RCPA Short course in Medical Genetics and Genetic Pathology
CLINICAL MANIFESTATIONS OF GENETIC MOSAICISM
Adjunct Prof Mac Gardner
Univ Otago & Northern Regional Genetics, NZ
DERIVATION OF THE WORD ‘MOSAICISM’
This word came from the ancient Greek mouseion meaning 'place holy to the Muses‘. This transmutated into the Latin musaeum meaning ‘decoration with small square stones‘, and the decorations may originally have been dedicated to the Muses.
In medieval Latin it was changed to musaicus/mosaicus and passed via Italian mosaico
and French mosaïque into English as mosaic. The starting position:
Mosaicism is common
Indeed, are we all mosaics?
Consider the mathematics:
A person ≈ 10,000,000,000,000 cells
This needed 9,999,999,999,999 mitoses
No QA system could get it right that many times!
MOSAICISM: 2 or more genetically different cell lineages in the constitution of one individual, derived from a single original zygote (one cell line usually normal)
Mendelian: Some cells/tissues carry a classic gene mutation
Chromosomal: 2 (or more) karyotypically different cell lines*:
Epigenetic: Some cells/tissues have a different imprinting state (cf. David Amor re somatic UPD11)
Immunogenetic: All that rearranging of immunogenes
CNVs: May be rather frequent
Mitochondrial: The whole question of heteroplasmy
* k
i
l id
Excluding:
Cancer:
Many/practically all cancers are chromosomally ‘mosaic’, but these arise post‐constitutionally in 46,XY and 46,XX people (a.k.a. normal men and women)
Example of highly complex cytogenetic aberrations in a case of high‐grade myxofibrosarcoma Willems SM, Debiec‐Rychter M, Szuhai K, Hogendoorn PC, Sciot R. Local recurrence of myxofibrosarcoma is associated with increase in tumour grade and cytogenetic aberrations, suggesting a multistep tumour progression model. Mod Pathol. 2006 Mar;19(3):407‐16.
Excluding:
Chromosomal loss with ageing: • The quite common finding of loss of an X or a Y chromosome in an occasional cell in an older female or male population (and more notably in centenarians) may reflect “normal” age‐related anaphase lag. Excluding:
X Chromosome inactivation in females: • Every 46,XX female is mosaic with respect to X chromosome activation
• An epigenetic form of mosaicism
• Each cell makes its own autonomous decision about which X – maternal or paternal – remains the active one, at a very early stage of embryonic existence
Excluding:
X Chromosome inactivation in females: • Every 46,XX female is mosaic with respect to X chromosome activation
• An epigenetic form of mosaicism
• Each cell makes its own autonomous decision about which X – maternal or paternal – remains the active one, at a very early stage of embryonic existence
Excluding:
Chimerism: The existence of 2 cell lines of different origins: (1) Fusion of 2 DZ twin embryos
(2) Iatrogenic engraftment
(3) Maternal residuum of fetal cells post partum, 46,XX//46,XY: A cause of maternal auto‐immune disease (systemic sclerosis/scleroderma)
Rak JM, et al. Male microchimerism and HLA compatibility in French women with sclerodema: a different profile in limited and diffuse subset. Rheumatology 2009 48(4):363‐6.
Excluding:
Chimerism: The existence of 2 cell lines of different origins: (1) Fusion of 2 DZ twin embryos
(2) Iatrogenic engraftment
(3) Maternal residuum of fetal cells post partum, 46,XX//46,XY: A cause of maternal auto‐immune disease (systemic sclerosis/scleroderma)
Rak JM, et al. Male microchimerism and HLA compatibility in French women with sclerodema: a different profile in limited and diffuse subset. Rheumatology 2009 48(4):363‐6.
MOSAICISM CAN BE:
• SOMATIC,
• GONADAL, or
Distribution
• SOMATIC‐GONADAL
and
• MENDELIAN, or
Genetic type
• CHROMOSOMAL
MENDELIAN SOMATIC MOSAICISM:
The genetic error arises • post‐zygotically, after initial differentiation of embryonic cell types
• in a cell destined to form a body lineage other than gonadal tissue.
Four examples:
• Neurofibromatosis type 1 (gene is neurofibromin)
• Neurofibromatosis type 2 (gene is NF2, a.k.a. merlin)
• Periventricular nodular heterotopia (one gene is • Atrial fibrillation (one gene may be GJA5)
FLNA)
NF1, a.k.a. VON RECKLINGHAUSEN’S DISEASE
Somatic mosaicism, also called ‘segmental’ mosaicism
Cutaneous signs confined to one region of the body
Tinschert S, Naumann I, Stegmann E, Buske A, Kaufmann D, Thiel G, Jenne DE.
