Id helix-loop-helix proteins are differentially expressed in gestational trophoblastic disease W C Xue, H C Feng, 1 K Y K Chan, P M Chiu, H Y S Ngan, 2 U S Khoo, S W Tsao, 1 K W Chan & A N Y Cheung Departments of Pathology, 1 Anatomy and 2 Obstetrics and Gynaecology, Queen Mary Hospital, The University of Hong Kong, Hong Kong, China Date of submission 8 December 2004 Accepted for publication 15 March 2005 Xue W C, Feng H C, Chan K Y K, Chiu P M, Ngan H Y S, Khoo U S, Tsao S W, Chan K W & Cheung A N Y (2005) Histopathology 47, 303–309 Id helix-loop-helix proteins are differentially expressed in gestational trophoblastic disease Aims: To assess the expression of Id proteins in trophoblastic tissues and to correlate this with clinical parameters, proliferative and apoptotic indices as well as to related oncogene expression. Methods and results: Immunohistochemistry for Id1, Id2, Id3 and Id4 was performed on 83 trophoblastic tissues including 17 normal first-trimester placentas, seven term placentas, 47 hydatidiform moles (HM), and 12 spontaneous miscarriages. The four Id proteins were predominantly expressed in the villous and implanta- tion site intermediate trophoblast. Expression of Id1 in HM was significantly higher than that in normal placenta (P ¼ 0.0006) and spontaneous miscarriage (P ¼ 0.0001) but did not correlate with subsequent development of gestational trophoblastic neoplasia (GTN). Id1 expression correlated with the proliferation index as assessed by MCM7 (P ¼ 0.003) and Ki67 (P ¼ 0.017) and with the apoptotic activity assessed by TUNEL (P ¼ 0.001) and M30 CytoDeath anti- body (P ¼ 0.013). Moreover, the expression of Id1 correlated with the expression of p53 (P ¼ 0.004), p21 WAF1 ⁄ CIP1 (P ¼ 0.003) but not with p16 (P ¼ 0.107). Conclusions: Id proteins may play a role in the regu- lation of proliferative and apoptotic activity in tro- phoblastic tissue and are potentially useful in differentiating molar and non-molar gestation, but are not helpful in predicting GTN. Keywords: apoptosis, gestational trophoblastic disease, Id proteins, proliferation Abbreviations: CM, complete mole; GTN(D), gestational trophoblastic neoplasia (disease); HLH, helix-loop-helix; HM, hydatidiform mole; PCNA, proliferating cell nuclear antigen; PM, partial mole Introduction Id proteins (inhibitor of differentiation or inhibitor of DNA binding) belong to a subfamily (class V) of helix- loop-helix (HLH) proteins. 1 Unlike most HLH transcrip- tion factors, Id proteins do not have the basic DNA binding domain. Four mammalian members of Id proteins (Id1–Id4) have been identified thus far. They act as dominant negative regulators of transcription factors by forming heterodimers with other basic HLH proteins, thus preventing transcription factors binding to the target genes. 2,3 Accumulating evidence supports the view that Id proteins are important mediators of critical cellular processes such as cell fate determin- ation, proliferation, differentiation, angiogenesis, cell death and cell invasion in a variety of cell types. 4–10 For example, expression of Id proteins is typically high in actively proliferating cells and is down-regulated as a prerequisite for exit from the cell cycle and terminal differentiation. 2,3,7 On the other hand, overexpression of Id genes can induce apoptosis in serum-deprived fibroblasts. 8 In addition, Id proteins may also function as oncoproteins and are considered as essential for the vascularization of tumours. 5,11,12 Expression of Id proteins has been demonstrated in a variety of human tissues. The four Id proteins differ substantially with respect to their expression patterns Address for correspondence: Dr Annie N Y Cheung, Department of Pathology, The University of Hong Kong, Queen Mary Hospital, Pokfulam Road, Hong Kong, China. e-mail: anycheun@hkucc.hku.hk Ó 2005 Blackwell Publishing Limited. Histopathology 2005, 47, 303–309. DOI: 10.1111/j.1365-2559.2005.02190.x