This is the second in a series about Helicos Biosciences, a next generation sequencing startup
Helicos Part I
Having familiarized ourselves with the NGS market, we are better positioned to take a look at Helicos and evaluate their recent IPO
In 2003 professor Stephen Quake, then at California Institute of Technology, published a paper in the Proceedings of the National Acedemy of Sciences describing that sequence information could be obtained from a single strand of DNA without amplification. Quake's group was able to overcome the hurdle of resolving individual bases by using DNA polymerase in combination with fluorescent nucleotides to image sequential incorporations during synthesis. Furthermore, this paper demonstrated a breakthrough application of single molecule theory in the field of DNA sequencing.
Dr. Quake subsequently met with Noubar Afeyan and Stanley Lapidus, then CEO and Partner at Flagship Ventures respectively, and they agreed to found a company to develop and commercialize the single molecule sequencing technology. Professor Eric Lander, Director of the Broad institute, played some sort of advisory role during the founding stages of the company. The company was incorporated in May 2003 and was renamed Helicos BioSciences in November of the same year.
Besides launching an IPO, highlights of Helicos since its inception include receiving a $2 million grant from NHGRI. In addition they have assembled a rather strategic management team and a prototype collaboration with, among others, Dr. Leroy Hood of the Institute for Systems Biology. Finally, during the 2Q investor conference Mr. Lapidus stated that the company is on track for a product launch to take place later in 2007.
price : n/a
MB/run : n/a
run length : ~100MB/hr
MB/day : ~1000MB
Read length : ~25
Raw base accuracy : n/a
The length of time between this blog entry and my last is largely due to the difficulties I encountered researching on the exact processes that the Heliscope employs to enable its "true Single Molecule Sequencing" (tSMS) technology. Needless to say that Helicos, despite the media fanfare, is playing their precise technology pretty close to the vest.
Using the original PNAS article, available patents, this chapter, (written by the two first authors on the PNAS paper for the Ohio U physics department) and some rumors, I will piece together what exactly we know about the Heliscope technology. Where applicable, I will add my suspicions to the best of my knowledge to fill in the gaps.
The Heliscope process in a nutshell is to shear the DNA and polyadenylate the fragments. These fragments, which also incorporate a dye molecule, are then attached randomly (via poly Ts) to the proprietary flowcell surface. Initial attachment locations are recorded via the dye molecule which is illuminated by laser excitation and recorded by a CCD camera connected to a microscope. After removing the dye, DNA polymerase and a dye-labeled nucleotide flow in and are then washed out. If the particular nucleotide-dye is complementary to any given fragment it will thus become incorporated into the growing strand. The camera will again mark the location of the fluorescence upon subsequent laser excitation. The dye molecule is then removed, washed away, and sequencing processes by repeatedly cycling through the four different nucleotides.
The benefits of sequencing single molecules of DNA are advantages of improved throughput and reduced cost. Compared to the other techniques discussed in my last entry, the Heliscope workflow is considerably less complicated, leading to shorter run times....there is no PCR, no beads, no microtiter plates etc. Of course this also equates to less reagents used, and conceivably, lower price per run.
However, the major challenge facing single molecule sequencing is that of sensitivity. Allow yourself to imagine the difference in signal intensity between a singly incorporated nucleotide on the Heliscope surface and the analgous millions of incorporated flourescent molecules on the Illumina system's flowcell surface. Therefore exploring how the Heliscope allegedly attains this sensitivity to the degree of six orders of magnitude over its competitors is essential to our assessment of the company.
