Show no LOH and retain one p53 wild-type allele

About half of the tumours that develop in p53+/- heterozygote mice exhibit LOH and lose the wild-type p53 allele. Some of the remaining tumours, however, show no LOH and retain one p53 wild-type allele. Which of the following genetic changes might explain the development of these p53+/- tumours?
Activating mutation in FLT3
MYC gene amplification
Mdm2 gene amplification
Activating mutation in H-Ras