Neurofibromatosis is a disease characterized by the appearance of benign or malignant tumors that affect the central or peripheral nerves. Another manifestation that characterizes this disease is the appearance of spots on the skin due to overpigmentation.
It is a disease of a genetic nature. It arises as a result of mutations in the genes that code for neurofibromin and merlin. Both molecules act as tumor suppressors by negatively regulating certain oncogenes .
Oncogenes are genes included in the human genome that regulate cell growth and proliferation. If these genes become out of control, the cell cycle can be out of control, which implies that the cell in question acquires characteristics of a tumor cell.
Types of neurofibromatosis
We distinguish several types of neurofibromatosis, which can be differentiated depending on the gene that is mutated.
Neurofibromatosis type I
Also known as von Recklinghausen’s disease. It is the most frequent of all types of neurofibromatosis, occurring in 1 in 3,000 people . It gives rise to neurological, cutaneous and even, sometimes, bony or soft tissue manifestations.
In this type, the mutated gene is the one that codes for neurofibromin (more than 1000 possible mutations have been identified, but not all of them give rise to a dysfunctional gene). It is an autosomal dominant disease, that is, mutations take place in non-sexual chromosomes and only one mutation is required in one of the two
Neurofibromatosis type II
It is much less common than type I, occurring in approximately 1 in 35,000 people. It gives rise to bilateral acoustic neuromas, that is to say, tumors that appear in the nerves that connect the ear with the encephalon.
In this type the mutated gene is the one that codes for the merlin (up to 200 possible mutations have been identified, but again, not all of them give rise to a dysfunctional gene). The vast majority of people suffering from type II neurofibromatosis have inherited the defective gene from one of their parents.
The manifestations of this disease were previously attributed to type II neurofibromatosis instead of being considered as a third type of neurofibromatosis. In this case, the mutated gene is SMARCB1, located in the same chromosomal arm as the gene mutated in neurofibromatosis type II. In the past, it was thought that the gene involved was the same as in the previous case, but advances in the field of sequencing have made it possible to show the different genes involved.
Schwannomatosis is characterized by the development of neuromas in the peripheral nerves that, in most cases, are truly painful . Type II neurofibromatosis is very similar, but in this case characteristic acoustic neuromas do not develop. In addition, the clinical picture differs between one type and another.
Peripheral or central tumors
The tumors that arise can appear in the peripheral or central nerves , there are many types depending on the nerve in which they develop and the clinical manifestations that they imply.
Tumors in peripheral nerves
They are very common in neurofibromatosis type I. They are characterized by appearing anywhere along the path of the peripheral nerves, which are the ones that connect the set of organs and muscles of the body with the central nervous system.
These tumors develop from the nerve sheaths (myelin sheaths), the type of tissue that lines the neuronal axons, protecting them and facilitating the transmission of the nerve impulse. If we analyze them under the microscope we could observe that they are constituted by a mixture of Schwann cells, fibroblasts, neural cells and mast cells. There are different forms of tumors in peripheral nerves:
- Cutaneous neurofibroma It is soft and meaty.
- Subcutaneous neurofibroma It is usually hard and has a nodular appearance.
- Nodular plexiform neurofibroma. They can grow into the intervertebral foramen, giving rise to intraspinal tumor masses that can compress the spinal cord , causing great pain.
- Diffuse plexiform neurofibroma. These are subcutaneous nodules caused by an excessive or amorphous growth of the bones or of the underlying Schwann cells.
- Schwannomas . Tumors that derive from Schwann cells. Its malignancy is very rare.
Tumors in central nerves
In this case they arise in the nerves located in the central nervous system (encephalon and spinal cord). They also come in several forms:
- Optic gliomas. They are the result of a lack of control in the cycle of cells called astrocytes (type of nerve cell). Most of the time they are asymptomatic, but if they grow enough they can compress the optic nerve and cause blindness.
- Acoustic neurons They arise in the nerves that connect the auditory system with the encephalon. They can cause dizziness, ataxia and even deafness. They are characteristic of type II neurofibromatosis.
- Meningiomas. These tumors develop in the membranes that protect the brain, called meninges . They are usually asymptomatic, however, if the tumor grows enough it can lead to severe headaches.