Genetic Basis of Acoustic Neuroma

An acoustic neuroma is a type of benign tumor that develops inside the inner ear and affects the nerves responsible for hearing and balance. As occurs with other types of tumors, the formation of an acoustic neuroma depends on the uncontrolled production of a specific type of cell, in this case the Schwann cells. Schwann cells are cells that normally grow around nerves, creating a layered structure - similar to the layers of an onion - and their function is to protect and give support to the nerve cells. In patients affected by acoustic neuroma, the production of Schwann cells around the nerves of the inner ear is somehow continuous and uncontrolled. This abnormal production of Schwann cells generates “clusters” of cells of various sizes, forming the tumor itself.

Acoustic neuroma symptoms do not appear in patients while the tumor is initially being formed, because often there are no direct effects on the nerves themselves. After several years, however, when the size of the tumor has become more significant and the tumor has started to press against the nerves, symptoms become apparent.

Some of the symptoms of acoustic neuroma include:

• gradual loss of hearing

• tinnitus, or ringing in the ear

• dizziness

• headaches

• inability to maintain balance

• pain in the face

• speech impediments

It is often difficult to diagnose a patient with acoustic neuroma based solely on these symptoms, as they are very “generic” in nature. Since the effects of an acoustic neuroma are visible only when the tumor becomes large enough to press against a nerve or nerves, acoustic neuroma symptoms usually do not appear before the age of 30. A careful ear examination, a physical examination and MRI of the head can help to indicate the presence of acoustic neuroma.

If an acoustic neuroma is not diagnosed early, the tumor may continue growing and it could become so large that it will start exerting pressure on the facial nerves or even on the brain; it may in some cases even become life threatening. Moreover, when the tumor is too large, surgical removal becomes harder or impossible, because there will be a higher risk of permanently damaging important facial nerves. When removal is no longer an option, doctors may decide to intervene with radiosurgery, which is a technique involving the use of radiation to focus on the tumor to reduce its size and, consequently, its effects.

Patients may suffer from either unilateral or bilateral acoustic neuroma. Those individuals suffering from unilateral acoustic neuroma exhibit a tumor in only one ear and the formation of the tumor in these patients is quite often random and sporadic and not necessarily linked to a genetic cause, though it often is. In patients with bilateral acoustic neuroma, both ears are affected and more often than not, the tumor can be shown to have a genetic cause.

Genetic analyses performed on patients suffering from acoustic neuroma have shown that these individuals are affected by a genetic disorder called Neurofibromatosis Type 2, or NF2. This disorder is caused by the absence or the lack of function of a gene on chromosome 22. This gene is believed to function in the suppression of the growth and formation of Schwann cells. In other words, when the gene does not correctly perform its job, Schwann cells are continuously being produced; this continuous production of cells is the hallmark of a tumor. While bilateral acoustic neuroma is believed to be caused by the absence of the gene on chromosome 22, unilateral acoustic neuroma is believed to be caused by the loss of function of the gene, even though the gene is still present. The reason as to why this gene would be turned off is still unknown.

Acoustic neuroma, NF2, follows an autosomal dominant pattern of inheritance, which means that a child from a parent affected by NF2 will have a 50% chance of inheriting the faulty gene.