A family with dominantly transmitted gynecomastia of pre-pubertal onset was first described in 1962 by Wallach and Garcia [
14]. After this initial report, several cases have been described [
5–
8,
15]. Laboratory examinations of the affected males revealed markedly elevated serum estrogen values and estrogen/androgen ratios and significantly increased aromatase activity in fibroblasts and lymphocytes [
5–
8,
15]. Linkage analyses in two families indicated a close association between
CYP19A1-flanking polymorphic markers and the disease phenotype [
5,
6]. Thus, the condition was assumed to be caused by gain-of-function mutations of
CYP19A1, and, therefore, the name of AEXS was coined for this condition [
7,
8]. However, since direct sequencing and Southern blotting analysis failed to detect mutations or copy number abnormalities in the coding region of
CYP19A1 [
5,
6], the molecular basis of this entity remained elusive until recently.
In 2003, Shozu et al. reported a father-son pair and a sporadic case with AEXS in whom they identified heterozygous chromosomal inversions of the chromosome 15 [
2]. Subsequently, Demura et al. performed detailed molecular studies for these cases and additional two cases and characterized four types of inversions affecting the 5′ region of
CYP19A1 [
3]. Each inversion has resulted in the formation of a chimeric gene consisting of
CYP19A1 coding exons and exon 1 of the widely expressed neighboring genes, that is,
CGNL1,
TMOD3,
MAPK6, and
TLN2. These data imply that overexpression of
CYP19A1 in the inversion-positive cases are caused by cryptic usage of constitutively active promoters. Consistent with this,
in silico analysis revealed the presence of promoter-compatible sequences around exon 1 of
CGN1,
TMOD3, and
MAPK6 in multiple cell types, although such sequences remain to be identified for noncoding exons of
TLN2 [
4].
We recently studied 18 males from six families with AEXS (families A–F) and identified three types of heterozygous cryptic genomic rearrangements in the upstream region of the
CYP19A1 coding exons () [
4]. In families A and B, we identified the same 79,156
bp tandem duplication encompassing seven of the 11 noncoding exons 1 of
CYP19A1. Notably, this duplication includes exon I.4 that functions as a major promoter for extragonadal tissues such as fat and skin; therefore,
CYP19A1 overexpression in these families would be explained by increasing the number of this promoter. Indeed, RT-PCR analysis detected a splice variant consisting of exon I.4 at the 5′ side and exon I.8 at the 3′ side in lymphoblastoid cell lines and skin fibroblasts of the patients, indicating that the duplicated exon I.4 at the distal nonphysiological position actually functions as transcription start sites. In family C, we identified a 211,631
bp deletion affecting exons 2–43 of
DMXL2 and exons 5–10 of
GLDN. This deletion appears to have caused
CYP19A1 overexpression because of cryptic usage of
DMXL2 exon 1 as an extra transcription start site for
CYP19A1. Indeed, RT-PCR revealed the presence of chimeric mRNA clones consisting of
DMXL2 exon 1 and
CYP19A1 exon 2, supporting the notion that aberrant splicing has occurred between these two exons. Such
DMXL2/
CYP19A1 chimeric mRNA accounted for 2–5% of
CYP19A1-containing transcripts from skin fibroblasts. In families D–F, we identified an identical 165,901
bp deletion including exons 2–43 of
DMXL2. RT-PCR identified the same chimeric mRNA as that detected in family C.
Collectively, three types of genomic rearrangements on 15q21 have been identified in AEXS to date, namely, inversion type (four subtypes), duplication type, and deletion type (two subtypes) () [
2–
4]. In this regard, sequence analyses for the breakpoints have indicated that (1) inversion types are formed by a repeat sequence-mediated nonallelic intrachromosomal or interchromosomal recombination or by a replication-based mechanism of fork stalling and template switching (FoSTeS) that occurs in the absence of repeat sequences and is often associated with microhomology [
16], (2) duplication type is generated by FoSTeS, and (3) deletions are produced by nonhomologous end joining that takes place between nonhomologous sequences and is frequently accompanied by an insertion of a short segment at the fusion point or by a nonallelic recombination [
16]. Thus, it appears that genomic sequence around
CYP19A1 harbors particular motifs that are vulnerable to replication- and recombination-mediated errors. The results provide novel mechanisms of gain-of-function mutations leading to human diseases.
Pituitary-