Segmental neurofibromatosis is caused by somatic mutation of the neurofibromatosis
type 1 (NF1) gene. Eur J Hum Genet. 2000 8:455-9.
Tinschert S, Naumann I, Stegmann E, Buske A, Kaufmann D, Thiel G, Jenne DE.
Segmental neurofibromatosis is caused by somatic mutation of the neurofibromatosis
type 1 (NF1) gene. Eur J Hum Genet. 2000 8:455-9.
NF1, a.k.a. VON RECKLINGHAUSEN’S DISEASE
Somatic mosaicism, also called ‘segmental’ mosaicism
Cutaneous signs confined to one region of the body
Skin fibroblasts from café au lait macule Æ deletion of the NF1
gene
Fibroblasts from skin elsewhere, and blood, Æ normal
Tinschert S, Naumann I, Stegmann E, Buske A, Kaufmann D, Thiel G, Jenne DE.
Segmental neurofibromatosis is caused by somatic mutation of the neurofibromatosis
type 1 (NF1) gene. Eur J Hum Genet. 2000 8:455-9.
Mutation happened in this cell?
Mutation happened in this cell?
Cardiac tube
What he (and we) looked
like at 3 weeks of age:
Neurofibromatosis type 2 (NF2)
a.k.a. Bilateral Vestibular Schwannoma
• 29 y/o female, quite sudden onset lower limb weakness and loss of bladder control
• Hearing grossly intact • MRI showed spinal and intracranial tumours:
Post-op
Pre-op
Schwannoma
Tumour
Vestibular schwannoma
Question:
Will our children
get this? What if
we have another
child?
Ç
NF2
NF2 and the neural crest ...
and Schwann cells
What she (and we) looked
like at 3 weeks of age:
Neural tube
Æ
Cardiac tube
Neural crest cells Æ
At day 20 of life:
genetically perfect
Neural
tube
Neural crest cells Æ
At day 21 of life: one
(somatic) cell mutates
z
z
“First hit”
zz
“First hit”
zzzz
zz
z zzz
zzzzzzzz
Orange cells = progeny cells from “first hit”
Cell can cross the
midline
zzzz
zz
z zzz
zzzzzzzz
z z
Orange cells = progeny cells from “first hit”
zzzz
zz
z z
zzzzzzzz
z z
Orange cells = progeny cells from “first hit”
Purple cell = cell having “second hit”
zzzz
zz
z z
zzzzzzzz
z z
Orange cells = progeny cells from “first hit”
Purple, blue cells = cells having “second hits”
zzzz
zz
z z
zzzzzzzz
z z
Orange cells = progeny cells from “first hit”
Purple, blue, red = cells having “second hits”
zzzz
zz
z z
zzzzzzzz
z z
Orange cells = progeny cells from “first hit”
Purple, blue, red = cells having “second hits”, whose
progeny, being homozygous (or cpd het), can form a
schwannoma
Y 448 –1 g>a
X gln320stop
zzzz
zz
z z
zzzzzzzz
Y methylation
z z
(Hypothetical) first & second hits at molecular level
How this knowledge, along with a basic understanding of embryology, can inform genetic counselling .....
Neural crest cells Æ
zzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzz
Å Precursor yolk sac cells, which will ⇒
blood, gametocytes
BILATERAL
TUMOURS
Neural crest cells Æ
zzzzzzzzz
zzzzzzzzzzzz
zzzzzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzz
Å Precursor yolk sac cells, which will ⇒
gametocytes
BILATERAL
TUMOURS
zzzzzzzzz
zzzzzzzzzzzz
zzzzzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzzzzzzzz
zzz
Neural crest cells Æ
zzz
zzz
Blood cells
zzz
Å Precursor yolk sac cells, which will ⇒
gametocytes
UNILATERAL
TUMOURS
Neural crest cells Æ
zzz
zzz
Å Precursor yolk sac cells, which will ⇒
gametocytes
MINIMUM ESTIMATES OF MOSAICISM IN
SPORADIC (-ve fam hx) CASES OF NF2
Bilateral vestib schwannoma:
33% +
Unilat
60% +
“
“
:
But what fraction of the mosaicism includes the gonad?
Transmission risks to offspring for isolated cases of NF2, after negative mutation testing on peripheral blood
Age at dx Risk (cf. baseline risk = 50% in nonmosaic)
____________________________________
VESTIBULAR SCHWANNOMA IS
____________BILAT________UNILAT____
< 20 30% 11% 20–29 16% 8%
30–39 11% 6%
40+
9%
5%
____________________________________
Mosaicism in neurofibromatosis type 2: an update of risk based on uni/bilaterality of vestibular schwannoma at presentation and sensitive mutation analysis including multiple ligation‐dependent probe amplification. D Gareth R Evans, R T Ramsden, A Shenton, C Gokhale, N L Bowers, S M Huson, G Pichert, A Wallace J. Med. Genet. 2007;44;424‐428.
