was underpinned by CD39 and P2X7 coexpression on intratumor myeloid subsets, an early signature of macrophage depletion, and active IL18 release that facilitated the significant expansion of intratumor effector T cells. More importantly, anti-CD39 facilitated infiltration into T cell-poor tumors and rescued anti-PD-1 resistance. Anti-human CD39 enhanced human T-cell proliferation and Th1 cytokine production and suppressed human B-cell lymphoma in the context of autologous Epstein-Barr virus-specific T-cell transfer. SIGNIFICANCE: Overall, these data describe a potent and novel mechanism of action of antibodies that block mouse or human CD39, triggering an eATP-P2X7-inflammasome-IL18 axis that reduces intratumor macrophage number, enhances intratumor T-cell effector function, overcomes anti-PD-1 resistance, and potentially enhances the efficacy of adoptive T-cell transfer.This article is highlighted in the In This Issue feature, p. 1631.

Reduced Incidence of Hepatocellular Carcinoma in Cirrhotic and Noncirrhotic Patients With Chronic Hepatitis B Treated With Tenofovir-A Propensity Score-Matched Study.

Background: The effect of newer oral anti-hepatitis B virus (HBV) medication, tenofovir disoproxil (TDF), on liver-related outcomes among Asians is limited. We examined the effect of TDF on the incidence of hepatocellular carcinoma (HCC) in an Asian population with chronic hepatitis B (CHB). Methods: This was a retrospective cohort study of 6914 adults with chronic HBV monoinfection and no history of transplantation who were recruited from 6 US referral, community medical centers and a community based Taiwan cohort for a total of 774 patients who received TDF and 6140 who were not treated. Propensity score matching (PSM) for age, sex, HBV e antigen status, HBV DNA level, alanine aminotransferase (ALT) level, baseline cirrhosis status, and follow-up time was performed to balance the groups, resulting in 591 treated individuals and 591 untreated individuals. Kaplan-Meier analysis was used to estimate the cumulative risk of HCC. Cox proportional hazards models were run to estimate the HCC risk between groups. Results: The 8-year cumulative HCC incidence was significantly higher in the PSM untreated group (20.13% vs 4.69%; P < .0001). Cirrhosis was a significant predictor for HCC (adjusted hazard ratio [aHR], 5.36; 95% confidence interval [CI], 2.73-10.51; P < .001). On multivariate analysis adjusted for age, sex, HBV DNA level, ALT level, and study site, TDF was associated with a 77% reduction in the risk of HCC (aHR, 0.23; 95% CI, .56-.92) in patients with cirrhosis and a 73% reduction (aHR, 0.27; 95% CI, .07-.98) in patients without cirrhosis. Conclusions: Among cirrhotic and noncirrhotic Asian patients with CHB, TDF therapy was significantly associated with a reduction in the 8-year HCC cumulative incidence rate.

Increasing co-morbidities in chronic hepatitis B patients: experience in primary care and referral practices during 2000-2015.

OBJECTIVES: Data on liver and non-liver co-morbidities in chronic hepatitis B (CHB) patients are limited. This study analyzes the prevalence of co-morbidities in a multicenter CHB cohort evaluated over 15 years. METHODS: This study included 2734 consecutive adult American CHB patients from a university medical center and several community primary care clinics. Data were analyzed by time periods (patients in each time period were unique without overlapping): 2000-2005 (n = 885), 2006-2010 (n = 888), and 2011-2015 (n = 961). Patients were identified via electronic query using diagnosis code with data confirmed and extracted via individual chart review. Most patients were male (57.9%) and Asian (89.6%). RESULTS: Mean age increased significantly from 43.3 ± 13.4 years during 2000-2005 to 49.1 ± 14.4 during 2011-2015 (p < 0.001). Between 2000-2005 and 2011-2015, fatty liver disease among new CHB patients increased from 1.6 to 6.8% (p < 0.001). Advanced liver diseases also increased (p < 0.001): cirrhosis (12.6-24.6%), hepatic decompensation (1.1-7.9%), and hepatocellular carcinoma (HCC) (4.9-9.1%). Similar trends were observed for non-liver co-morbidities (p < 0.001). Specifically, diabetes increased almost fivefold (4.9-22.9%), hypertension increased threefold (12.3-36.1%) and chronic kidney disease increased 4.5-fold (4.4-19.7%). Prevalence of osteopenia and osteoporosis also increased in CHB patients: 5.4-13.4% (p < 0.001) and 2.9-8.7% (p < 0.001), respectively. These trends were observed in both liver clinics and primary care clinics (except for advanced liver disease), treated and untreated patients, and for both sexes. CONCLUSIONS: The CHB patient population is aging and now presents with significantly more co-morbidities. Early diagnosis and linkage to care is needed to prevent and mitigate liver as well as non-liver co-morbidities.

Lower Observed Hepatocellular Carcinoma Incidence in Chronic Hepatitis B Patients Treated With Entecavir: Results of the ENUMERATE Study.

