Against all odds: blended phenotypes of three single-gene defects

Whole-exome sequencing allows for an unbiased and comprehensive mutation screening. Although successfully used to facilitate the diagnosis of single-gene disorders, the genetic cause(s) of a substantial proportion of presumed monogenic diseases remain to be identified. We used whole-exome sequencing to examine offspring from a consanguineous marriage featuring a novel combination of congenital hypothyroidism, hypomagnesemia and hypercholesterolemia. Rather than identifying one causative variant, we report the first instance in which three independent autosomal-recessive single-gene disorders were identified in one patient. Together, the causal variants give rise to a blended and seemingly novel phenotype: we experimentally characterized a novel splice variant in the thyroglobulin gene (c.638+5G>A), resulting in skipping of exon 5, and detected a pathogenic splice variant in the magnesium transporter gene TRPM6 (c.2667+1G>A), causing familial hypomagnesemia. Based on the third variant, a stop variant in ABCG5 (p.(Arg446*)), we established a diagnosis of sitosterolemia, confirmed by elevated blood plant sterol levels and successfully initiated targeted lipid-lowering treatment. We propose that blended phenotypes resulting from several concomitant single-gene disorders in the same patient likely account for a proportion of presumed monogenic disorders of currently unknown cause and contribute to variable genotype-phenotype correlations.