Unexpected high intrafamilial phenotypic variability observed in hypophosphatasia

Area of Science:

• Genetics
• Metabolic Disorders
• Pediatrics

Background:

• Hypophosphatasia (HPP) is a rare, inherited disorder of bone and mineral metabolism.
• It exhibits significant clinical and allelic heterogeneity linked to the ALPL gene.

Purpose of the Study:

• To investigate the phenotypic variability in a family with HPP.
• To correlate genotype with phenotype and functional enzyme activity.

Main Methods:

• Genetic analysis of a family with HPP, including parents and four children.
• In vitro functional studies of ALPL mutations (p.Glu191Lys and p.Gly334Asp).
• Assessment of residual alkaline phosphatase (AP) activity.

Main Results:

• Observed significant phenotypic variability among siblings with compound heterozygous ALPL mutations.
• In vitro studies revealed differential effects of mutations: p.Glu191Lys (mild, 68% activity) and p.Gly334Asp (severe, 1.2% activity).
• Simulated genetic status showed 29% residual AP activity, with comparable serum AP levels in affected children.

Conclusions:

• ALPL mutation location alone is insufficient for predicting HPP clinical severity.
• Prenatal diagnosis and counseling for HPP require cautious interpretation and multidisciplinary expertise.
• Further research into confounding factors influencing HPP phenotype is necessary.