Pedigree with Skewed X Inactivation

In X-linked recessive disease, heterozygous females are usually protected because roughly half their cells express the normal allele after random X inactivation. Skewed inactivation can shift this balance. If the proband's active X is predominantly paternal (85:15), and the paternal X carries the normal allele, that would predict protection, not symptoms. The key is which X carries the mutation: if the maternal X carries the variant and is mostly inactive, she should be protected. Symptoms suggest that in relevant tissues the mutant allele is expressed in a sufficient fraction of cells, or that the blood skewing does not reflect the disease-relevant tissue. The aunt's 70:30 skewing favoring the mutant-bearing active X explains her moderate symptoms. The grandmother's 50:50 skewing explains her lack of symptoms. Blood methylation is a surrogate; tissue-specific skewing can differ, so the proband's blood result must be interpreted cautiously.

The proband is a heterozygous carrier of an X-linked recessive variant. Each of her sons has a 50% chance of inheriting the variant and being affected; each daughter has a 50% chance of being a carrier, and carrier daughters may be symptomatic depending on skewing. The unaffected brother inherited his single X from the mother. If he is unaffected, he did not inherit the variant-bearing X (or the variant is not fully penetrant, which is unlikely for a clear missense variant in males). His daughters would therefore not be carriers via him, and his sons are unaffected. Recurrence risk for the brother's offspring is essentially the population background, not the family variant, assuming his carrier status is excluded by testing. Genetic counseling should include testing the brother's X-linked variant status if he is considering children.

First, confirm the phase of the variant relative to the active X in the proband using a transcribed polymorphism or allele-specific expression in disease-relevant tissue, not just blood. Second, measure X inactivation in the relevant tissue (e.g., skin fibroblasts, muscle) because blood skewing may not reflect the target tissue. Third, test whether the variant protein has residual function; a hypomorphic allele with partial activity would explain mild symptoms even with modest skewing. Fourth, examine the mother's and aunt's tissues for concordance between skewing direction and symptoms. Finally, consider whether the variant is truly causative using segregation, in silico prediction, and functional assays. Consistency across tissues and a plausible mechanism linking mutant allele expression to phenotype would strengthen the causal claim.