by ecancer reporter Clare Sansom

The most common form of liver cancer, hepatocellular carcinoma (HCC) is particularly prevalent in China, where approximately 55% of the over 600,000 cases diagnosed worldwide occur each year.

The most important risk factor for this disease is infection with the hepatitis B virus (HBV), which is endemic in China.

Studies of families at high risk for this disease indicate that there is a strong genetic component to the link between HBV and hepatocellular carcinogenesis, but very little is known about the exact nature of this relationship.

One recent genome-wide association study (GWAS) in a relatively small Chinese population identified a risk factor for HCC development in HBV carriers at chromosome 1p36.22, but the size of this study prevented the identification of further factors.

A large group of researchers led by Long Yu from Fudan University, Shanghai, China has now carried out a larger, multi-stage GWAS study in a population of chronic HBV carriers of Chinese descent.

In the first stage of the GWAS, a total of 2,689 Chinese HBV carriers – 1,212 diagnosed with HCC (cases) and 1,477 without cancer (controls) – were genotyped at 733,202 known SNP locations across the whole genome.

An initial analysis of this data showed that cases and controls shared a common genetic background, and genotypes at over two million further SNPs were inferred based on Chinese and Japanese HapMap samples.

A total of 35 SNPs from all regions of the genome were selected for replication and further analysis.

Two of these were found to be significantly associated with the risk of developing HCC: rs7574865 at chromosome 2q32.2-2q32.3 and the imputed SNP rs9275319 at 6p21.3.

These two SNPs were then evaluated further in five independent, geographically diverse populations of Chinese HBV carriers including, between them, 3,737 HCC cases and 4,294 controls.

The statistical evidence for association between these two SNPs and the risk of developing HCC was highly significant, with overall p-values for the meta-analysis of 1.66×10⁻¹¹ for rs7574865 and 8.65×10^–19 for rs9275319; neither of these had previously been reported as a risk factor for HBV-related HCC.

The SNP rs7574865 on chromosome 2 is located in the third intron of the gene STAT4, which encodes a transcription factor involved in regulating immune responses.

This transcription factor transmits signals from the cytokines interleukin-12 and interferon a/b to induce production of gamma interferon (IFN-g), which has a critical role in protection against viral infection and carcinogenesis; reduced STAT4 expression levels will therefore tend to promote cancer development by reducing IFN-g.

Individuals with HBV-associated hepatocellular carcinoma (cases) who were homozygous for the risk-associated G allele at this SNP were found to have lower levels of STAT4 RNA than those with other genotypes; expression levels were also found to be lower in tumour tissue than in adjacent normal liver tissue.

Consistent with the role of STAT4 in immune defence, high expression levels of this gene have been associated with autoimmune diseases; in these populations, also, the G allele at rs7574865 also seemed to confer some protection against these diseases.

The SNP rs9275319 is located in the human leukocyte antigen (HLA) class II region on chromosome 6, between genes HLA-DQB1 and HLA-DQA2.

These genes have a primary role in many diseases that are mediated through the immue system, including some cancers and liver diseases.

The researchers then sequenced the HLA-DQB1 and HLA-DQA2 genes in a subset of one of the cohorts from the second part of the GWAS study that included 478 HCC cases and 458 controls, but found no significant HCC risk associated with any of the 33 alleles observed in this population.

Finally, the researchers genotyped 4,585 subjects without HCV infection recruited from geographical areas corresponding to the infected subjects at these two positions, and found that the “non-risk” allele at rs9275319, but not that at rs7574865, was significantly associated with a risk of HCV infection.

In summary, this three-stage GWAS has identified two previously unknown genetic variants, both involved in the immune response, that are linked to hepatocellular carcinoma in HBV-infected individuals: one in gene STAT4 on chromosome 2 and the other in the HLA-DQ region on chromosome 6.

Reference

 Jiang, D-K., Sun, J., Cao, G. and 61 others (2012).  Genetic variants in STAT4 and HLA-DQ genes confer risk of hepatitis B virus–related hepatocellular carcinoma. Nature Genetics, published online ahead of print 16 December 2012. doi:10.1038/ng.2483