Monitoring multiple myeloma by quantification of recurrent mutations in serum

Circulating tumor DNA (ctDNA) is a promising biomarker to monitor tumor load and genome alterations. We have explored the presence of ctDNA in multiple myeloma patients and its relation to disease activity during long-term follow-up. We used digital droplet PCR to monitor recurrent mutations, mainly in mitogen activated protein kinase pathway genes NRAS, KRAS and BRAF. Mutations were identified by next generation sequencing or PCR of bone marrow plasma cells, and their presence analyzed in 251 archived serum samples obtained from 20 patients during up to 7 years. In 17 of 18 patients, mutations identified in bone marrow during active disease were also found in a time-matched serum sample. The concentration of mutated alleles in serum correlated with the fraction in bone marrow plasma cells (r=0.507, n=34, p<0.002). There was a striking covariation between circulating mutation levels and M protein in 10 out of 11 patients with sequential samples. When relapse evaluation by ctDNA and M protein could be directly compared, ctDNA showed relapse earlier in two patients (3 and 9 months), later in one patient (4 months) and in three patients there was no difference. In three patients with transformation to aggressive disease, the concentrations of mutations in serum increased up to 400 times, an increase that was not seen for the M protein. In conclusion, ctDNA in myeloma is a multi-faceted biomarker reflecting mutated cells, total tumor mass and transformation to a more aggressive disease. Its properties are both similar and complementary to M protein.