What is the difference between Jacobs (XYY) syndrome and Turner syndrome in a patient with 45,X/46,XY mosaicism?

Jacobs Syndrome versus Turner Syndrome

Jacobs syndrome (47,XYY) and Turner syndrome (45,X) are fundamentally different chromosomal conditions: Jacobs syndrome affects phenotypic males with an extra Y chromosome, while Turner syndrome affects phenotypic females with a missing or structurally abnormal X chromosome.

Jacobs Syndrome (47,XYY)

Chromosomal Pattern and Phenotype

• Jacobs syndrome is characterized by a 47,XYY karyotype where males have an extra Y chromosome, resulting in a phenotypically normal male appearance 1.
• This condition represents a rare chromosomal abnormality that can be associated with male hypogonadism, though it is classified among "uncommon causes" of primary hypogonadism 1.

Clinical Manifestations

• Males with 47,XYY typically present with normal male external genitalia and may go undiagnosed unless fertility issues or other concerns prompt karyotype analysis 1.
• Array-based testing may miss XYY syndrome if the wrong gender control is used, representing an important technical limitation of chromosomal microarray analysis 1.
• The condition is associated with potential fertility impairment, and karyotype testing should be recommended for males with primary infertility and azoospermia or sperm concentration <5 million sperm/mL when accompanied by elevated FSH or testicular atrophy 1.

Turner Syndrome (45,X)

Chromosomal Pattern and Phenotype

• Turner syndrome is characterized by complete or partial absence of one X chromosome (45,X karyotype) in phenotypic females, resulting in short stature, gonadal dysgenesis, and various somatic features 2.
• Mosaic forms exist, including 45,X/46,XX mosaicism, which may present with variable phenotypic severity 1.

Clinical Manifestations

• Classic features include short stature, ovarian dysgenesis leading to primary amenorrhea, and Turner stigmata such as webbed neck, low posterior hairline, and cardiac anomalies 2, 3.
• Females with Turner syndrome typically require puberty induction due to gonadal failure and are at risk for associated conditions including cardiac defects, renal anomalies, and hearing loss 4.
• Noninvasive prenatal screening (NIPS) for monosomy X has a positive predictive value of only 29.5%, lower than other sex chromosome aneuploidies, partly due to higher rates of placental mosaicism and maternal mosaicism for 45,X 1.

The Special Case: 45,X/46,XY Mosaicism

Clinical Spectrum

• 45,X/46,XY mosaicism represents a distinct entity that bridges the phenotypic spectrum between Turner syndrome and normal male development, accounting for 10-12% of Turner syndrome cases 2.
• This karyotype presents with highly variable phenotypes: approximately 60% present with ambiguous genitalia, 11-12% present as phenotypically normal males with bilateral descended testes, and some present as phenotypic females with Turner features 3, 5, 6.

Sex Assignment and Clinical Features

• Of patients with 45,X/46,XY mosaicism and genital anomalies, approximately 61% are raised male and 39% female, with management individualized based on anatomical assessment 5.
• Phenotypic males with 45,X/46,XY may present with short stature, Turner-like stigmata, and normal male external genitalia, often going unrecognized until evaluation for growth retardation 3.
• Females with 45,X/46,XY are significantly shorter than males with the same karyotype, and height Z-score decreases with age for both genders 4.

Gonadal Pathology and Tumor Risk

• The presence of Y chromosome material in 45,X/46,XY mosaicism confers significant gonadoblastoma risk, particularly in individuals with gonadal dysgenesis 1.
• Gonadoblastoma risk in individuals with 46,XY karyotype and gonadal dysgenesis exceeds 40%, with occurrences reported from infancy through young adulthood 1.
• Prophylactic gonadectomy is generally recommended for dysgenetic gonads in 45,X/46,XY individuals, particularly those raised female or with intra-abdominal gonads 2, 5, 4.
• Gonadal histology in 45,X/46,XY is highly variable: intra-abdominal gonads are typically dysgenetic testes or streak gonads, while palpable scrotal gonads may include histologically normal testes, dysgenetic testes, or streak gonads 5.

Fertility Considerations

• Males with 45,X/46,XY mosaicism may develop azoospermia, as documented in case reports of phenotypically normal males with this karyotype 3.
• All females with 45,X/46,XY typically require puberty induction, contrasting with the majority of males who undergo spontaneous puberty 4.

Surveillance Requirements

• Boys with 45,X/46,XY mosaicism require thorough clinical evaluation similar to girls with Turner syndrome, including cardiac and renal screening, as approximately 58% exhibit at least one Turner syndrome stigmata 3, 4.
• Growth hormone treatment in 45,X/46,XY mosaicism has shown variable results, with some studies reporting poor response 3, 4.
• Routine follow-up is essential for monitoring late-onset abnormalities including infertility and gonadal tumors in phenotypically normal males with this karyotype 3.

Key Diagnostic Pitfalls

• Conventional karyotyping remains superior to chromosomal microarray for detecting sex chromosome aneuploidies when rapid diagnosis is needed or when specific aneuploidies are suspected 1.
• Boys with unexplained short stature who are short for their families should undergo karyotype analysis, as 45,X/46,XY mosaicism may present solely with growth retardation in phenotypically normal males 3.
• The clinical spectrum of 45,X/46,XY does not conform to a set pattern, requiring individualized anatomical and histological assessment rather than assumptions based on karyotype alone 5, 6.