Branchiootorenal Syndrome Panel

Branchiootorenal syndrome is an autosomal dominant disorder characterized by branchial arch defects (branchial fistulas or cysts), hearing loss (sensorineural, conductive, or mixed hearing loss), and renal anomalies (from mild renal hypoplasia to bilateral renal agenesis) (Smith 2013). Common clinical features include malformations of the outer, middle, and inner ear; preauricular pits and tags; facial asymmetry and palate abnormalities. Some patients develop end-stage renal disease (ESRD) later in life. The clinical spectrum of Branchiootorenal syndrome is wide and substantial phenotypic variability can occur even within the same family. Age of onset varies from early childhood to young adulthood.

Branchiootorenal syndrome is an autosomal dominant disorder that can be caused by defects in the EYA1, SIX1 or SIX5 genes (Abdelhak et al. 1997; Krug et al. 2011).

EYA1 has 16 coding exons that encode a member of the eyes absent (EYA) family of proteins, which may play a role in development of the kidney, branchial arches, eye, and ear. EYA1 defects represent the major cause of Branchiootorenal syndrome, accounting for about 40% of cases. Pathogenic EYA1 variants have been found to include missense and nonsense substitutions, splicing variants, and small indels (Human Gene Mutation Database). Large deletions, duplications and complex rearrangements involving the EYA1 gene have also been commonly reported. These pathogenic variants have been found to cluster within or in the immediate vicinity of the highly conserved 271 amino acid C-terminal region (eyaHR), which is encoded by exons 9 to 16 (Abdelhak et al. 1997).

SIX1 has two coding exons that encode a homeobox protein critical to organogenesis. SIX1 defects represent a less frequent cause of Branchiootorenal syndrome. To date, pathogenic SIX1 variants have been found to include missense substitutions, a splicing variant, a small indel and a large deletion (Human Gene Mutation Database).

SIX5 has three coding exons that encode a homeodomain-containing transcription factor functioning in the regulation of organogenesis. SIX5 defects represent a minor cause of Branchiootorenal syndrome. To date, only missense pathogenic variants have been found in the SIX5 gene (Human Gene Mutation Database).

Pathogenic EYA1 variants that can be detected via DNA sequencing were found in approximately 30% of patients with Branchiootorenal syndrome (Orten et al. 2008; Krug et al. 2011).

Pathogenic SIX1 variants were found in approximately 4% (10/247) of patients with Branchiootorenal syndrome (Kochhar et al. 2008).

SIX5 pathogenic variants were found in approximately 5% of patients with Branchiootorenal syndrome (Hoskins et al. 2007) while no SIX5 pathogenic variants have been found in another cohort of 140 patients from 124 families with Branchiootorenal syndrome (Krug et al. 2011).

The detection rate of large deletions and duplications involving the EYA1 gene is varied. Multi-exon deletions within EYA1, which can be detected by aCGH, were found in 15% (3/20) of unrelated patients with Branchiootorenal syndrome (Abdelhak et al. 1997). In another study, 4 out of 5 families were found to have complex rearrangements including a large inversion and deletions (Vervoort et al. 2002).

A large segmental duplication involving the SIX1 gene was reported in a patient with Branchiootorenal syndrome and oculoauriculovertebral spectrum features (Ou et al. 2008). However, the frequency of exon-level copy number changes has not been reported in a large cohort of patients with Branchiootorenal syndrome.

Thus far, no large deletions or duplications involving the SIX5 gene have been reported (Human Gene Mutation Database).

This test is performed using Next-Gen sequencing with additional Sanger sequencing as necessary.

This panel provides 100% coverage of all coding exons of the genes plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define coverage as ≥20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).