Nanopore ctDNA SIMMA preprint
A preprint called SIMMA describes single‑molecule nanopore ctDNA analysis detecting minimal residual disease down to about 100 parts per million and SNVs at 0.0125% variant allele fraction, with a reported cost near £33 per sample. The method reportedly identifies ecDNA, copy‑number changes, methylation, and SNVs from 1 mL plasma, suggesting a broad molecular readout from very small samples. (x.com)
Cancer cells shed tiny DNA fragments into blood, but the tumor signal can be vanishingly small. A medRxiv preprint posted April 8 says a new nanopore sequencing method called SIMMA can read those fragments one molecule at a time from low-input samples. (medrxiv.org) The paper, led by researchers at the European Molecular Biology Laboratory European Bioinformatics Institute, the University of Birmingham, the Institute of Cancer Research, and several National Health Service hospitals, analyzed 792 plasma and cerebrospinal fluid samples from 277 pediatric patients with brain and extracranial tumors. The authors say the assay combines whole-genome and targeted sequencing on the same cell-free DNA sample. (medrxiv.org) Cell-free DNA is the debris released by dying cells, and tumor-derived circulating tumor DNA is often less than 5% of that mixture and can fall to 0.1% or lower. That low abundance is why liquid-biopsy tests often trade breadth for sensitivity, or sensitivity for cost. (nature.com; springer.com) Nanopore sequencing works by threading DNA through a pore and reading changes in electrical current, which can preserve long-range features such as fragment length and methylation marks. Reviews of the field say that makes it attractive for multimodal blood tests, but accuracy at very low variant levels has been a persistent hurdle. (springer.com; nature.com) SIMMA is aimed at that problem by using single-molecule consensus reads from low-input cell-free DNA and then layering mutation, copy-number, fragment-size, and methylation analysis onto the same sample. In the abstract, the authors say the method enabled tumor diagnosis, driver-mutation detection, and identification of extrachromosomal DNA months before clinical relapse. (medrxiv.org; medrxiv.org) Extrachromosomal DNA is DNA that sits outside chromosomes, often in circular pieces that can carry amplified cancer genes such as MYCN. The full preprint says SIMMA detected more than 800 copies of a complex extrachromosomal DNA molecule carrying a MYCN amplification in one neuroblastoma cell-free DNA sample at about 0.6-fold genome-wide depth. (medrxiv.org) The supplementary figures also show the method separating short mono-, di-, and tri-nucleosomal fragments and ultralong fragments above 1,000 base pairs, then using those size bands to sharpen copy-number calls. Those fragment-length patterns matter because tumor-derived cell-free DNA is not chopped up the same way as DNA from healthy cells. (medrxiv.org; springer.com) The preprint is still unreviewed, and medRxiv labels it as research that “should not be used to guide clinical practice.” The next test is whether outside groups can reproduce the sensitivity, cost, and turnaround claims in prospective clinical studies. (medrxiv.org)
