Epstein-Barr virus-associated gastric cancer reveals intratumoral heterogeneity of PIK3CA mutations

Background

Recent whole-genome sequencing identified four molecular subtypes of gastric cancer (GC), of which the subgroup of Epstein–Barr virus-associated GC (EBVaGC) showed a significant enrichment of PIK3CA mutations. We here aimed to validate independently the enrichment of PIK3CA mutations in EBVaGC of a Central European GC cohort, to correlate EBV status with clinico-pathological patient characteristics and to test for a major issue of GC, intratumoral heterogeneity.

Patients and methods

In a first step, 484 GCs were screened for EBV and PIK3CA hot spot mutations of exon 9/20 using EBER in situ hybridization and pyrosequencing, respectively. Secondly, an extended sequencing of PIK3CA also utilizing next generation sequencing was carried out in all EBVaGCs and 96 corresponding lymph node metastases.

Results

Twenty-two GCs were EBER-positive, all being of latency type I. Intratumoral heterogeneity of EBER-positivity was found in 18% of EBVaGCs. Twenty-three GCs held PIK3CA mutations in hot spot regions of exon 9 or 20, being significantly more common in EBVaGCs (P < 0.001). Subsequent extended sequencing of PIK3CA of EBVaGCs showed that 14% harvested three to five different PIK3CA genotypes (including wildtype) in the same primary tumor, albeit in histologically and spatially distinct tumor areas, and that intratumoral heterogeneity of PIK3CA was also present in the corresponding lymph node metastases.

Conclusions

Our findings unravel issues of tumor heterogeneity and illustrate that the assessment of the EBV status in tissue biopsies might carry the risk of sampling errors, which may significantly hamper adequate molecular tumor classification in a more clinical setting. Moreover, this is the first report of intratumoral heterogeneity of PIK3CA mutations in GC, and our findings lead to the conclusion that PIK3CA mutant and -wildtype tumor subclones are skilled to metastasize independently to different regional lymph nodes.

Electrical properties of the vertically growing root tip of Lepidium sativum L

Electrical transmembrane potential differences and resistances in different tissues of intact root tips of Lepidium sativum L. were investigated in a humid atmosphere by conventional glass-microelectrode techniques with the reference electrode at the surface (apoplast) of the root. The resting potential (inside negative) in cells of the root cap rose from-80 mV in external cell layers (secretion cells) to approx.-140 mV in central cells (statocytes). Measurements of the electric input resistance within the apoplast of the root tip (calyptra, meristem and elongation zone) yielded a preference for longitudinal contact (resistance per length of tissue approx. 3.4 GOhm m(-1)) compared with transversal contact (approx. 14 GOhm m(-1)). Similarly, the symplastic coupling expressed as the characteristic length (L) where a signal is reduced to 1/c compared with the origin yielded L y =390 μm in the longitudinal (y) direction and L x =140 μm in the transversal (x) direction. Cable analytical treatment of the symplastic input resistances (approx. 10 MOhm) resulted in low membrane resistances in the y-direction at the ends of cells compared with the membrane resistances in the x-direction (approx. 0.2 Ohm m(2)) of the lateral membranes in the approximately cylindrical cells. This anisotropy is discussed in terms of model calculations. The resistivity of the symplast was calculated to be about 2.5 Ohm m. The input current-voltage relationship displayed a slight curvature with increasing slope for the more negative membrane potential typical of membranes with electrogenic pumps. Even after massive electrical stimulation in the range from-50 to-150mV carried out to trace current-voltage curves, electrical excitations (action potentials) were not detected in the cells investigated.

