Germline Alterations: Acc and Hereditary tumour Syndromes
ACC can occur in the context of several tumour susceptibility syndromes (Table 1).
Table1. Main genetic predispositions to adrenocortical tumours and the molecular genetics of sporadic ACC. The table describes the main hereditary syndromes associated with adrenocortical tumours for which the locus and/ or genes have been identified at the germline level. The alterations of these genes and chromosomal regions as somatic defect observed on tumour DNA of sporadic tumours are listed.
Historically, ACC has been first described as part of Li– Fraumeni syndrome in 1969, which was later linked to germline TP53 mutations. This syndrome displays dominant inheritance and confers susceptibility to breast cancer, soft- tissue sarcoma, brain tumours, osteosarcoma, leukaemia, and ACC. Germline mutations in TP53 have been observed in 50– 80% of children with apparently sporadic ACC in North America and Europe. In Southern Brazil, a founder germline mutation, R337H in exon 10, is observed in almost all paediatric cases. Germline TP53 mutations are found much more rarely in adult ACC (<5%).
Another historic germline alteration is observed in the imprinted 11p15 region, carrying IGF2. This locus is altered most often through uniparental disomy, with loss of the maternal allele, responsible for IGF2 overexpression, implicated in Beckwith– Wiedemann syndrome. This overgrowth disorder is characterized by macrosomia, macroglossia, organomegaly and developmental ab normalities (in particular abdominal wall defects with exomphalos), nephroblastoma, ACC, neuroblastoma, and hepatoblastoma.
11q13 locus, is a tumour suppressor gene. A heterozygous inactivating germline mutation of MEN1 is found in about 90% of families affected by multiple endocrine neoplasia type 1. The principal clinical features of this autosomal dominant syndrome include para thyroid (95%), endocrine pancreas (45%) and pituitary (45%) tumours and thymic carcinoids. Benign adrenocortical tumours and/ or hyperplasia are common (25– 40%), whereas ACC is rarely observed in MEN1 patients (<2%). Lynch syndrome is dominantly inherited and due to germline mutations in DNA mismatch repair genes MLH1, MSH2, MSH6, and PMS2. This syndrome typically confers an increased risk for colorectal, endometrial, small bowel, and upper tract urothelial cancers (Amsterdam Criteria), but also for sebaceous tumours, ovarian and pancreatic cancers, and ACC. Lynch syndrome accounts for up to 5% of ACC cases.
ACC has been described in several other hereditary tumour syndromes, including APC, NF1, PRKAR1A, FLCN, or BRCA2 germline mutations.
Overall, germline predisposition affects 10% of adult ACC patients. Therefore, patient with newly diagnosed ACC should be considered for a genetic evaluation. In the absence of obvious inherited genetic syndrome, a next generation sequencing panel including MSH2, MSH6, PMS2, MLH1, EPCAM, MEN1, APC, and TP53 could be proposed.
Somatic Alterations: IGF2, tP53, and Wnt/ β- catenin Pathways
In various cancers, the study of chromosomal rearrangement led to the identification of the oncogenes or tumour suppressor genes involved in their development. However, so far in ACC, such genes have been mostly identified by candidate gene approaches through the study of familial diseases responsible for adrenocortical tumours. Nevertheless, the loci of these genes are frequently altered in sporadic ACC, suggesting the importance of these loci and genes in the development of these tumours (Table 1). Studies using microsatellite markers in ACC have demonstrated a high percentage of loss of heterozygosity (LOH) or allelic imbalance at 11p15 (≥ 90%) and 17p13 (≥ 85%), corresponding respectively to IGF2 and TP53 loci. Indeed, IGF2 is strongly overexpressed in 90% of sporadic ACC and somatic mutations of TP53 are found in 25% of adult ACC.
Genetic alterations of the Wnt- signalling pathway were initially identified in familial adenomatous polyposis coli and have been extended to a variety of cancers. Furthermore, familial adenomatous polyposis coli patients with germline mutations of the APC (adenomatous polyposis coli) gene that lead to an activation of the Wnt- signalling pathway, may develop adrenocortical tumours. The Wnt- signalling pathway is normally activated during embryonic development. ß- catenin is a key component of this signalling pathway. In ACC, ß- catenin de localization can be observed, consistent with an abnormal activation of the Wnt- signalling pathway. This is explained in a subset of ACC (25%) by somatic activating mutations of CTNNB1, the ß- catenin gene. Recently, exome sequencing and single- nucleotide poly morphism (SNP) array studies revealed alterations— mostly homozygous deletions— of ZNRF3 in 20% of sporadic ACC. ZNRF3 encodes an E3- ubiquitin ligase and is a known inhibitor of the Wnt/ ß- Catenin pathway, by regulating the Wnt receptor turnover.
Genomics: Recent Advances towards a new classification of Acc
The development of genomics in the last decade has been a source of new progress both for tumour classification and pathogenesis understanding.
Gene expression profile (i.e. transcriptome), of benign tumours differs markedly from that of ACC. IGF2 appeared as one of the most highly expressed genes in ACC. Adenomas display a steroidogenic signature, whereas ACC are characterized by a proliferative signature, suggesting that a dedifferentiation process might occur during malignant transformation. The differences in gene expression profiles offer new diagnostic tools to discriminate benign from malignant adrenocortical tumours.
Moreover, studies analysing genomic (exome and chromosome alterations), epigenomic (micro- RNA expression and methylation) and transcriptomic (gene expression) profiles provided a complete high- throughput molecular characterization of adrenal tumours, in particular two large scale, integrated genomic studies, one from the European Network for the Study of Adrenal Tumours (ENSAT) network, and the other from The Cancer Genome Atlas (TCGA) programme. These studies converge in an unsupervised classification of ACC into three groups, characterized by distinct molecular alterations and associated with very different outcomes (Figure 1). The first group is characterized by the combination a ‘C1A’ transcriptome profile (with upregulation of proliferative genes), a CpG island hypermethylation, a ‘noisy’ chromosome alteration profile (i.e. numerous and anarchic alterations), and an accumulation of mutations in cell- cycle and Wnt/ ß- catenin related genes. This subgroup is associated with very poor outcome. Conversely, a second group of ACCs is characterized by a ‘C1B’ transcriptome profile (enriched in immune- related genes), no hypermethylation, a ‘chromosomal’ genome profile (i.e. extended patterns of LOH), and low mutation rate. This subgroup is associated with a better outcome. Finally, a third group of ACCs is characterized by ‘C1A’ transcriptome, and either hypermethylation or ‘noisy’ profile but not both, and shows an intermediate prognosis. These findings led to the development of new molecular markers for the prognostication of ACC.
Fig1. Genomic classification of adrenocortical cancer. This figure presents a schematic view of the genomic classification of ACC. Tumours are divided into three groups based on their transcriptome ‘C1A’ or ‘C1B’, chromosome alterations ‘Noisy’ or ‘Chromosomal’, and methylome— CIMP (CpG islands methylator phenotype) or non- CIMP profiles. These three groups are associated with very different prognosis. IGF2 is overexpressed in 90% of ACC in all molecular groups. Activation of the Wnt- βcatenin pathway (including CTNNB1 and ZNRF3 alterations) and alterations in cell- cycle genes (including TP53 and CDKN2A) are found mostly in aggressive ACC subgroups.