Neurofibromatosis type 2 (NF2): a clinical and molecular review. Evans DGR. Orphanet J Rare Dis. 2009 19:16.
Question:
Will our children
get this? What if
we have another
child?
Ç N.
Answer:
Probably, no
NF2
• Periventricular nodular heterotopia ‐‐ a cause of epilepsy in females
Might some, mutation‐negative on blood, be due to localised (periventricular neurone) mutation in FLNA (filamin‐A)?
• Atrial fibrillation
Some may be due to localised (cardiac myocyte) mutation in ‘gap junction protein’ gene GJA5 (connexin‐40)
Gollob MH et al. Somatic mutations in the connexin 40 gene (GJA5) in atrial fibrillation.
New Engl J Med 354: 2677-2688, 2006.
Recognition of somatic mosaicism: representative monogenic disorders
Immune dysfunction
Adenosine deaminase deficiency
Wiskott–Aldrich syndrome
Clotting disorders
Haemophilia A
Haemophilia B
Skeletal disorders
Achondroplasia
Osteogenesis imperfecta
Marfan syndrome
Pseudoachondroplasia
Muscle disorders
Duchenne muscular dystrophy
Congenital myotonic dystrophy
Chromosomal instability
Bloom syndrome
Fanconi anaemia
Tumour suppressor
Neurofibromatosis type I
Neurofibromatosis type II
Tuberous sclerosis
Skin disorders
Bullous ichthysiform erythroderma
Incontinentia pigmenti
Endocrine disorders
Androgen insensitivity
Nervous-system
Friedreich ataxia
Metabolic disorders
Tyrosinaemia type I
Lesch–Nyhan
Youssoufian & Pyeritz 2002
How much rare variation?
“The total somatic mutational load must be enormous. For example, the
intestinal epithelium contains approximately 106 independent stem cells,
each of which generates transient daughter cells every week or two. Thus,
the intestinal epithelium of a 60-y-old is expected to harbor >109
independent mutations. This implies that, not far beyond the age of
60y, nearly every genomic site is likely to have acquired a mutation in
at least one cell in this single organ.”
SOMATIC ‘REVERTANT’ MOSAICISM OF LATER ORIGIN
The genetic ‘correction’ arises • post‐zygotically
• well into embryonic/fetal development
• in a differentiated cell, destined to form just one body lineage
One example:
• Epidermolysis bullosa ‐‐ a blistering/dystrophic skin disorder
SOMATIC ‘MOSAICISM’ OF VERY EARLY ORIGIN:
MZ TWIN DISCORDANCE
The genetic error arises • post‐zygotically
• at or before the blastocyst/inner cell mass stage
• after splitting of embryo into 2
• in a cell destined to form all or almost all body lineages
Three examples:
• Dravet syndrome of severe epilepsy (gene is sodium channel SCN1A)
• NF1 (gene is neurofibromin)
Mendelian
• Trisomy 21 (may be ‘correction’, rather than an error)
Chromosomal
MZ
MZ
Twin with epilepsy →
← Unaffected twin
(Dravet syndrome)
SCN1A mutation in all
these tested tissues
Normal SCN1A gene
in all these tested
tissues
Timing of de novo mutagenesis‐‐a twin study of sodium‐channel mutations. Vadlamudi L, Dibbens LM, Lawrence KM, Iona X, McMahon JM, Murrell W, Mackay‐Sim A, Scheffer IE, Berkovic SF. N Engl J Med 2010 363:1335‐40.
ONE TWIN WITH DRAVET SYNDROME
Normal allele
mutates to an
SCN1A allele
Zygote is
normal at
SCN1A locus
MZ
Monozygotic twins discordant for neurofibromatosis type 1 due to a
postzygotic NF1 gene mutation.
Vogt J, Kohlhase J, Morlot S, Kluwe L, Mautner VF, Cooper DN, Kehrer-Sawatzki H.
Institute of Human Genetics, University of Ulm, Ulm, Germany.
“Whereas DNA sequence analysis indicated somatic mosaicism for
the NF1 nonsense mutation, c.4108C>T (p.Q1370X), in the affected
twin, this lesion was apparently absent in his unaffected brother. . . . .
.
