Keywords
split hand/foot malformation - chromosomal microarray - microduplication
Introduction
Split hand/foot malformation (SHFM), also known as ectrodactyly, is characterized
by aplasia or hypoplasia of the phalanges, metacarpals, and metatarsals, leading to
a deep median cleft of the hand and/or foot. SHFM can occur as an isolated condition,
as part of a syndrome, or in association with other congenital anomalies.[1] Here, we report a likely pathogenic microduplication in the 10q24.32 region of chromosome
10 in a family with multiple affected members, including three fetuses.
Case Report
A non-consanguineous couple presented to the Genetics Department due to a history
of three previous pregnancies affected by SHFMs. The husband and several members of
his family, including his father, brother, and niece, also had similar deformities
([Fig. 1A]). None of the affected family members exhibited skin abnormalities, seizures, intellectual
disability, or visual/hearing deficits.
Fig. 1 (A) Pedigree of the family. (B, C) Ultrasound and fetal autopsy findings of monodactyly in hands with lobster deformity
in the foot in one of the fetuses. (D) Absent 1st and 2nd metacarpals, and hypoplastic 3rd metacarpal in both hands. Only
the index finger is present in the right hand, and the index finger with a hypoplastic
middle finger in the left hand, in the husband. (E) Absent middle three toes with hypoplasia and syndactyly of the middle three metatarsals
in both feet in the husband. (F) Chromosomal microarray revealed a duplication of chromosome 10 encompassing the
10q24.31q24.32 region in the husband and one of the fetuses.
All three fetuses displayed monodactyly of the hands and lobster claw deformities
of the feet. An autopsy was performed on the second fetus ([Fig. 1B, C]). The husband presented with bilateral absence of the first and second metacarpals
and hypoplastic third metacarpals. He had only the index finger on his right hand,
while his left hand exhibited an index finger and a hypoplastic middle finger ([Fig. 1D]). In the feet, the middle three toes were absent, with hypoplasia and syndactyly
of the corresponding metatarsals ([Fig. 1E]). These findings suggested an autosomal dominant, non-syndromic genetic etiology.
Whole-exome sequencing (WES) and chromosomal microarray analysis were performed first
on the husband, followed by chromosomal microarray analysis of the second fetus, whose
DNA had been preserved.
Results
WES of the husband did not identify any pathogenic variants. However, chromosomal
microarray analysis revealed a 481-kb likely pathogenic duplication on chromosome
10, encompassing the 10q24.31q24.32 region, which is associated with SHFM type 3 (SHFM3).
Similarly, chromosomal microarray analysis of the second fetus identified a 479-kb
duplication in the 10q24.32 region ([Fig. 1F]). This duplicated region includes several OMIM genes: LBX1, BTRC, POLL, DPCD, and FBXW4.
The couple was counseled that the phenotype is due to contiguous gene duplication
in the 10q24 region. They were informed that the recurrence risk for SHFM in each
pregnancy is 50%, consistent with autosomal dominant inheritance. Various reproductive
options, including advanced reproductive techniques, were discussed to help them make
informed decisions.
Discussion
SHFM is a rare condition with a prevalence of 1 to 5 per 100,000 live births, accounting
for approximately 15% of all limb reduction defects. While most cases are sporadic,
familial occurrences are relatively uncommon. SHFM is both clinically and genetically
heterogeneous, presenting in various forms: as an isolated anomaly, in association
with other malformations, or as part of a syndrome.[1]
[Table 1] provides an overview of the different types of SHFM, along with associated findings
and syndromes.[2]
Table 1
Different types of split hand foot malformation (SHFM)
|
Type
|
Chromosome/gene involved
|
Pattern of inheritance
|
Associated findings/syndromes
|
|
SHFM1
|
DLX5 gene
Deletion/duplication/rearrangement of the 7q21.3 region
|
Autosomal dominant
|
Ectrodactyly, ectodermal dysplasia, and cleft lip palate (EEC)
Sensorineural hearing loss
Intellectual disability
Triphalangeal thumb
|
|
SHFM1 with sensorineural hearing loss
|
DLX5 gene
|
Autosomal recessive
|
Sensorineural hearing loss
|
|
SHFM2
|
Xq26
|
X-linked
|
–
|
|
SHFM3
|
Duplication 10q24.31q24.32
|
Autosomal dominant
|
Triphalangeal/duplicated thumbs
Facial dysmorphism
Intellectual disability
Other congenital anomalies
|
|
SHFM4
|
TP63 gene
|
Autosomal dominant
|
Triphalangeal/duplicated thumbs
|
|
SHFM5
|
Deletion 2q31
|
Autosomal dominant
|
Microcephaly
Microphthalmia
|
|
SHFM6
|
WNT10B gene
|
Autosomal recessive
|
Sparse hair and interrupted eyebrows
|
|
SHFM7 with mesoaxial polydactyly
|
ZAK gene
|
Autosomal recessive
|
Hearing impairment
|
|
SHFM8
|
EPS15L1 gene
|
Autosomal recessive
|
–
|
|
SHFLD1
|
1q42.2q43
|
Autosomal dominant
|
Tibial aplasia/hypoplasia
|
|
SHFLD2
|
6q14.1
|
Autosomal dominant
|
|
SHFLD3
|
Duplication 17p13.3
Microduplications involving BHLHA9
|
Autosomal dominant
|
Abbreviation: SHFLD, split hand foot malformation with long bone deficiency.
