FGFR2, FGF8, FGF10 and BMP7 as candidate genes for hypospadias

New horizons at the caudal embryos: coordinated urogenital/reproductive organ formation by growth factor signaling

The cloaca/urogenital sinus and its adjacent region differentiate into the urogenital/reproductive organs. Caudal regression syndrome (CRS; including mermaid syndrome), a type of severe cloacal malformation displays hindlimb fusion and urogenital organ defects, thus suggesting that such defects are caused by several morphogenetic alterations during early development. The attenuation of bone morphogenetic protein (Bmp) signaling at the posterior primitive streak of embryos leads to the caudal dysmorphogenesis including the cloaca and fusion of both hindlimbs. Genetic tissue lineage studies indicate the presence of coordinated organogenesis. Hedgehog (HH)-responding cells derived from peri-cloacal mesenchyme (PCM) contribute to the urogenital/reproductive organs. These findings indicate the existence of developmental programs for the coordinated organogenesis of urogenital/reproductive tissues based on growth factor function and crosstalk.

The spectrum of phenotypes associated with mutations in steroidogenic factor 1 (SF-1, NR5A1, Ad4BP) includes severe penoscrotal hypospadias in 46,XY males without adrenal insufficiency

OBJECTIVE

Hypospadias is a frequent congenital anomaly but in most cases an underlying cause is not found. Steroidogenic factor 1 (SF-1, NR5A1, Ad4BP) is a key regulator of human sex development and an increasing number of SF-1 (NR5A1) mutations are reported in 46,XY disorders of sex development (DSD). We hypothesized that NR5A1 mutations could be identified in boys with hypospadias.

DESIGN AND METHODS

Mutational analysis of NR5A1 in 60 individuals with varying degrees of hypospadias from the German DSD network.

RESULTS

Heterozygous NR5A1 mutations were found in three out of 60 cases. These three individuals represented the most severe end of the spectrum studied as they presented with penoscrotal hypospadias, variable androgenization of the phallus and undescended testes (three out of 20 cases (15%) with this phenotype). Testosterone was low in all three patients and inhibin B/anti-Müllerian hormone (AMH) were low in two patients. Two patients had a clear male gender assignment. Gender re-assignment to male occurred in the third case. Two patients harbored heterozygous nonsense mutations (p.Q107X/WT, p.E11X/WT). One patient had a heterozygous splice site mutation in intron 2 (c.103-3A/WT) predicted to disrupt the main DNA-binding motif. Functional studies of the nonsense mutants showed impaired transcriptional activation of an SF-1-responsive promoter (Cyp11a). To date, adrenal insufficiency has not occurred in any of the patients.

CONCLUSIONS

SF-1 (NR5A1) mutations should be considered in 46,XY individuals with severe (penoscrotal) hypospadias, especially if undescended testes, low testosterone, or low inhibin B/AMH levels are present. SF-1 mutations in milder forms of idiopathic hypospadias are unlikely to be common.

Clinical dilemmas and surgical treatment of penoscrotal, scrotal and perineal hypospadias

Hypospadia is the most common congenital malformation of the urinary tract. It is a malformation with the opening of the urethra proximally from the usual site. The meatal opening can be anywhere alone the shaft of the penis, or in more severe forms, within the scrotum, or in the perineum. Consequently the hypospadias can be distal, medial and proximal. The proximal ones can be penoscrotal (PS), scrotal (SC) and perineal (PE). The cause of hypospadias is largely unknown; however, current epidemiology and laboratory studies have shed new light into the etiology of hypospadias. With recent advancements in molecular biology, microarray technology, it appears that hypospadias is potentially related to disrupted gene expression. Currently, the only available treatment is surgery. The aim of this study was to present our results of the surgical correction of hypospadias and methods used to answer the clinical dilemmas about the gender. Authors have used two methods for a surgical resolution of the hypospadia - one-step operation suggested by Snodgrass and two-step operation, employing free graft suggested by Bracka. Clinical dilemmas regarding the gender were answered using cytogenetic assessment through lymphocyte cultivation method, suggested by Seabright. The cytogenetic assessment was carried in 23 patients with proximal hypospadia (penoscrotal, scrotal and perineal). Characteristic male cariotype (46, XY) was found in 22 patients. In one patient, with scrotal hypospadia, we found the characteristic female cariotype. This patient had testicles. The patient with female cariotype had a TDG gene that determines the differentiation of the testicles. Although surgery remains the only therapy for the treatment of the hypospadias, better understanding of the molecular and hormonal mechanisms behind the diseases may contribute to the prevention and the decrease in the incidence of the malformation. Cytogenetic testing in patients with unclear gender is important in planning further treatment.

