DPYS and hyperinsulinemic hypoglycemia, familial, 4: Furthermore, it has been demonstrated that patients with partial DHP deficiency are prone to develop severe 5-FU toxicity (van Kuilenburg et al 2003), and heterozygous mutations in the UPB1 gene may impair uracil catabolism (Fidlerova et al 2012; Thomas et al 2008).