OBJECTIVES: Data from the United States are lacking regarding the impact of entecavir (ETV) on the risk of hepatocellular carcinoma (HCC). Our aim is to determine whether treatment with ETV is associated with a reduced HCC risk by calculating the expected HCC incidence based on the Risk Estimation for Hepatocellular Carcinoma in Chronic Hepatitis B (REACH-B) model and comparing it with the observed HCC incidence. METHODS: The incidence of HCC in US patients treated with ETV between 2005 and 2013 in a retrospective cohort was obtained. The predicted HCC incidence was calculated using the REACH-B model. The standardized incidence ratios (SIRs) were calculated as a ratio of observed over predicted HCC cases. RESULTS: Of 841 patients, 646 (65% male, 84% Asian, median age 47 years, 36% hepatitis B e antigen positive, 9.4% with cirrhosis) met the inclusion criteria. Over a median follow-up of 4 years, 17 (2.6%) cases of HCC were diagnosed, including 8 out of 61 (13.1%) patients with cirrhosis and 9 out of 585 (1.5%) without cirrhosis. Compared with those without HCC, the 17 patients with HCC were older at 53 years vs. 47 years and more likely to have cirrhosis at 47.1% vs. 8.4%. Among patients without cirrhosis, the observed HCC incidence was significantly lower than predicted by the fourth year (SIR, 0.37; 95% confidence interval: 0.166-0.82). A sensitivity analysis that comprised all patients, including those with cirrhosis, showed that at the maximum follow-up time of 8.2 years, a significantly lower than predicted HCC incidence was noted with an SIR of 0.56 (95% confidence interval: 0.35-0.905). CONCLUSIONS: Based on the REACH-B model, long-term ETV therapy was associated with a lower than predicted HCC incidence. However, the risk of HCC persisted, and careful HCC surveillance remains warranted despite the anti-viral treatment.

Entire-volume serial histological examination for detection of micrometastases in lymph nodes of colorectal cancers.

The purpose of this study was to accurately detect lymph-node micrometastases, i.e., metastatic cancer foci that have a size between 2.0 and 0.2 mm, in nodes excised from colorectal cancer (CRC) patients, and to determine how frequently micrometastases might be missed when standard histological examination procedures are used. A total of 311 lymph nodes were removed and examined from 90 patients with Stage I to IV CRC. The number of slices of histology sections ranged from 6 to 75 per node (average = 25.5; SD = 11.1), which provided a total of 7,943 slices. Lymph nodes were examined in their entire volume at every 50-µm and 100-µm intervals for nodes smaller and larger than 5 mm respectively. The total number of thin sections examined in each node and the number of thin sections where metastatic foci were present were counted. The number of thin sections with metastatic foci and the total number of slices was determined for each node. In addition, the presence or absence of metastatic foci in the "central" slice was determined. Micrometastases were found in 12/311 (3.9%) of all lymph nodes. In the 12 lymph nodes with micrometastases, the rate of metastatic slices over all slices was 39.4% (range = 6.3 to 81.3%; SD = 25.8%) In the central slice of each node, micrometastases were present only in 6 of 12 lymph nodes (50%); accordingly, they were not present in the central slice for half the micrometastatic nodes. These 6 nodes represented 1.9% of the 311 nodes and 11.1% of the 54 metastatic nodes. This study suggests that a significant fraction of micrometastases can be missed by traditional singleslice sectioning; half of the micrometastases would have been overlooked in our data set of 311 nodes.

The angiotensin II receptor (Agtr1a): functional regulatory polymorphisms in a locus genetically linked to blood pressure variation in the mouse.

Hypertension is a complex trait with multiple genetic determinants. A previous genome-wide linkage study of systolic blood pressure in a mouse genetic backcross implicated a region of chromosome 13 (LOD = 3.3 at 16.0 cM) as a determinant of blood pressure differences between a hereditary low blood pressure strain of Mus musculus (BPL/1) and Mus spretus (SPRET); at this locus, the unexpected effect of the BPL/1 allele was to increase blood pressure. A plausible candidate locus encoding angiotensin II receptor isoform 1a (Agtr1a) is also located at 16.0 cM on chromosome 13. We therefore investigated structural and functional differences at Agtr1a between BPL/1 and SPRET, as well as the BPH/2 strain. Resequencing Agtr1a in the three strains established the exon/intron and proximal promoter structure of the mouse gene. Coding exon 3 spanned 1,960 bp (with 26 SNPs), including the 1,077-bp/359-amino acid ORF (with 5 cSNPs, all of which were synonymous). Promoter sequences revealed a consensus TATA box, conserved G/C-rich regions, and a striking, lengthy simple sequence repeat region, composed of di-, tri-, tetra-, and penta-nucleotide repeats, whose overall length varied markedly among the strains. Twenty-five other SNPs and three single nucleotide deletions differentiated the strains' promoters, six of which were in likely functional promoter motifs. Agtr1a mRNA abundance in the adrenal gland in vivo was greater (P < 0.05) in BPL/1 than SPRET, consistent with the predicted effect of the BPL/1 allele to confer higher blood pressure. When Agtr1a promoters were subcloned into luciferase reporter plasmids and transfected into PC12 chromaffin cells, basal promoter expression was higher (P < 0.001) in BPL/1 than in SPRET, consistent with the endogenous mRNA results. In summary, Agtr1a on chromosome 13 is highly polymorphic between mouse strains, although the amino acid sequence specified by the ORF is invariant, even across mouse species. We conclude that polymorphisms in the Agtr1a promoter account for differences in gene expression in vivo between BPL/1 and SPRET, in a way consistent with the effects of alleles at this locus on chromosome 13 to affect blood pressure in the mouse genome-wide linkage study.