Membrane-potential responses following gravistimulation in roots of Lepidium sativum L

Membrane potentials were measured in lateral statocytes of vertically and nonvertically growing roots of Lepidium sativum L. using conventional glass-microelectrode techniques. Statocytes in vertically growing roots showed a stable resting potential of-118±5.9 mV without spontaneous fluctuations. Upon tilting the root 45° from the vertical, an electrical asymmetry was observed. Statocytes on the physically lower side of the root depolarized by approx. 25 mV. This depolarization occurred following a latent period of 8 s reaching a minimum (approx.-93 mV) after 170 s. This depolarization is the earliest event in graviperception ever recorded. After this depolarization, the cell repolarized within 60 s to a potential approx. 10 mV more positive than the original resting potential. Statocytes on the upper flank showed a slow hyperpolarization (t 1/2h=half time for hyperpolarization=168 s) reaching a final, stable potential at a level 10 mV more negative. These effects of gravistimulation were statenchyma-specific, since cells in the cortex and rhizodermis showed no similar effects. The gravi-electrical responses were observed in 25% of all roots tested. Roots which showed no gravi-electrical response had a reduced elongation growth, lacked gravity-induced bending and lacked the typical structural polarity in punctured statocytes. This observed transition from a symmetrical pattern of resting potential in the statenchyma to an asymmetrical pattern following gravistimulation supports the results observed with external current measurements (Behrens et al., Plant Physiol. 70, 1079-1083, 1982) and extends these results to the cellular level and to considerably improved temporal resolution. The asymmetry in the gravi-electrical response extends the graviperception model of Sievers and Volkmann (Planta 102, 160-172, 1972) which comprises an asymmetrical sedimentation of the amyloplasts on the distal endoplasmic reticulum of statocytes. This generates an intraorgan signal which then must be transmitted to the growth zone.

Can a Ca2+ pump in the endoplasmic reticulum of the Lepidium root be the trigger for rapid changes in membrane potential after gravistimulation?

Since gravistimulation is followed by alterations in the external current symmetry (Behrens et al., 1982), the effect of gravistimulation on cellular membrane potential was investigated using conventional glass microelectrode techniques. The resting potential of statocytes in a vertically oriented root is approx. -118 mV. Upon gravistimulation, the membrane potential is temporarily depolarized (lag time = 2 s) to a potential of approx. -93 mV. This depolarization is only observed in statocytes located on the physically lower root flank while those on the corresponding upper flank become weakly hyperpolarized (approx. -13 mV). These results reflect altered ion fluxes across the plasma membrane. The perception of gravistimulus was suggested to result from a pressure of the amyloplasts on the distal endoplasmic reticulum (ER) of the statocytes (Sievers and Volkmann, 1972). A causal relationship between changes in ER-amyloplast interactions and the rapid alterations in plasma membrane potential described above is not known. A candidate for such an intracellular messenger is Ca2+. As a first step in establishing the validity of such an assumption, we have isolated ER membranes from roots. When incubated with micromolar concentrations of Ca2+, the vesicular membrane fraction accumulates Ca2+. The accumulation is ATP-dependent and -specific and is directly coupled to ATP hydrolysis since a protonophore shows no inhibitory effect. Thus, in analogy to the sarcoplasmic reticulum of muscle, regulation of an ER-localized Ca2+ compartment might be an important step in such complex processes as stimulus-transduction in gravitropism.

Rapid Changes in the Pattern of Electric Current around the Root Tip of Lepidium sativum L. following Gravistimulation

Using a highly sensitive vibrating electrode, the pattern of naturally occurring electric currents around 1-day-old primary roots of Lepidium sativum L. growing vertically downward and the current pattern following gravistimulation of the root has been examined. A more or less symmetrical pattern of current was found around vertically oriented, downward growing roots. Current entered the root at the root cap, the meristem, and the beginning of the elongation zone and left the root along most of the elongation zone and in the root hair zone. After the root was tilted to a horizontal position, we observed current flowing acropetally at the upper side of the root cap and basipetally at the lower side within about 30 seconds in most cases. After a delay of several minutes, acropetally oriented current was also found flowing along the upper side of the meristematic zone. The apparent density of the acropetal current in the root cap region increased and then decreased with time. Gravitropic curvature was first visible approximately 10 minutes after tilting of the root to the horizontal position. Since the change in the pattern of current in the root cap region precedes bending of the root and is different for the upper and lower side, a close connection is suggested between the current and the transduction of information from the root cap to the elongation zone following graviperception in the cap.