Characterization of neuroendocrine tumors in heterozygous mutant MENX rats: a novel model of invasive medullary thyroid carcinoma [thyroid]. Rattus norvegicus

CDKN1B (p27) was formally established as a tumor suppressor gene (tsg) following the identification of inactivating germline mutations in rats (MENX syndrome) and patients (MEN4 syndrome) developing multiple neuroendocrine tumors (NETs). MENX-affected rats are homozygous for the predisposing p27 mutation, suggesting a canonical tsg function. In contrast, mice heterozygous for a defective Cdkn1b allele are already predisposed to tumor formation (haploinsufficiency). We here report that heterozygous mutant rats (p27+/mut) develop the same NETs seen in the homozygous (p27mut/mut) animals but with slower progression. In the tumors of p27+/mut rats, the wild-type allele is neither lost nor silenced, implying that p27 is haploinsufficient for tumor suppression also in this model. Transcriptome profiling of rat NETs having different p27 dosages revealed a tissue-specific, dose-dependent effect of p27 on gene expression. In p27+/mut rats, thyroid neoplasms progress to invasive and metastatic medullary thyroid carcinomas (MTCs) accompanied by increased calcitonin levels, as in humans. Comparison of expression signatures of late-stage versus early-stage MTCs from p27+/mut rats identified genes potentially involved in tumor aggressiveness. The expression of a subset of these genes was evaluated in human MTCs, and found associated with aggressive RET-M918T-positive tumors. Altogether, p27 haploinsufficiency in MENX rats uncovered a novel, representative model of invasive/metastatic MTC, exploitable for translational studies of this aggressive and often incurable cancer. Overall design: We compared medullary thyroid carcinomas from heterozygous and homozygous p27Kip1/Cdkn1b mutant rats. Heterozygous animals were analyzed at the age of 9 and 18 months

CDKN1B（p27）正式被确立为抑癌基因（tumor suppressor gene, TSG），此前研究者在罹患MENX综合征的大鼠以及罹患MEN4综合征的多发性神经内分泌肿瘤（neuroendocrine tumors, NETs）患者体内，均鉴定出了失活性生殖系突变。携带易感p27突变的MENX综合征模型大鼠为纯合子，这提示p27发挥经典抑癌基因功能。与之相反，携带缺陷型Cdkn1b等位基因的杂合小鼠已表现出肿瘤易感性，即单倍剂量不足（haploinsufficiency）。本研究报道，杂合突变型大鼠（p27+/mut）会罹患与纯合突变型大鼠（p27mut/mut）一致的多发性神经内分泌肿瘤，但肿瘤进展更为缓慢。在p27+/mut大鼠的肿瘤组织中，野生型等位基因既未发生丢失也未被沉默，这表明在该模型中，p27发挥肿瘤抑制功能同样依赖单倍剂量不足效应。对携带不同p27剂量的大鼠神经内分泌肿瘤进行转录组谱分析（transcriptome profiling）后发现，p27对基因表达的调控具有组织特异性且呈剂量依赖性。在p27+/mut大鼠中，甲状腺肿瘤可进展为侵袭性及转移性甲状腺髓样癌（medullary thyroid carcinomas, MTCs），并伴随降钙素水平升高，这与人类患者的临床表现一致。对比p27+/mut大鼠晚期与早期甲状腺髓样癌的基因表达特征，研究者鉴定出了潜在参与肿瘤侵袭性进展的相关基因。对其中部分基因在人类甲状腺髓样癌中的表达水平进行检测后发现，其表达与侵袭性RET-M918T阳性肿瘤显著相关。综上，MENX综合征大鼠的p27单倍剂量不足表型，构建了一种全新且具有代表性的侵袭/转移性甲状腺髓样癌模型，可用于该侵袭性且常难以治愈的癌症的转化研究。实验设计：本研究对比了杂合及纯合p27Kip1/Cdkn1b突变大鼠的甲状腺髓样癌样本。其中杂合模型大鼠分别于9月龄及18月龄时被取样分析。