by ecancer reporter Clare Sansom

Many cell types secrete tiny membrane-enclosed, protein-enriched vesicles known as exosomes into their environment.

Any consistent differences between the protein content of exosomes derived from cancer cells and those from normal cells could provide a useful, non-invasive diagnostic tool for cancer.

Pancreatic cancer has one of the worst prognoses of all cancer types, with a five-year survival after diagnosis of only 5%.

One reason for its poor prognosis is that it is usually diagnosed at a late stage; the five-year survival rises to 12-14% in patients diagnosed with Stage I disease.

A non-invasive tool to detect this cancer at an early stage should therefore benefit a significant number of patients.

A membrane-anchored protein, glypican-1 (GPC1) is known to be over-expressed in breast and pancreatic cancer cell lines compared to normal tissue from the same organs.

A group of researchers led by Raghu Kalluri of the University of Texas MD Anderson Cancer Center, Houston, TX, USA identified GPC1 as a marker of cancer-derived exosomes [1], firstly by using a mouse model.

Kalluri and his co-workers injected tumour cells from the MDA-MB-231 cell line into the mammary fat pads of nude mice.

The mice were bled before inoculation and when the developing tumours reached 300, 550, 1000 and 1350 mm3 in volume and circulating exosomes (crExos) in the blood samples were assessed for GPC1 using mass spectrometry.

Only circulating exosomes derived from tumours were found to contain GPC1, and the percentage of GPC1 exosomes in the blood increased with tumour size and overall tumour burden.

The researchers then obtained samples of serum from 32 patients with breast cancer; 190 patients with the most common type of pancreatic cancer, pancreatic ductal adenocarcinoma; and 100 healthy volunteers, isolated crExos from the samples and tested them for GPC1.

All the patients with pancreatic cancer and 75% of the breast cancer patients were found to have higher levels of GPC1 crExos in their serum than the healthy donors.

Many patients with pancreatic cancer carry mutations in the KRAS gene, and transcripts of mutant KRAS have been found in their circulation.

The researchers therefore analysed crExos from the pancreatic cancer patients for mRNA bearing KRAS mutations.

They found wild type KRAS mRNA in both GPC1 and GPC1- crExos, but mutated mRNA only in the GPC1 exosomes.

In all cases, the mutation observed in the crExos was exactly the same as that in the patient’s tumour; the most commonly observed mutation was G12D.

Kalluri and his co-workers also investigated serum samples from 26 patients with benign pancreatic disease – either pancreatitis or cystic adenomas – and 5 who had been diagnosed with a pancreatic cancer precursor lesion (PCPL).

They found that levels of GPC1 crExos were consistently higher in patients with pre-cancerous lesions than either those with benign pancreatic disease or healthy volunteers, which suggested that GPC1 might be a useful biomarker for early stage pancreatic cancer.

Furthermore, this test was more specific and sensitive in distinguishing PCPL from benign pancreatic disease than CA19-9, which is the most common biomarker in current use.

All these results were validated in a smaller independent cohort that also included patients with benign pancreatic disease.

Levels of GPC1 crExos were found to correlate with disease stage and with the burden of metastatic disease, and, unlike CA19-9 levels, to decrease after surgery in both PDAC and PCPL patients.

Patients whose GPC1 crExos levels decreased more than the median survived for longer after surgery than those with decreases less than the median (26.2 months and 15.5 months respectively).

Taken together, these results suggest that measuring GPC1 in circulating exosomes might provide a reliable biomarker for both the detection of early stage pancreatic cancer and the staging of the disease.

Writing in a News and Views commentary in the same issue of Nature [2], Clotilde Théry of the Institut Curie, Paris, France described the potential implications of these results for the early diagnosis of this devastating disease as ‘huge’, while cautioning that they should be repeated in a larger patient cohort and expressing disappointment that the results in breast cancer patients had been much more ambiguous.

References:

[1]: Melo, S.A., Luecke, L.B., Kahlert, C. and 10 others (2015). Glypican-1 identifies cancer exosomes and detects early pancreatic cancer. Nature, published online ahead of print 24 June 2015. doi:10.1038/nature14581

[2]: Théry, C. (2015). Diagnosis by extracellular vesicles.  Nature, published online ahead of print 24 June 2015. doi:10.1038/nature14626