** Shares of cryptocurrency and blockchain-related firms sink premarket as bitcoin tumbles 9.5% to $42,745.25

** Bitcoin bank Silvergate Capital Corp down 5.9%; cryptocurrency miners Riot Blockchain and Marathon Patent Group slide 6.9% and 9%, respectively

** China-based Future Fintech Group Inc slips 3%

** Shares of Overstock.com , an online retailer and blockchain tech investor, down 3.1%; China-focused SOS drops 6% and shares of crypto exchange Coinbase Global fall 4.6%

** MicroStrategy Inc, a highly leveraged BTC buyer and business intelligence software firm, tumbles 7.2%

** Bitcoin mining machine makers Ebang International and Canaan Inc shed 5.6% and 6.4%, respectively

** Bitcoin falls as much as 9.7% to $42,660, its lowest since Aug. 7 hurt by a broad risk-off sentiment

** BTC has risen ~46% YTD and has largely lifted the market values of crypto-related stocks with it

(Reporting by Aaron Saldanha) ((Aaron.Saldanha@thomsonreuters.com; +91 9810644993; Reuters Messaging: Aaron.Saldanha@thomsonreuters.com@reuters.net))