Kallmann syndrome is the most common form of genetically disease, with autosomal recessive, X-linked, and autosomal dominant forms of inheritance. Clinically, it is characterized by its
association with anosmia or hyposmia; 85% of the cases are autosomal
. The X-linked form (KAL1) is caused by mutations of the KAL1 gene at Xp22.3. This leads to failure of olfactory
axons and GnRH-expressing neurons to migrate from their common
origin in the olfactory placode to the brain. The KAL gene product
anosmin-1, an extracellular 95 kDa matrix glycoprotein, facilitates
neuronal growth and migration. The KAL gene is also expressed in
various parts of the brain, facial mesenchyme, and mesonephros and
metanephros, thus explaining some of the associated findings in
patients with Kallmann syndrome, such as synkinesia (mirror movements), hearing loss, midfacial defects, and
renal agenesis.
Some kindreds contain anosmic individuals with or without hypogonadism; others contain hypogonadal individuals who are anosmic. Cleft lip and palate, hypotelorism, median facial clefts, sensorineural
hearing loss, unilateral renal aplasia,
neurologic deficits, and other
findings occur in some affected patients. When Kallmann syndrome is
caused by terminal or interstitial deletions of the Xp22.3 region, it may
be associated with other contiguous gene syndromes, such as steroid
sulfatase deficiency,
chondrodysplasia punctata, X-linked ichthyosis,
or ocular albinism.
The autosomal dominant form of Kallmann syndrome (KAL2)
occurs in up to 10% of patients, and is caused by a loss of function
mutation in the
fibroblast growth factor receptor 1 (FGFR1) gene. Cleft
lip and palate are associated with KAL2 but not with KAL1. Oligodontia and hearing loss may occur with both KAL1 and KAL2.
A variety of other genes, including FGF8, PROK2/PROKR2, NELF,
CHD7 (responsible for CHARGE [coloboma of the eye, heart anomaly,
choanal atresia, retardation, and genital and ear anomalies] syndrome,
which includes
hypogonadism in its phenotype), HS6ST1, WDR11,
and SEMA3A, are associated with defects in neuronal migration that
can result in Kallmann syndrome, but in most patients the affected
gene remains undefined