Screening of a large cohort of blepharophimosis, ptosis, and epicanthus inversus syndrome patients reveals a very strong paternal inheritance bias and a wide spectrum of novel FOXL2 mutations

Abstract

Blepharophimosis, Ptosis, and Epicanthus inversus Syndrome (BPES) is caused by autosomal dominant mutations in FOXL2. There are two forms of BPES: type I (with primary ovarian insufficiency (POI)) and type II (without POI). Data are presented from a large cohort of 177 BPES probands. Diagnostic testing identified a wide range of mutations in 119 mutation-positive patients (including 38 novel mutations). Although FOXL2 mutations are distributed throughout the gene, over 50% were frameshift mutations within a hotspot region of the gene that can be detected using a single primer pair to provide a cost-effective and rapid screening method. There was a significant proportion of de novo cases in this study, although in 7% there may be undetected parental mosaicism. There was an excess of female compared to male probands and a highly significant bias in the parental original of inherited mutations, with 20/21 found to be paternal in origin (95%). This could be because BPES in a female is more likely to come to clinical attention and because there is a generalised and more widespread clinical effect on fertility, in addition to the established association with POI. This study demonstrates the importance of cascade screening and provides new information on inheritance and parental mosaicism in BPES which will aid genetic counselling and accurate risk management.

Introduction

The clinical manifestations of blepharophimosis, ptosis, and epicanthus inversus syndrome (BPES) were first described over 100 years ago (Vignes, 1889). In addition to the titular symptoms of blepharophimosis, ptosis, and epicanthus inversus, other features include low nasal bridge and primary ovarian insufficiency (POI; Sacrez et al., 1963; Johnson, 1964; Smith, 1970; Townes and Muechler, 1979). There are two forms of BPES: type I (with POI) and type II (without POI) (Zlotogora et al., 1983). Both are caused by autosomal dominant mutations in the FOXL2 gene on chromosome 3q22. The BPES critical region was originally narrowed down following the mapping of a translocation (Lawson et al., 1995), and FOXL2 was identified as the cause of BPES when point mutations within the gene were reported a few years later (Crisponi et al., 2001).

FOXL2 encodes a protein of 376 amino acids with the critical forkhead domain between codons 53 and 139 (pfam.xfam.org/protein/P58012) and a polyalanine tract of unknown function at p.Ala221_Ala234. The BPES subtype can be influenced by the mutation type and its location. Mutations predicted to result in a protein truncated before the polyalanine tract preferentially lead to BPES type I (including POI), polyalanine expansions preferentially lead to BPES type II, while mutations that predict a truncated or extended protein containing an intact forkhead and polyalanine tract are not known to have a genotype-phenotype correlation (De Baere et al., 2003). There are currently no known genotype/phenotype correlations for FOXL2 missense mutations (De Baere et al., 2003; Beysen et al., 2008a). The FOXL2 protein is a transcriptional repressor of key genes in ovarian follicle differentiation, and it is thought that heterodimer formation between a wild-type and mutant FOXL2 protein leads to POI (Kuo et al., 2011).

For patients with a strong clinical diagnosis of BPES, previous studies have shown a high FOXL2 mutation detection rate (over 80%) with 27–30 base pair expansions within the polyalanine repeat tract accounting for around 30% of mutations (De Baere et al., 2003; Beysen et al., 2008a, 2009). A second mutation hotspot was also reported, with the c.843_859dup; p.(Pro287ArgfsTer75) mutation accounting for 13% of mutations in one study (Beysen et al., 2009), but other mutations were distributed widely throughout FOXL2. FOXL2 whole gene deletions and submicroscopic genomic rearrangements were also seen in a significant proportion of cases (12% and 5% respectively; Beysen et al., 2009), so testing strategies cannot be limited to point mutation testing alone.

A large cohort of 177 BPES probands referred to the Wessex Regional Genetics Laboratory for diagnostic testing of FOXL2 underwent Sanger sequencing of the FOXL2 coding sequence and dosage analysis by MLPA (Schouten et al., 2002). The resulting mutation data are presented here and represent a valuable resource regarding FOXL2 mutation types, mutation hot-spots and inheritance patterns.