Lightcycler system

For qRT-PCR of mRNA targets, total RNA was extracted from cancer cells using RNAiso Plus (TaKaRa, 09108B). cDNA synthesis was performed according to the manufacturer’s instructions (TaKaRa, DRR047A), and qRT-PCR was performed with SYBR Premix Ex Taq II (TaKaRa, DRR081A) using a LightCycler system (Roche). The PCR reaction conditions for all of the assays were 94 °C for 30 seconds, followed by 40 cycles of amplification (94 °C for 5 seconds, 60 °C for 30 seconds and 72 °C for 30 seconds). GAPDH mRNA was used to normalize RNA inputs. The qRT-PCR primers are listed in Additional file 1: Table S2.
For qRT-PCR of miRNAs, small RNAs were extracted from cancer cells or tumor tissues using RNAiso for small RNAs (TaKaRa, D340A). miRNAs were converted to cDNA using a cDNA synthesis kit (TaKaRa, DRR047A), and qRT-PCR was performed with SYBR Premix Ex Taq II (TaKaRa, DRR081A) using a LightCycler system (Roche). The PCR reaction conditions for all of the assays were 95 °C for 20 seconds, followed by 40 cycles of amplification (95 °C for 10 seconds, 60 °C for 20 seconds and 70 °C for 5 seconds). U6 was used to normalize the RNA inputs. All of the primers were from the Bulge-Loop™ miRNA qRT-PCR primer set (RiboBio, MQP-0102, China).

Swabs were briefly vortexed in 500 µL PBS and viral RNA was extracted from 140 µL in accordance with the manufacturer’s instructions (QIamp viral RNA; Qiagen, Toronto, ON, Canada). RNA extractions were performed within the 48 h following sampling (swabs were kept at 4 °C in the meantime). RT-qPCR was performed on 96-well plates with a final volume of 20 µL in a Light Cycler system (Roche, Penzberg, Germany). The mixes were prepared in accordance with the manufacturer’s instructions (QuantiNova Probe; Qiagen, Toronto, ON, Canada) with 2 µL of RNA and primers that targeted the E-gene. To obtain human-derived viral RNA and perform phylogenetic analysis, the owner from case 3 self-performed a nasal swab which was treated as a feline one. To check whether the material in the oropharyngeal and rectal swabs originated from felines, RT-qPCR targeting the feline 40S ribosomal protein S7 (RPS7) gene was also performed. SARS-CoV-2-positive swabs were further tested for human ribosomal protein L30 (RPL30) mRNA expression, to rule out human contamination. RPS7 and RPL30 RT-qPCR were performed on 96-well plates in a final volume of 10 µL in a Light Cycler system (Roche, Penzberg, Germany). Mixes were prepared according to the manufacturer’s instructions (QuantiFast; Qiagen, Toronto, ON, Canada). The primers’ sequences are listed in Supplementary Table S2.