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In this study, we present 3 cases of Down syndrome (DS) associated with disorders/differences of sex development (DSD) and review the literature on this topic. Case 1: 1-year-old child with male genitalia and DS phenotype, 47,XX,+21 karyotype and testicular DSD. Case 2: 11-month-old child with male genitalia and few DS dysmorphisms, 45,X/47,XY,+21 karyotype, and mixed gonadal dysgenesis. Case 3: 4-month-old child with female genitalia and DS phenotype, 47,XY,+21 karyotype and XY complete gonadal dysgenesis. In the literature, among 188 patients, 107 (57%) had Klinefelter syndrome and 61 (33%) Turner syndrome, 12 (6%) had mixed gonadal dysgenesis, 2 (1%) had partial androgen insensitivity, 2 (1%) ovotesticular DSD, and the others had congenital adrenal hyperplasia, XY partial gonadal dysgenesis, XY complete gonadal dysgenesis, and complete androgen insensitivity (1 case each). A typical DS phenotype was found in all individuals of the revision, with the exception of one case, but DSD features were not always reported. In conclusion, the association of DS with sex chromosome DSD is the most frequently observed, whereas associations with 46,XX and 46,XY DSD is extremely rare.

Historically, the terms partial (PGD) and mixed gonadal dysgenesis (MGD) have been used to describe incomplete testicular differentiation in individuals with 46,XY or 45,X/46,XY karyotypes, respectively. However, it is currently unclear to what extent clinical features actually differ between these individuals. The aim of this study was to compare clinical, laboratory, and histological findings in these 2 groups. Patients with testicular dysgenesis seen in our service between 1989 and 2013 were selected. Sixty-one patients met the inclusion criteria. Individuals with 46,XY and 45,X/46,XY karyotypes were compared regarding genital features, gonadal histology and function, growth, and associated conditions. Twenty-five had mosaicism with a 45,X cell line (MGD), while a 46,XY karyotype (PGD) was found in 36 cases belonging to 32 families. Mutations in NR5A1, WT1, and SRY genes associated with testicular dysgenesis were found in 12 families. There were no significant differences regarding parental consanguinity, degree of external androgenization, gonadal location, histology, and function, and associated conditions. However, in the MGD group, the presence of a uterus, lower birth weight and length, and short stature were more often observed. Therefore, the use of histological features to classify PDG and MGD should be abandoned and replaced by classification based on karyotype.

Denys-Drash syndrome (DDS) is characterized by nephropathy, genital abnormalities, and predisposition to Wilms' tumor. DDS patients usually present heterozygous de novo germline WT1 mutations. The WT1 gene comprises 10 exons encoding the N-terminal transactivation and the C-terminal DNA-binding regions. Two unrelated patients with genital ambiguity and Wilms' tumor were analyzed by sequencing of the WT1 gene, and 3 mutations in exon 1 were identified of which 2 are novel. Patient 1 carried a c.555delC mutation that causes a frameshift and a premature stop codon. Patient 2 carried both c.421A>C and c.424C>T aberrations that lead to the missense p.Lys141Gln and the nonsense p.Lys142* mutation, respectively. As both patients were heterozygous for the mutations, we tested their parents who did not carry any mutation. Therefore, the 3 WT1 mutations occurred de novo in both patients. Heterozygous mutations result in WT1 haploinsufficiency as they impair protein production. They are associated with a milder DDS phenotype as observed in the patients studied here.

Mutations in the NR5A1 gene, which encodes the steroidogenic factor 1 (SF1), are responsible for different phenotypes of disorders of sex development (DSD), such as bilateral anorchia and hypospadias. Furthermore, they can be associated with primary amenorrhea, premature ovarian failure, male infertility, adrenal tumors, and endometriosis. Direct sequencing of the 7 NR5A1 exons including ∼1,000 bp of the 5′-upstream and 3′-downstream regions and all intron-exon boundaries was performed in patients with DSD. Three different in silico tools were used to assess the consequences of a splice site mutation. As a result, 3 novel NR5A1 mutations were identified in 3 patients with 46,XY partial gonadal dysgenesis: p.Lys38* and p.Leu80Trpfs*8 lead to premature translation termination codons within the SF1 DNA-binding domain, and the intronic nucleotide substitution c.1138+1G>T at the intron 6 donor splice site is considered to modify correct splicing. We assume that the anomalous mRNA produced as a result of p.Lys38* and p.Leu80Trpfs*8 will be degraded by nonsense-mediated mRNA decay even before translation, leading to SF1 haploinsufficiency. The c.1138+1G>T mutation is expected to produce a truncated protein. Heterozygous SF1 loss-of-function mutations in these cases resulted in mild DSD manifestations, such as dysgenetic testes, spontaneous puberty, and preserved adrenal function.