A new form of nucleoside phosphorylase deficiency in two brothers with defective T-cell function

Two brothers, age 9 and 11, respectively, have marked deficiency of nucleoside phosphorylase associated with defective T-cell function and normal B-cell function. Unlike the previously described five patients with this syndrome, each of these children has sufficient NP catalytic activity in their red blood cells (below 1% of the normal level) to be visualized after electrophoresis and staining for the enzyme. Their healthy sibling has normal NP activity and a normal isozyme pattern. The nonconsanguineous parents have about half-normal NP activity, but their electrophoretic patterns differ from each other's and from those of their affected children. These findings are consistent with genetic heterogeneity at the NP structural gene locus, resulting in compound heterozygosity for two different abnormal alleles.