CMT4J: Charcot–Marie–Tooth disorder caused by mutations in FIG4 PRESENTED BY SAIRA FATIMA ROLL NO: 31 MSc 4 [2018-2020] Department of MicroBiology & Molecular Genetics University of the Punjab Lahore, Pakistan

1. CMT4J: Charcot–Marie–
Tooth disorder caused by
mutations in FIG4
PRESENTED BY SAIRA FATIMA
ROLL NO: 31
MSC 4 [2018-2020]
Department of MicroBiology & Molecular Genetics
University of the Punjab
Lahore, Pakistan

2. CMT4J: Charcot–
Marie–Tooth
disorder caused
by mutations in
FIG4
i. CHARCOT–MARIE–TOOTH (CMT)
DISORDER IS A MONOGENETIC
DISORDER OF HETEROGENEOUS
ETIOLOGY WITH A PREVALENCE OF
ONE IN 2500.
ii. IT IS CHARACTERIZED BY
PROGRESSIVE DISTAL MUSCLE
WEAKNESS AND ATROPHY AND LEADS
TO SENSORY DEFICITS OF THE
EXTREMITIES (1, 2).
iii. MUTATION OF FIG4 CAUSES
NEURODEGENERATION IN THE PALE
TREMOR MOUSE AND PATIENTS WITH
CMT4J.

3. CMT can be classified into four main types:
• CMT1 is a primary demyelinating form.
• CMT2 is caused by primary axonal neuropathy.
• CMT3 is a severe form of Dejerine – Sottas disease or
congenital hypo-myelinating neuropathy.
• CMT4 is characterized by a less common autosomal
recessive demyelinating neuropathy.

4. In this article from Chow et al., they identify a novel form of CMT4, with
causative mutations in the FIG4 gene in patients.
• This investigation was spurred on by a novel spontaneous mouse
mutant, the ‘pale tremor’ mouse.
• The pale tremor phenotype consists of diluted pigmentation, severe
tremors starting early in life, and abnormal gait.
• The inheritance pattern was autosomal recessive.
• Initially using microsatellite markers to identify candidate genes,
reverse transcriptase-polymerase chain reaction (PCR) from brain
ribonucleic acid was performed and transcripts from the FIG4 gene
were shown to be missing exons 19–23.
• Genomic PCR for this region indicated that it was a transposon
insertion, not a genomic deletion, which resulted in the abnormal
transcript.

5. Neuropathy
• In pale tremor mice revealed phenotypes similar to CMT, including large vacuoles within
neurons indicative of late-stage endosomal accumulation, as well as slowed nerve
conduction velocity (NCV) in peripheral nerves and a complete lack of response from tail
sensory fibers.
• The vacuoles are observed in the trigeminal ganglia by post-natal day 1 (P1), the dorsal
root ganglia by P7, and the spinal motor neurons by 6 weeks of age.
• The large vacuoles within neurons preceded neuronal loss. Within the brain, neuropathy
is observed as early as P7 in the thalamus, pons, medulla, cortical layers V and VI, and
the olfactory bulb – many areas of which are involved in motor control, emphasizing
similarity to CMT.
• The diluted pigmentation phenotype of the mice, which is not observed in human
patients, was shown to be because of altered biosynthesis of melanosomes and
clumping of melanosomes in hair shafts, again implicating dysfunction in late-
endosome trafficking.

6. Mouse phenotype similarity to CMT
 The mouse phenotype similarity to CMT prompted the authors to study
individuals with apparent autosomal recessive CMT but with no known
mutations.
 Four patients were found with an I41T missense mutation in FIG4 (0/590
control chromosomes, 4/190 CMT-patient chromosomes; p ¼ 0.003) in
combination with either a frame-shift truncation mutation or a
nonsense mutation (R183X).
 The patients with the compound frame-shift mutations had very low
NCVs of 2–7 m/s vs a normal velocity of 40–50 m/s and had developed
CMT as early as 5 years of age.
 This phenotype of early disease onset and drastically low NCVs are
common pathologies of demyelinating CMT4.

7. FIG4’s role in CMT comes from
analysis of the I41T mutation in yeast
FIG4 has two functions in yeast:
1. cleavage of phosphatidylinositol-triphosphate (PIP3) to phosphatidylinositol-(3,5)-bisphosphate [PI(3,5)P2]
2. activation of the Fab1/ PIKfyve kinase
 When yeast containing the equivalent mutation (I59T) of the I41T mutation found in patients were placed under
hyperosmotic shock conditions, an increase in large vacuoles as well as decreased PI(3,5)P2 levels were observed,
demonstrating that this loss-of-function mutation does cause defects in the conserved biochemical and cellular
roles of FIG4 (Fig. 3) on next slide.
 Importantly, this yeast assay also provides an easy and efficient bioassay for testing putative mutations in FIG4.

8. Fig. 3. The phenotypes for FIG4 mutations in yeast, mice and humans are indicated. Although
not yet tested, it is likely that humans will also show altered phosphatidyl-inositol-(3,5)-
bisphosphate [PI(3,5)P2] levels and increased vacuole size in neuronal tissue. The diluted
pigmentation and spleen cell loss phenotypes (in gray text) seem to be mouse specific at this
point.

9.  Perhaps overexpression studies of FIG4 may find that it can be neuroprotective
because it is likely that a neuronal stressor may also be involved in the etiology of
CMT4J.
 Indeed, the lack of phenotype in I59T yeast under normal conditions as well as other
studies on the control of PI(3,5)P2 levels and vacuole size in yeast by FIG4 hint at the
involvement of neuronal stressors. This could explain the differential neuronal
sensitivity to loss of this protein observed by Chow et al.
 Insights into phosphoinositide signaling will lead to a better understanding of CMT
and perhaps lead to a therapeutic to stop the disease progression seen in CMT4J.
 CMT4 classification does imply that phosphoinositide signaling is important for
interactions with Schwann cells and oligodendrocytes in maintaining myelin.

10. Thank You