The heterogeneity of the clinical phenotypes results mainly from the great variety of mutations in the PAH gene. Null alleles eliminate almost all the enzyme's activity and cause classic PKU, while mutations with residual PAH activity result in milder forms. Like many single-gene disorders, genotype-phenotype correlations exist in most but not all cases. Environmental factors and/or modifier genes can also play a role in the clinical manifestations of the disease. The correlations of the most common mutations and their biochemical and clinical phenotypes are summarized in Table 8-1.
UREA CYCLE DISORDERS (ORNITHINE TRANSCARBAMYLASE DEFICIENCY)
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