In the field of single molecule imaging, the largest challenge is that of increasing both the resolution and sensitivity of a given signal past the limitations of the detecting instrument. This effort is primarily approached by increasing the signal to noise ratio. Given that the efficiency of the flourophores are maximal this is best accomplished by reducing the noise in the system. The Heliscope, apparently based on the method developed in Dr. Quake's laboratory, accomplishes this in two major ways. First, it uses a method called Total Internal Reflection Microscopy (TIRM) whereby only the flourophores within ~150nm of the flowcell surface are illuminated. This leads to a dramatic reduction of the noise from the bulk fluids. In addition, this method increases theoretical speed of readout as no scanning is involved. However, while TIRM reduces noise from objects in the solution far away from the surface, it does not reduce noise from surface bound impurities. In this manner, Dr. Quake's group overcame the second challenge of eliminating non-specifically bound surface dye molecules using a method known as Flourescent Resonant Energy Transfer (FRET). In this method, not one but actually two flourescent dyes are used. Requirements are that one dye, (Cy3) termed the donor, has an emission spectra that overlaps with absorption spectra of a second dye (Cy5), termed the acceptor. Thus when the donor molecule is within proximity to the acceptor, usually less than 10nm, and is excited at its specific excitation wavelength, it will transfer this energy to the acceptor dye which in turn becomes excited and emits a photon of lower energy. Consider it to be a kind of baton passing between the donor and the acceptor in a molecular relay race. The PNAS paper describes FRET being used by placing the donor molecule (Cy3) on the existing DNA strand and the acceptor (Cy5) on the incorporated nucleotide. In this manner, only the polymerase incorporated nucleotide-Cy5 molecules emit a signal while the non-specifically bound surface nucleotide-Cy5s remain dark because they are not within 10nm of the attached DNA containing a Cy3 donor. This combination of TIRM with FRET provide an unparalleled increase in the signal to noise ratio of single molecule detection and was indeed a groundbreaking application in DNA sequencing. Presumably this is the technology which makes the Heliscope possible, but how has Helicos improved, if at all, upon the technology?
Three fundamental questions remain which I will endeavor to answer in turn. How does the Heliscope deal with long stretches of repeat nucleotides? Does the Heliscope still use FRET and where exactly is the donor? How is the dye molecule "removed" after each incorporation cycle?
To begin, an initial issue with this method of DNA sequencing was a problem with accurately sequencing large numbers of repeat nucleotides, so called homopolymer regions. The problem can be realized if one imagines that during a cycle of a given nucleotide say dGMP the instrument would need to be able to detect 1, 2, 3 or more simultaneous incorporations if the template has a string of cytosines. As the problems explained above with signal to noise concerned simply detecting the presence of a fluorescent molecule it can be understood that detecting the difference between one and two incorporations is possible, but higher orders are out of the question. After a period of uncertainty it seems that Helicos has solved this issue. The press release on Feb 9th 2007 states "The proprietary nucleotide analogs contained in these unique formulations control accurate base-by-base extension through chemical means." I assume that they refer to their patent issued on Jan 30 2007. This patent simply describes using a dye-conjugated nucleotide in conjunction with DNA polymerase kinetics in such a way that one or two (but statistically insignificant amounts of higher) nucleotides are incorporated per relatively short reaction cycle.
Does the Heliscope use FRET? I ask because on the website the "technology" shows one dye molecule only. The short answer is that I don't know for sure, but I'm pretty sure. The original PNAS paper was published in 2003 which wasn't that long ago and all the single molecule people I know are still using FRET for the unsurpassed resolution. Furthermore, page 12 of this presentation (by the chairman of research core facilities and technology at the Mayo clinic) shows FRET as part of the process and furthermore has the donor attached to the end of the polyadenylated tail. This makes additional sense in light of the rumored ~25bp read length because at 3.4nm per turn and 10.5 bases per turn, that puts about 30bp within the 10nm range of FRET acceptor absorbance. It should be noted that there has been discussion in the original paper, the patents, and elsewhere of putting the donor molecule on the DNA polymerase itself, an area of research that I have no doubt that Helicos is actively pursuing for its later generation sequencers.
Finally, is the acceptor molecule removed and if so, how? On this topic I am unable to find any definitive information. Traditionally, the acceptor molecule is photobleached after detection using specific laser illumination at its absorbance. This leaves the donor molecule relatively unharmed, and capable of donating to another fresh acceptor. One drawback to this technique however, is that the acceptor molecule is not removed and therefore successive incorporations are compromised via steric interactions. In addition, successive photobleaching of the acceptor molecules will eventually also bleach the donor. Perhaps Helicos has developed a more robust donor, and an uncompromising acceptor, in this anything is possible. Alternatively, in the single molecule literature there are many examples of cleavable dyes, either chemically or photocleavable. That the dye is in fact cleaved off and removed seems indicated on the Helicos website, but I am not confident about the specific details of those slides. Certainly cleaving and removing the dye is the preferred method in this instance and if Helicos does not yet employ this method I am sure it is another development for future machines.
Phew! I hope you are still with me after all that. There is no doubt that it has been a challenge to disentangle the methodology from the hype regarding Helicos. However, the benefit is that we get to make an assessment based on quite a bit of science, long before S&P even touches it. At this point I still have some misgivings regarding whether or not the Heliscope will live up to expectations, but we will address these issues next time, as well as go over the financials.
Disclosure: I am long shares of HLCS