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.

[Metatropic dysplasia in a girl with c.1811_1812delinsAT mutation in exon 11 of the TRPV4 gene not previously reported]. / Displasia metatrópica en una niña con mutación c.1811_1812delinsAT en el exón 11 del gen TRPV4 no informada previamente.

Metatropic dysplasia is a skeletal disorder with clinical heterogeneity, characterized by craniofacial dysmorphy including frontal bossing and midface hypoplasia, short trunk,progressive kyphoscoliosis and shortened limbs. The TRPV4 gene is located on 12q24.11, coding a cation channel with nonselective permeability to calcium; it is expressed and involved in many physiological processes through responses to different stimuli. Over 50 mutations in TRPV4 have been described. We present a seven months old girl with heterozygous mutation c.1811_1812delinsAT; p.I604N in intron 11 not previously reported in the TRPV4 gene and with clinical findings compatible with metatropic dysplasia.

Displasia metatrópica en una niña con mutación c. 1811_1812delinsAT en el exón 11 del gen TRPV4 no informada previamente / Metatropic dysplasia in a girl with c. 1811_1812delinsAT mutation in exon 11 of the TRPV4 gene not previously reported

La displasia metatrópica es una alteración esquelética con heterogeneidad clínica, caracterizada por dismorfias craneofaciales, que incluyen prominencia frontal e hipoplasia medio facial, tronco corto con cifoescoliosis progresiva y acortamiento de las extremidades. El gen TRPV4 se localiza en 12q24.11; codifica a un canal de catión con permeabilidad no selectiva al calcio, el cual se expresa y participa en muchos procesos fisiológicos en respuesta a diversos estímulos. Más de 50 mutaciones en TRPV4 han sido descritas. Se presenta el caso de una niña de 7 meses de edad con mutación heterocigota c.1811_1812delinsAT; p.I604N en el intrón 11 no informada previamente en el gen TRPV4 y con hallazgos clínicos compatibles con displasia metatrópica.

Metatropic dysplasia is a skeletal disorder with clinical heterogeneity, characterized by craniofacial dysmorphy including frontal bossing and midface hypoplasia, short trunk, progressive kyphoscoliosis and shortened limbs. The TRPV4 gene is located on 12q24.11, coding a cation channel with non-selective permeability to calcium; it is expressed and involved in many physiological processes through responses to different stimuli. Over 50 mutations in TRPV4 have been described. We present a seven months old girl with heterozygous mutation c.1811_1812delinsAT; p.I604N in intron 11 not previously reported in the TRPV4 gene and with clinical findings compatible with metatropic dysplasia

RIN2 deficiency results in macrocephaly, alopecia, cutis laxa, and scoliosis: MACS syndrome.

Inherited disorders of elastic tissue represent a complex and heterogeneous group of diseases, characterized often by sagging skin and occasionally by life-threatening visceral complications. In the present study, we report on an autosomal-recessive disorder that we have termed MACS syndrome (macrocephaly, alopecia, cutis laxa, and scoliosis). The disorder was mapped to chromosome 20p11.21-p11.23, and a homozygous frameshift mutation in RIN2 was found to segregate with the disease phenotype in a large consanguineous kindred. The mutation identified results in decreased expression of RIN2, a ubiquitously expressed protein that interacts with Rab5 and is involved in the regulation of endocytic trafficking. RIN2 deficiency was found to be associated with paucity of dermal microfibrils and deficiency of fibulin-5, which may underlie the abnormal skin phenotype displayed by the patients.

Sall1, sall2, and sall4 are required for neural tube closure in mice.

Four homologs to the Drosophila homeotic gene spalt (sal) exist in both humans and mice (SALL1 to SALL4/Sall1 to Sall4, respectively). Mutations in both SALL1 and SALL4 result in the autosomal-dominant developmental disorders Townes-Brocks and Okihiro syndrome, respectively. In contrast, no human diseases have been associated with SALL2 to date, and Sall2-deficient mice have shown no apparent abnormal phenotype. We generated mice deficient in Sall2 and, contrary to previous reports, 11% of our Sall2-deficient mice showed background-specific neural tube defects, suggesting that Sall2 has a role in neurogenesis. To investigate whether Sall4 may compensate for the absence of Sall2, we generated compound Sall2 knockout/Sall4 genetrap mutant mice. In these mutants, the incidence of neural tube defects was significantly increased. Furthermore, we found a similar phenotype in compound Sall1/4 mutant mice, and in vitro studies showed that SALL1, SALL2, and SALL4 all co-localized in the nucleus. We therefore suggest a fundamental and redundant function of the Sall proteins in murine neurulation, with the heterozygous loss of a particular SALL protein also possibly compensated in humans during development.