by ecancer reporter Clare Sansom

Cancer of the gallbladder is relatively rare worldwide, but its geographical spread is uneven and it is more prevalent in some ethnic groups, including East Asians.

It has a poor prognosis, largely because it is often diagnosed late; the median survival time from diagnosis is less than a year and only about one in 10 patients survive for five years.

Risk factors for developing gallbladder cancer include gallstones and chronic inflammation of the gallbladder (cholecystitis), and patients with a family history of gallstones are also at increased risk.

Little is known about the genetic changes that give rise to this condition, although gallbladder cancer cells are known to be highly metastatic and the well-known tumour suppressor TP53 has been identified as a frequently mutated gene.

A large group of researchers led by Yingbin Liu of Shanghai Jiao Tong University, Shanghai, China performed whole-exome sequencing of paired samples of gallbladder tumour and matched normal gallbladder tissue from 32 patients.

A total of 1,450 somatic single nucleotide polymorphisms and 34 somatic insertions or deletions were observed in these samples, with a C>T/G>A change the most common point mutation.

A total of 23 genes were found to have non-synonymous somatic mutations in three or more samples, although only two of these – TP53 and CS – were found to be statistically significant.

The researchers then designed a panel of cancer-related genes that included all these, and sequenced the genes in the panel in 51 paired gallbladder tumour – normal samples using an ultra-deep targeted sequencing technique.

Twenty-five of these paired samples had already been analysed using whole-exome sequencing, making 57 sample sets in all.

A total of 217 non-synonymous single nucleotide variations were found in this sample, with TP53, KRAS and ERBB3 the most frequently mutated genes.

Somatic mutations of TP53 and KRAS are very common in many tumour types.

The gene ERBB3 is a member of the epidermal growth factor receptor (EGFR or ErbB) family of receptor tyrosine kinases, and other members of this family were also found to be mutated in this set of tumours.

Activating mutations of ERBB3 and ERBB2 (which is also known as EGFR) have been implicated in the development of several tumour types but not previously in gallbladder cancer.

The researchers over-expressed the ERBB2 and ERBB3 proteins bearing the mutations that had been observed in these tumour samples in human gallbladder cancer cell lines, and found that each mutation led to increased proliferation of at least one cell line.

The oncogenic potential of mutations in these genes was confirmed by inhibiting their expression using RNAi.

A further 25 genes were found to be mutated less frequently in these cancers, and, interestingly, this gene set did not overlap with a set of genes that had previously been found to be mutated in cancer of the bile duct (cholangiocarcinoma).

Liu and his co-workers then investigated the prevalence of these mutations in the core signalling pathways, and discovered that the ErbB pathway was the most frequently mutated; this sample of tumours contained mutations in 15 of the genes in this pathway.

Most, but not all, of the mutations of genes in this pathway were found to be mutually exclusive.

Tumours located on the gallbladder neck were more likely to have mutations in one or more ErbB pathway genes than those on other sections of the gallbladder.

More strikingly, ErbB pathway mutations were strongly associated with a poor prognosis: patients with tumours bearing these mutations survived for a median of 8 months after diagnosis, compared to 13 months for those whose tumours did not carry these mutations.

Taken together, these results implicate mutations in the ErbB pathway in the development of gallbladder cancer, and suggest that patients whose gallbladder tumours bear these mutations might benefit from the drugs targeted to these genes that are in the clinic or in development.

Reference

Li, M., Zhang, Z.,, Li, M. and 41 others (2014). Whole-exome and targeted gene sequencing of gallbladder carcinoma identifies recurrent mutations in the ErbB pathway. Nature Genetics, published online ahead of print 6 July 2014.