We conclude that the twinning event, which would have taken place
within three days post-fertilization, must have preceded the
c.4108C>T mutation which is therefore predicted to have occurred
during the blastocyst stage, leading to somatic mosaicism with
normal cells lacking the mutation.”
ONE TWIN WITH NF1
Zygote is
normal at
NF1 locus
Twining occurs
by day 3-4
morula/blastocyst
MZ
Hatching
blastocyst →
Hatched
blastocyst →
Jansen RPS, Bowman MC, de Boer KA, Leigh DA, Lieberman DB, McArthur SJ. What next for preimplantation
genetic screening (PGS)? Experience with blastocyst biopsy and testing for aneuploidy. Hum Reprod 23: 14761478, 2008.
ONE TWIN WITH NF1
Normal allele
mutates to an
NF1 allele, in
one cell of day
4 or 5
blastocyst
Zygote is
normal at
NF1 locus
Twining occurs
by day 3-4
morula/blastocyst
MZ
ONE TWIN WITH TRISOMY 21
‘Third’ chromo
21 lost
Zygote is
trisomy 21
MZ
47,XX,+21
46,XX
SOMATIC MOSAICISM IN THE EARLY EMBRYO ~ CHROMOSOMAL
The genetic error arises • post‐zygotically, • in 1 or more cells in a preimplantation embryo
Four examples:
• Trisomy 21 mosaicism – a classic e.g. (As above, but didn’t twin)
• The concept of ‘chaotic mosaicism’ in the IVF embryo (but in vivo?)
• Fetal Trisomy 3
• ‘Hypomelanosis of Ito’ associated with mosaicism for a structural rearrangement
SOMATIC MOSAICISM IN THE EARLY EMBRYO ~ CHROMOSOMAL
The genetic error arises • post‐zygotically, • in 1 or more cells in a preimplantation embryo
Four examples:
• Trisomy 21 mosaicism – a classic e.g. (As above, but didn’t twin)
• The concept of ‘chaotic mosaicism’ in the IVF embryo (but in vivo?)
• Fetal Trisomy 3
• ‘Hypomelanosis of Ito’ associated with mosaicism for a structural rearrangement
PGD -- Preimplantation Genetic Diagnosis
‘Chaotic mosiaicism’
in a day-3 embryo
Studied blastomeres
Hypothetical derivative
lysed blastomere
3
1,18,X,Y
biopsied
blastomere
Courtesy: Lucille Voullaire
-
−3+5+13+14+18−19+20+22+X+Y
+1+3−18+X+Y
−1+3+18−X−Y
+3
−3
+3
−3
N
To the placenta: ‘confined placental mosaicism’
+3
−3
†
N
To the inner cell mass: true mosaicism of the embryo
+3
−3
†
N
SOMATIC MOSAICISM IN THE EARLY EMBRYO ~ CHROMOSOMAL
The genetic error arises • post‐zygotically, • in 1 or more cells in a preimplantation embryo
Four examples:
• Trisomy 21 mosaicism – a classic e.g. (As above, but didn’t twin)
• The concept of ‘chaotic mosaicism’ in the IVF embryo (but in vivo?)
• Fetal mosaic Trisomy 3 (majority of cells trisomic)
• ‘Hypomelanosis of Ito’ associated with mosaicism for a structural rearrangement
FETAL mosaic TRISOMY 3
FETAL mosaic TRISOMY 3
FETAL TRISOMY 3
SOMATIC MOSAICISM IN THE EARLY EMBRYO ~ CHROMOSOMAL
The genetic error arises • post‐zygotically, • in 1 or more cells in a preimplantation embryo
Four examples:
• Trisomy 21 mosaicism – a classic e.g. (As above, but didn’t twin)
• The concept of ‘chaotic mosaicism’ in the IVF embryo (but in vivo?)
• Fetal mosaic Trisomy 3 (majority of cells trisomic)
• ‘Hypomelanosis of Ito’ associated with mosaicism for a structural rearrangement
HYPOMELANOSIS OF ITO
Mosaicism for a duplication of terminal 3q:
3q26.3→3qter
SOMATIC‐GONADAL MOSAICISM ~ CHROMOSOMAL
The genetic error arises • post‐zygotically, • in a cell destined to form somatic and gonadal tissue.
One example:
• Parental low‐level mosaicism for a deletion of chromosome 1q
blood
GONADAL MOSAICISM
The genetic error arises • post‐zygotically, • in a mitotic (pre‐meiotic) gametocyte
• Consider the seminiferous tubule total length of 1 km . . . . 1,000/1,000,000,000 = 1 millionth of the tubule has this abnormality
Close to 1 km of tram
track in view
A small ($2) coin
comprises 1/50,000 of
the distance.