Mutation analysis of FOXF2 in patients with disorders of sex development (DSD) in combination with cleft palate

In contrast to disorders of sexual differentiation caused by lack of androgen production or inhibited androgen action, defects affecting development of the bipotent genital anlagen have rarely been investigated in humans. We have previously documented that the transcription factor FOXF2 is highly expressed in human foreskin. Moreover, Foxf2 knockout mice present with cleft palate in combination with hypoplasia of the genital tubercle. We hypothesized that humans with disorders of sex development (DSD) in combination with cleft palate could have mutations in the FOXF2 gene. Eighteen children with DSD and cleft palate were identified in the Lübeck DSD database (about 1,500 entries). Genomic DNA sequence analysis of the FOXF2 gene was performed and compared with 10 normal female and 10 normal male controls, respectively. Two heterozygous DNA sequence variations were solely present in one single patient each but in none of the 20 normal controls: a duplication of GCC (c.97GCC[9]+[10]) resulting in an extra alanine within exon 1 and a 25*G>A substitution in the 3'-untranslated region. Two patients carried a c.262G>A sequence variation predicting for an Ala88Thr exchange which was also detected in 2 normal controls. Two silent mutations, c.1272C>T (Ser424Ser) and c.1284T>C (Tyr428Tyr), respectively, occurred in the coding region of exon 2, again in both patients and normal controls. In conclusion, the majority of the detected sequence alterations were polymorphisms without obvious functional relevance. However, it cannot be excluded that the 2 unique DNA sequence alterations could have affected FOXF2 on the mRNA or protein level thus contributing to the observed disturbances in genital and palate development.

Is hypospadias a genetic, endocrine or environmental disease, or still an unexplained malformation?

Hypospadias is one of the most frequent genital malformations in the male newborn and results from an abnormal penile and urethral development. This process requires a correct genetic programme, time- and space-adapted cellular differentiation, complex tissue interactions, and hormonal mediation through enzymatic activities and hormonal transduction signals. Any disturbance in these regulations may induce a defect in the virilization of the external genitalia and hypospadias. This malformation thus appears to be at the crossroads of various mechanisms implicating genetic and environmental factors. The genes of penile development (HOX, FGF, Shh) and testicular determination (WT1, SRY) and those regulating the synthesis [luteinizing hormone (LH) receptor] and action of androgen (5alpha reductase, androgen receptor) can cause hypospadias if altered. Several chromosomal abnormalities and malformative syndromes include hypospadias, from anterior to penoscrotal forms. More recently, CXorf6 and ATF3 have been reported to be involved. Besides these genomic and hormonal factors, multiple substances found in the environment can also potentially interfere with male genital development because of their similarity to hormones. The proportion of hypospadias cases for which an aetiology is detected varies with the authors but it nevertheless remains low, especially for less severe cases. An interaction between genetic background and environment is likely.

Gene expression analyses on embryonic external genitalia: identification of regulatory genes possibly involved in masculinization processes

Androgen plays a crucial role in initiating and maintaining the expression of male sexual characteristics in mammals. In humans and mice, any defects along the pathway of androgen functions result in congenital urogenital abnormalities. The genital tubercle (GT), an anlage of the external genitalia, differentiates into a penis in males and a clitoris in females. Although masculinization of the external genitalia is androgen-dependent, the molecular pathway of its potential downstream genes is largely unclear. To identify the genes involved in mouse GT masculinization, we performed gene expression analyses, such as real-time quantitative polymerase chain reaction and section in situ hybridization analysis. From our studies we have identified candidate genes, Cyp1b1, Fkbp51 and MafB as potential androgen targets during mouse GT masculinization.

Abnormal urethra formation in mouse models of split-hand/split-foot malformation type 1 and type 4

Urogenital birth defects are one of the common phenotypes observed in hereditary human disorders. In particular, limb malformations are often associated with urogenital developmental abnormalities, as the case for Hand-foot-genital syndrome displaying similar hypoplasia/agenesis of limbs and external genitalia. Split-hand/split-foot malformation (SHFM) is a syndromic limb disorder affecting the central rays of the autopod with median clefts of the hands and feet, missing central fingers and often fusion of the remaining ones. SHFM type 1 (SHFM1) is linked to genomic deletions or rearrangements, which includes the distal-less-related homeogenes DLX5 and DLX6 as well as DSS1. SHFM type 4 (SHFM4) is associated with mutations in p63, which encodes a p53-related transcription factor. To understand that SHFM is associated with urogenital birth defects, we performed gene expression analysis and gene knockout mouse model analyses. We show here that Dlx5, Dlx6, p63 and Bmp7, one of the p63 downstream candidate genes, are all expressed in the developing urethral plate (UP) and that targeted inactivation of these genes in the mouse results in UP defects leading to abnormal urethra formation. These results suggested that different set of transcription factors and growth factor genes play similar developmental functions during embryonic urethra formation. Human SHFM syndromes display multiple phenotypes with variations in addition to split hand foot limb phenotype. These results suggest that different genes associated with human SHFM could also be involved in the aetiogenesis of hypospadias pointing toward a common molecular origin of these congenital malformations.