ALPL Mutations With Dominant-Negative Effect in Infantile Hypophosphatasia Monozygotic Twins

Hypophosphatasia (HPP) is a rare inborn error of metabolism caused by ALPL gene mutations, resulting in deficient tissue-nonspecific alkaline phosphatase (ALP) activity. We investigated genotype-phenotype correlations in a monozygotic female twin pair with infantile HPP.

Peripheral blood samples were collected from two female twins with HPP and their family members. Genomic DNA was extracted, and variations were detected using whole exome sequencing. Zygosity was confirmed via KING 2.3.1 software. Pathogenic variants were validated using Sanger sequencing, mutation analyses, and bioinformatics.

The twins presented with bulging anterior fontanel at 3 months of age. At 6 months, serum ALP levels decreased, and skeletal dysplasia, hypercalcemia, and nephrocalcinosis developed. One twin died of pneumonia at 11 months; the other remained alive beyond 15 months. Monochorionic diamniotic placentation and a twin pair kinship coefficient (0.4879) confirmed monozygosity. Exome sequencing revealed that the twins carried compound heterozygous ALPL mutations c.299C>T (p.Thr100Met) and c.1271T>C (p.Val424Ala). The maternally inherited allele c.1271T>C, which was suspected to be pathogenic according to the American College of Medical Genetics and Genomics (ACMG) guidelines, was reported in a homozygous Chinese adult with HPP. The paternally inherited allele c.299C>T was rated as pathogenic by ACMG. In vitro, ALPL c.299C>T-related mutants exhibited reduced residual ALP activity owing to a dominant-negative effect. Analysis of c.299C>T-related mutations in the ALPL Gene Variant Database revealed statistically significant differences in a dominant-negative effect and lower residual ALP activity of allelic mutants in early-onset versus late-onset HPP (p < 0.05). The dominant-negative effect correlated positively with residual ALP activity (r _s = 0.889, p < 0.05).

Compound heterozygous ALPL mutations c.299C>T (p.Thr100Met) and c.1271T>C (p.Val424Ala) were causative factors of infantile HPP in the monozygotic twins, providing insights into how dominant-negative effects influence HPP severity.