It may come as a surprise that a condition marked by delayed puberty and loss of smell can be traced back to processes occurring in the womb. Kallmann syndrome, a rare genetic disorder, stems from disruptions in a critical development process called neuronal migration. This is a process in which nerve cells move precisely to their correct locations in the brain from the locations in which they were created. 

Understanding the connection between the two developmental stages offers valuable insight into not only the syndrome itself, but the broader relationship between early brain development and later hormonal function. 

Overview

Kallmann syndrome is a rare genetic disorder that affects both sexual development and sense of smell. This is because it is a type of hypogonadotropic hypogonadism, or in simpler terms, it is a condition that halts or delays sexual development due to a lack of the production of certain sex hormones. This leads to delayed or absent puberty, and in many cases, infertility.

At the root of Kallmann syndrome is a process called neuronal migration, the process by which neurons move to their correct spots in the brain during development. The genetic mutations seen in Kallmann syndrome tend to prevent this process, mainly for the hormone-regulating neurons in the brain, thus leading to the syndrome’s main symptoms. 

Understanding neuronal migration

After neurons are created in the growing foetus, they must move from their place of origin to their final destination. This process is called neuronal migration. Neuronal migration is essentially, as the name implies, the process of the movement of neurons. It is essential in the development of the human nervous system, as it is responsible for putting the neurons in the place they need to be to work properly as a whole system. In a metaphorical sense, this process is comparable to the movement of a series of different wires to form a complete circuit. There are many ways to form a functional circuit; however, only a specific configuration exists in which the kitchen light switch does not turn on the bathroom light. 

This process occurs during early development, when the foetus is still developing in the womb. ^1. It is also tightly regulated by genetic instructions. Hence, if something goes wrong during the stages of neural migration, it can have a major effect on the connections formed by the brain, leading to significant consequences later on in life.  

The genes associated with kallmann syndrome 

In about 30% of Kallmann syndrome cases, genetic mutations of specific known genes are inherited from the parents. The genes that are linked to Kallmann syndrome will be explored below. 

ANOS1

ANOS1 (formerly known as KAL1) is a gene responsible for making a protein called anosmin-1, which works like scaffolding in the movement of neurons. 

CHD7

CHD7 is a gene responsible for the making of the protein chromodomain helicase DNA binding protein 7, which is responsible for unravelling the DNA at certain points to facilitate the making of other proteins.

PROK2

PROK2 is a gene responsible for the making of the protein prokineticin 2, which is key in the migration of Gonadotropin Releasing Hormone (GnRH) synthesising neurons and neurons involved in the development of the sense of smell. 

PROKR2

PROKR2 is the gene that makes a protein called prokineticin receptor 2, which interacts with and is essential for the function of prokineticin 2. 

FGF8

FGF8 is a gene responsible for the making of the protein fibroblast growth factor 8, which has many different functions. These functions include signalling that is essential in the migration of Gonadotropin Releasing Hormone-synthesising neurons and olfactory neurons. 

FGFR1

FGFR1 is a gene responsible for the making of the protein fibroblast growth factor receptor 1. This receptor interacts with fibroblast growth factors. 

How neuronal migration defects cause kallmann syndrome

Mutations in the genes mentioned above lead to the disruption of the movement of specific neurons called GnRH and olfactory neurons (neurons responsible for the sense of smell).² 

The normal path of migration for the GnRH-synthesising neurons in the foetus begins in the olfactory placodes (a region behind the nose) and ends in the hypothalamus (an area in the brain that is responsible for maintaining stability). During the migration stage, olfactory neurons must move to the olfactory bulb, located at the bottom of the brain. 

Proper migration of these neurons leads to their proper connection. If migration is stopped by the absence of one or more proteins that are involved in the process, the result is their improper function. In the context of Kallmann syndrome, this includes issues with:

• The development of the olfactory (smell) system 
• The release of gonadotropins (Follicle-stimulating hormone and luteinising hormone), which are responsible for the release of sex hormones. This, in turn, causes:
  • Delays in puberty and sexual development
  • Low sex drive
  • Abnormalities in the development of genitalia
  • Infertility

Neurological and developmental effects of kallmann syndrome

Main effects

The main effects associated with Kallmann syndrome are hypogonadotropic hypogonadism (the production of little to no sex hormones along with a complete or reduced sense of smell. Kallmann syndrome symptoms may include:

• Small penis size
• Lack of testicular enlargement during puberty
• Absence of secondary sex characteristics (like facial or body hair or breasts)
• Delayed growth spurt
• Lack of sex drive

Some individuals may experience

Depending on the affected genes, some people with Kallmann syndrome can experience other symptoms. These are associated with improper migration of other sets of nerves in addition to the GnRH-synthesising and olfactory neurons. Other additional symptoms include:

• Facial abnormalities like a cleft lip
• Finger abnormalities like an especially short fourth finger
• Colour blindness  
• Bimanual synkinesis - Mirrored movement of the hands

Furthermore, those with a mutation in the CHD7 gene may experience a syndrome called CHARGE syndrome. This syndrome comes with additional symptoms like:

• Abnormal ear shape
• Heart defects
• Impaired hearing

It is important to note the variability of symptoms in those with Kallmann syndrome. This variability is a direct consequence of the severity of neuronal migration deficits. 

Diagnosis and treatment implications

Diagnosis of the syndrome begins with a physical examination and questions about family history, in relation to previous family members who have experienced the same symptoms. 

Following this, the doctor may perform additional tests like:

• Tests for hormone levels in the blood
• MRI scans of the brain
• Genetic tests to determine any mutations 

In terms of treatment, the doctor may prescribe hormone replacement therapy to induce puberty, like as hormone injections and patches. 

Summary

Kallmann syndrome is a complex condition rooted in the disrupted migration of neurons, which specifically affects the neurons responsible for regulating hormone secretion and the sense of smell. The genetic mutations involved in this process prevent key neurons from reaching their proper destination in the developing brain, leading to a vast range of symptoms. These include: delayed or absent puberty, complete or partial loss of smell, and, in some cases, other physical and developmental abnormalities. While the presentation of symptoms can vary greatly between individuals, diagnosis through hormone testing and imaging techniques, along with effective treatment with hormone replacement therapy, can significantly improve quality of life. Understanding the underlying cause of Kallmann syndrome not only helps those directly affected but also deepens our knowledge of how the brain develops in early life.