GONADAL MOSAICISM ~ CHROMOSOMAL
One example:
• Parental low‐level mosaicism for a duplication ‐‐ dup(21)
mos dup(21)
• Couple, 3 previous miscarriages, 2 previous failed IVF cycles
• IVF/PGD using FISH, 13 embryos, 2 transfers, 0 pregnancies
• Of 13 embryos, 4 showed apparent dup(21)(q22)
• Of 1002 sperm, 66 showed the same apparent dup(21)
Somprasit et al., RBM Online 9, 225,
Cell from embryo
Sperm cells
Reprod Biomed Online 9, 225, 2004
4
26
6% of the
track
between
here and the
tram
GONADAL MOSAICISM ~ MENDELIAN:
Two examples:
• Duchenne muscular dystrophy (gene is dystrophin)
• Achondroplasia (gene is FGFR3)
Gonadal mosaicism for Duchenne muscular dystrophy
(gene is dystrophin)
Melis MA, Cau M, Congiu R, Puddu R, Muntoni F, Cao A. Germinal mosaicism in a Duchenne muscular dystrophy family:
implications for genetic counselling. Clin Genet. 1993 43:247-9.
Gonadal mosaicism for achondroplasia (gene is FGFR3)
Peripheral
blood:
Sperm:
Natacci F, Baffico M, Cavallari U, Bedeschi MF, Mura I, Paffoni A, Setti PL, Baldi M, Lalatta F. Germline mosaicism in
achondroplasia detected in sperm DNA of the father of three affected sibs. Am J Med Genet 2008 146A:784-6.
PATHOGENIC MOSAICISM CAN BE:
• SOMATIC
e.g. Segmental NF1, Hypomelanosis of Ito
• overtly SOMATIC‐GONADAL
e.g. Father carries deleted no. 1 chromosome • GONADAL
e.g. grandfather carries Duchenne, father carries abn chromosome 21
END
Eugen von Hippel (1867 ‐ 1939), a German ophthalmologist, first described “angiomatosis retinae” in 1911.
Arvid Lindau (1892 – 1958), a Swedish pathologist, who described the link between the retinal, cerebellar and visceral components of a disease, which he called “angiomatosis of the central nervous system”. VHL ? Somatic mosaic
• 49 y/o man, onset giddiness, ataxia, vomiting Æ loss of consciousness
• MRI Æ cerebellar tumour
• Histo confirms haemangioblastoma
• Second small lesion in R cerebellar hemisphere
• Renal and pancreatic cysts
Cerebellar haemangioblastoma
P
• No retinal angiomas • MRI r/v Æ normal adrenal, inner ear anatomy
• Plasma metanephrine normal
VHL:
• Cerebellar/spinal hemangioblastoma 9 9
• Retinal angioma 8
• Pheochromocytoma (incl. extra‐adrenal) 8
• Renal cancer 8
• Renal cysts 9 9 9 9 9 9 9 9 9 9
• Pancreatic cysts 9 9
• Epididymal cysts 8
• Endolymphatic sac tumour 8
• +ve family hx 8
• But: on VHL gene sequencing no mutation; and
on MLPA, no large deletion/duplication
Note: gene testing has high (97%) detection rate
Why no mutation found? Possible explanations:
• A VHL‐like ‘phenocopy’ ? 8
• True VHL, mutation not able to be detected ? 8
• Somatic (or somatic‐gonadal) mosaicism ? 9
…… distinction important, in order to be able to advise son
Youssoufian & Pyeritz 2002
a | Asymmetric fibrous dysplasia (non-neoplastic developmental disease of osseous
tissue) of bones and irregular skin hyperpigmentation in a female with MCCUNE–
ALBRIGHT SYNDROME101. (Reproduced with permission from REF. 131 © (1993)
Oxford University Press).
Youssoufian & Pyeritz 2002
*
b | Regenerating liver nodules (asterisk) that have a reversion of an
inherited mutation in the fumarylacetoacetate hydrolase gene, which
causes HEREDITARY TYROSINAEMIA TYPE I.
(Reproduced with permission from REF. 22 © (1998) Elsevier Science
(USA)
Cytogenetic abnormalities and fragile-x syndrome in Autism Spectrum Disorder
Kavita S Reddy
Genzyme Genetics, Orange CA 92868, USA
author email corresponding author email
BMC Medical Genetics 2005, 6:3
J Med Genet. 1996 Apr;33(4):338-40.
Mosaicism for the fragile X syndrome full mutation and deletions within the CGG repeat of the FMR1 gene.
Milà M, Castellví-Bel S, Sánchez A, Lázaro C, Villa M, Estivill X.