A436: Glutamine synthetase immunohistochemical staining patterns correlate with molecular abnormalities of b-catenin pathway in hepatocellular adenoma

ABSTRACT BODY: Background : β-catenin mutated hepatocellular adenoma with inflammatory features (b-IHCA) or without (b-HCA) are known to be prone to malignancy. Recently, 3 levels of β-catenin activation (high, moderate and low) were described in HCA, related to specific molecular abnormalities of CTNNB1 gene, occurring in exons 3, 7 and 8, with different risk of malignancy (1). The aim of this study was to look if glutamine synthetase (GS) immunostaining could be used as a surrogate marker to identify the 3 different levels of β-catenin activation. Design: We retrieved 58 cases of b-HCA (25) and b-IHCA (33) and reviewed their GS staining pattern. In a first step, knowing the underlying molecular abnormality of CTNNB1 gene in a series of 30 cases, we found 3 GS patterns related to the level of b-catenin activation: 1-strong/diffuse/homogeneous (exon 3 other than S45); 2-moderate to strong/more or less diffuse/heterogeneous (exon 3-S45); 3-other patterns mainly small patches, sometimes around veins (exon 7/8). In a second step, we classify the whole series for GS pattern, blindly of molecular data, and we analyzed H&E, reticulin, b-catenin and CD34 stainings, together with the presence of features borderline with malignancy, and of HCC foci within the HCA. Results: Group 1 of GS (8 b-HCA and 14 b-IHCA) was easily identified, even in 2 cases of IHCA in which only a β-catenin mutated focus was present. Group 2 (8 b-HCA and 5 b-IHCA) corresponding to S45 exon 3 mutation was often very characteristic on GS, except in one b-IHCA which was wrongly classified as 7/8. Group 3 (9 b-HCA and 14 b-IHCA) corresponding to exon 7/8 mutations was more difficult to identify, particularly for b-IHCA; one b-IHCA and one b-HCA were wrongly identified as S45 and unclassified HCA respectively. Aberrant β-catenin nuclear staining was usual in group 1, rare in group 2, absent in group 3. In groups 2 and 3, GS was positive along the border of the HCA in 75% of cases. In addition, CD34 was diffusely expressed in 80% of cases in group 2 and occasionally in group 3, except at the HCA border. Cholestasis was more frequently observed in groups 1 and 2. No or only minor atypia were present in group 3, contrary to groups 1 and 2 where borderline lesions or HCC foci were present in 7/8 b-HCA, 4/14 b-IHCA and 2/8 b-HCA, 2/5 b-IHCA respectively. Conclusion: GS staining pattern, combined to H&E and CD34, is a valuable method to identify the 3 molecular subgroups of β-catenin mutated HCA (exon 3, S45, 7/8). Knowing that they differ in terms of malignancy risk, very rare in exon 7/8, high in exon 3, including S45 mutation, it represents an important tool for patient management.