EDNRB mutations cause Waardenburg syndrome type II in the heterozygous state.

Waardenburg syndrome (WS) is a genetic disorder characterized by sensorineural hearing loss and pigmentation anomalies. The clinical definition of four WS types is based on additional features due to defects in structures mostly arising from the neural crest, with type I and type II being the most frequent. While type I is tightly associated to PAX3 mutations, WS type II (WS2) remains partly enigmatic with mutations in known genes (MITF, SOX10) accounting for only 30% of the cases. We performed exome sequencing in a WS2 index case and identified a heterozygous missense variation in EDNRB. Interestingly, homozygous (and very rare heterozygous) EDNRB mutations are already described in type IV WS (i.e., in association with Hirschsprung disease [HD]) and heterozygous mutations in isolated HD. Screening of a WS2 cohort led to the identification of an overall of six heterozygous EDNRB variations. Clinical phenotypes, pedigrees and molecular segregation investigations unraveled a dominant mode of inheritance with incomplete penetrance. In parallel, cellular and functional studies showed that each of the mutations impairs the subcellular localization of the receptor or induces a defective downstream signaling pathway. Based on our results, we now estimate EDNRB mutations to be responsible for 5%-6% of WS2.

Estimation of the difference in HbF expression due to loss of the 5' δ-globin BCL11A binding region.

BCL11A was the focus of recent studies on its inhibiting effect when bound onto the ß-globin cluster in the mechanism of hemoglobin switching and HbF downregulation. We examined a cohort of 10 patients displaying different HbF levels and short deletions within the γß-δ intergenic region to find a possible correlation with the BCL11A binding site located 5' to the δ-globin gene. Precise characterization of deletions was achieved using a custom DNA-array chip and breakpoint sequencing. The α-globin cluster and major SNP associated with HbF expression were genotyped. Our results show that the loss of the BCL11A binding domain located 5' to the δ-globin gene is correlated with a strong HbF difference (mean+2.7 g/dL, ratio 2.81). This result provides evidence for the use of BCL11A level down-regulation or this domain blockage for new therapies in sickle cell disease and ß-thalassemia major patients.

A second observation of the rare frameshift mutation in the ß-globin gene: codon 46 (+A) (Hbb:c.138_139insA).

The preparation of a prenatal diagnosis in a family of North-African origin in which a child received a bone marrow transplant for ß-thalassemia major (ß-TM), prompted us to make the molecular diagnosis in the parents and siblings. Molecular and phenotype assays were carried on blood samples from the parents and the proband's sister. The father, a 45-year-old man, was found to be heterozygous for a rare mutation in exon 2 [codon 46 (+A), HBB:c.138_139insA] creating a frameshift, while the mother and sister were found to be carriers of the common codon 39 (C>T) stop mutation (HBB:c.118C>T). Because of the bone marrow transplant, proband genotyping was done from a buccal swab and revealed that he is a compound heterozygote for both the codon 46 and codon 39 mutations. In the parents and sister, hematological parameters were those of a thalassemia minor in agreement with the two ß(0) mutations found in the family.