Neurofibromatosis type II


Neurofibromatosis type II is a genetic condition which may be inherited or may arise spontaneously. The main manifestation of the condition is the development of bilateral benign brain tumors in the nerve sheath of the cranial nerve VIII, which is the "auditory-vestibular nerve" that transmits sensory information from the inner ear to the brain. Besides, other benign brain and spinal tumors occur. Symptoms depend on the presence, localisation and growth of the tumor. Many people with this condition also experience visual problems. Neurofibromatosis type II is caused by mutations of the "Merlin" gene, which seems to influence the form and movement of cells. The principal treatments consist of neurosurgical removal of the tumors and surgical treatment of the eye lesions. Historically the underlying disorder has not had any therapy due to the cell function caused by the genetic mutation.

Classification

NF2 is an inheritable disorder with an autosomal dominant mode of transmission.
There are two forms of the NF2:
NF2 is caused by inactivating mutations in the NF2 gene located at 22q12.2 of chromosome 22, type of mutations vary and include protein-truncating alterations, splice-site mutations, missense mutations and others. Deletions, too, in the NH2-terminal domain of merlin proteins have been associated with early tumor onset and poor prognosis in people with NF2. Protein truncating mutations correlate with more severe phenotype. There is a broad clinical spectrum known, but all people with the condition who have been checked have been found to have some mutation of the same gene on chromosome 22. Through statistics, it is suspected that one-half of cases are inherited, and one-half are the result of new, de novo mutations.

Pathogenesis

NF2 is caused by a defect in the gene that normally gives rise to a product called Merlin or Schwannomin, located on chromosome 22 band q11-13.1. Merlin was first discovered as a structural protein functioning as an actin cytoskeleton regulator. Later merlin's tumour suppressant role was described. Merlin regulates multiple proliferative signalling cascades such as receptor tyrosine kinase signalling, p21-activated kinase signalling, Ras signalling, MEK-ERK cascade, MST-YAP cascade. In a normal cell, the concentrations of active merlin are controlled by processes such as cell adhesion. It has been shown that Merlin inhibits Rac1 which is crucial for cell motility and tumour invasion. Also, merlin's interaction with cyclin D was described. It is known that Merlin's deficiency can result in unmediated progression through the cell cycle due to the lack of contact-mediated tumour suppression, mainly because of the cell:cell junction disruption, sufficient to result in the tumors characteristic of Neurofibromatosis type II. Recent studies showed that besides its cytoskeletal and cytoplasmic functions Merlin also translocates to the nucleus and suppresses proliferation by inhibiting E3 ubiquitin ligase CRL4. Finally, most recent studies indicated that Merlin also plays important role in energy metabolism regulation. Mutations of NF2 is presumed to result in either a failure to synthesize Merlin or the production of a defective peptide that lacks the normal tumor-suppressive effect. The Schwannomin-peptide consists of 595 amino acids. Comparison of Schwannomin with other proteins shows similarities to proteins that connect the cytoskeleton to the cell membrane. Mutations in the Schwannomin-gene are thought to alter the movement and shape of affected cells with loss of contact inhibition.
Ependymomas are tumors arising from the ependyma, an epithelium-like tissue of the central nervous system. In people with NF2 and ependymomas, the tumor suppressant function of Merlin may be compromised. Loss of function mutations occurring in chromosome 22q, where Merlin proteins are coded, can promote tumorigenesis, or the creation of new tumorous cells. Deletions, too, in the NH2-terminal domain of merlin proteins have been associated with early tumor onset and poor prognosis in affected people.

Pathology

The so-called acoustic neuroma of NF2 is in fact a schwannoma of the nervus vestibularis, or vestibular schwannoma. The misnomer of acoustic neuroma is still often used. The vestibular schwannomas grow slowly at the inner entrance of the internal auditory meatus. They derive from the nerve sheaths of the upper part of the nervus vestibularis in the region between the central and peripheral myelin within the area of the porus acousticus, 1 cm from the brainstem.

Genotype-phenotype-correlation

Many people with NF2 were included in studies that were designed to compare disease type and progression with exact determination of the associated mutation. The goal of such comparisons of genotype and phenotype is to determine whether specific mutations cause respective combinations of symptoms. This would be extremely valuable for the prediction of disease progression and the planning of therapy starting at a young age. The results of such studies are the following:
These results suggest that other factors will probably determine the clinical outcome.

Diagnosis

Prenatal

Bilateral vestibular schwannomas are diagnostic of NF2.

Postnatal

Ferner et al. give three sets of diagnostic criteria for NF2:
  1. Bilateral vestibular schwannoma or family history of NF2 plus Unilateral VS or any two of: meningioma, glioma, neurofibroma, schwannoma, posterior subcapsular lenticular opacities
  2. Unilateral VS plus any two of meningioma, glioma, neurofibroma, schwannoma, posterior subcapsular lenticular opacities
  3. Two or more meningioma plus unilateral VS or any two of glioma, schwannoma and cataract.
Another set of diagnostic criteria is the following:
The criteria have varied over time.
The last revision of the NF2 criteria was done by M.J. Smith in 2017. This included the consideration of a LZTR1 mutation instead of NF2 and excluded bilateral vestibular schwannomas that occur after 70 years of age.

Treatment

Surgery

There are several different surgical techniques for the removal of acoustic neuroma. The choice of approach is determined by size of the tumour, hearing capability, and general clinical condition of the person.
Larger tumors can be treated by either the translabyrinthine approach or the retrosigmoid approach, depending upon the experience of the surgical team. With large tumors, the chance of hearing preservation is small with any approach. When hearing is already poor, the translabyrinthine approach may be used for even small tumors. Small, lateralized tumours in people with NF2 with good hearing should have the middle fossa approach. When the location of the tumour is more medial a retrosigmoid approach may be better.
Auditory canal decompression is another surgical technique that can prolong usable hearing when a vestibular schwannoma has grown too large to remove without damage to the cochlear nerve. In the IAC decompression, a middle fossa approach is employed to expose the bony roof of the IAC without any attempt to remove the tumor. The bone overlying the acoustic nerve is removed, allowing the tumour to expand upward into the middle cranial fossa. In this way, pressure on the cochlear nerve is relieved, reducing the risk of further hearing loss from direct compression or obstruction of vascular supply to the nerve.
Radiosurgery is a conservative alternative to cranial base or other intracranial surgery. With conformal radiosurgical techniques, therapeutic radiation focused on the tumour, sparing exposure to surrounding normal tissues. Although radiosurgery can seldom completely destroy a tumor, it can often arrest its growth or reduce its size. While radiation is less immediately damaging than conventional surgery, it incurs a higher risk of subsequent malignant change in the irradiated tissues, and this risk is higher in NF2 than in sporadic lesions.

Medications

There are no prescription medicines currently indicated for reduction in tumor burden for NF2 patients, although in patient studies Bevacizumab has resulted in reduction in tumor growth rates and hearing improvements in some patients.

Hearing loss

Because hearing loss in those with NF2 almost always occurs after acquisition of verbal language skills, people with NF2 do not always integrate well into Deaf culture and are more likely to resort to auditory assistive technology.
One of these devices is the cochlear implant, which can sometimes restore a high level of auditory function even when natural hearing is totally lost. However, the amount of destruction to the cochlear nerve caused by the typical NF2 schwannoma often precludes the use of such an implant. In these cases, an auditory brainstem implant can restore some level of hearing, supplemented by lip reading.

Prevalence

Incidence of the condition is about 1 in 60,000.