#!/bin/bash
s='xxxxxxx-000-387-159 < 50 null null_27 KCNQ1 1 1.430 0.773 0.656 1.724
xxxxxxx-000-375-285 < 50 < 50 100 - 500_0 KCNQ1 2 1.409 0.786 0.623 1.801
xxxxxxx-000-523-531 50 - 100 50 - 100 100 - 500_31 KCNQ1 3 1.326 0.848 0.479 1.557'
sed -E 's/^(.{19})[^K]*(KCNQ)/\1 \2/' <<< "$s"
IyEvYmluL2Jhc2gKcz0neHh4eHh4eC0wMDAtMzg3LTE1OSA8IDUwIG51bGwgbnVsbF8yNyBLQ05RMSAxIDEuNDMwIDAuNzczIDAuNjU2IDEuNzI0Cnh4eHh4eHgtMDAwLTM3NS0yODUgPCA1MCA8IDUwIDEwMCAtIDUwMF8wIEtDTlExIDIgMS40MDkgMC43ODYgMC42MjMgMS44MDEKeHh4eHh4eC0wMDAtNTIzLTUzMSA1MCAtIDEwMCA1MCAtIDEwMCAxMDAgLSA1MDBfMzEgS0NOUTEgMyAxLjMyNiAwLjg0OCAwLjQ3OSAxLjU1NycKc2VkIC1FICdzL14oLnsxOX0pW15LXSooS0NOUSkvXDEgXDIvJyA8PDwgIiRzIgo=