Neurofibromatosis
This
relatively common disorder affects about 1 in 3000 people and is inherited as
an autosomal dominant trait. There are two main types: von Recklinghausen’s
neurofibromatosis (NF1; which accounts for 85% of all
cases) and bilateral acoustic neurofibromatosis (NF2); these
are phenotypically and genetically distinct.
The
NF1
gene has been localized to chromosome 17q11.1. It is unusually large (300 kb)
and many dif-ferent mutations within it have now been identified. The NF1
gene is a tumour suppressor gene, the product of which, neurofibromin,
interacts with the product of the RAS proto-oncogene. This may explain
the sus-ceptibility of NF1 patients to a variety of tumours.
The inheritance of NF1 is as an autosomal dominant trait
but about one-half of index cases have no preced-ing family history.
The
inheritance of NF2
is also autosomal domin-ant. Mapping to chromosome 22q12.2 followed the
observation of changes in chromosome 22 in menin-giomas as these tumours may be
seen in NF2.
This gene also normally functions as a tumour-suppressor gene, the product of
which is known as schwannomin.
The physical signs include the following
•
Six or more café au
lait patches (light brown oval macules; Fig. 21.1), usually
developing in the first year of life.
•
Axillary freckling (Fig. 21.2) in
two-thirds of affected individuals.
•
Variable numbers of skin
neurofibromas, some small and superficial, others larger and deeper, ranging
from flesh-coloured to pink, purple or brown (Fig. 21.1). Most are dome-like
nodules, but others are irregular raised plaques. Some are firm, some soft and
com-pressible through a deficient dermis (‘button-hole’ sign); others feel
‘knotty’ or ‘wormy’. Neurofibromas may not appear until puberty and become
larger and more numerous with age.
•
Small circular pigmented hamartomas
of the iris (Lisch nodules; Fig. 21.3), appear in early childhood.
Nearly all NF1 patients meet the criteria for dia-gnosis by the age of 8 years, and all do so by 20 years. The usual order of appearance of the clinical features is café au lait macules, axillary freckling, Lisch nodules and neurofibromas.
•
Bilateral acoustic neuromas.
•
Few, if any, cutaneous
manifestations.
•
No Lisch nodules.
The
café au
lait marks, axillary freckling and Lisch nodules should be
looked for, as they appear before the skin neurofibromas. A segmental form of
NF1 is caused by a postzygotic mutation. Isolated neurofi-bromas are not
uncommon in individuals without neurofibromatosis and are of little consequence
unless they are painful.
A
neurofibroma will occasionally change into a neuro-fibrosarcoma. Other
associated features may include kyphoscoliosis, mental deficiency, epilepsy,
renal artery stenosis and an association with phaeochromocy-toma. Forme fruste
variants occur, e.g. segmental neurofibromatosis.
Other
tumours of the central nervous system may occur, especially meningiomas and
gliomas.
Ugly
or painful lesions, and any suspected of undergo-ing malignant change, should
be removed. The chance of a child of an affected adult developing the disorder
is 1 in 2aparents should be advised about this.
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