The autosomal dominant mode of inheritance is the most common in SHFM, with the exception
of SHFM types 6, 7, and 8, which follow an autosomal recessive pattern, and SHFM type
2, which is X-linked. A total of 36 cases of SHFM have been reported from India; however,
only 7 cases each of SHFM1 and SHFM6, and 5 cases of SHFM3 have been molecularly confirmed.[3]
[4]
[5] SHFM exhibits high intrafamilial and inter-individual variability, as well as reduced
penetrance. Additionally, variability can manifest as differing patterns of anomalies
across the limbs of the same patient.[6] The clinical phenotype ranges from mild to severe, including hypoplasia of a single
phalanx, aplasia of one or more central digits, or even monodactyly.[5] These variations may be influenced by epigenetic and/or environmental factors. Due
to the marked clinical heterogeneity, establishing a specific diagnosis based solely
on clinical presentation remains challenging.
SHFM is caused by abnormalities in the Wnt-BMP-FGF signaling pathway, which plays
a crucial role in the development of the central portion of the apical ectodermal
ridge (AER). To date, mutations in CDH3, DLX5, EVX2, FGFR1/2, HOXD, MAP3K20, TP63, and WNT10B genes have been identified as being associated with SHFM. Additionally, microdeletions
in the 2q31 and 17q25 regions, as well as microduplications in the 10q24 and 17p13.3
regions, have been linked to the condition. In prenatal cases, deletions in the 2q21.33
region, a terminal deletion at 7q31, and a 22q11.2 deletion have been identified in
fetuses with SHFM accompanied by congenital anomalies.[7]
[8] Duplications at 17p13.3 encompassing the BHLHA9 gene are associated with SHFM with long bone deficiency. The BHLHA9 gene encodes a basic helix-loop-helix (bHLH) transcription factor that regulates
target genes critical for the proliferative zone and maintenance of AER. These duplications
are characterized by variable expressivity and a high degree of non-penetrance, particularly
in females, suggesting possible sex-influenced modifiers or epigenetic regulation
or environmental factors may modulate the phenotypic outcome.[9]
SHFM3 is one of the most common causes of SHFM and follows an autosomal dominant inheritance
pattern.[1] It is characterized by complete penetrance and variable expressivity. In SHFM3,
preaxial involvement of the upper extremities, such as triphalangeal thumb or preaxial
polydactyly, is frequently observed.[6]
[10] Complex polydactyly has been reported in one case and cutaneous syndactyly has also
been reported in a few cases.[5]
[11]
[12]
Duplications of the 10q24 region are the most common cause of SHFM, accounting for
approximately 20% of cases, followed by 17p13.3 duplications in 13% of cases.[1] The phenotype of SHFM3 is attributed to the involvement of several OMIM genes within
or near the duplicated region, including TLX1, LBX1, BTRC, POLL, FBXW4, DPCD, FBXW4, and FGF8. The size of duplication varies from 325 to 650 Kb. In most cases, the chromosomal
duplication encompasses an intergenic segment extending from a region centromeric
to LBX1 to a region telomeric to FBXW4.[12]
[13]
According to Dimitrov et al, duplication in the 10q24.31q24.32 region can result in
a syndromic form of SHFM that includes intellectual disability, seizures, hearing
loss, and congenital anomalies.[2]
[13] In addition, gonadal mosaicism was reported in one family. However, in our family,
no other abnormalities or intellectual disability were observed.
The exact reason why 10q24 duplications cause isolated SHFM3 in some patients while
being associated with additional anomalies and intellectual disability in others remains
unknown. Variability in intellectual disability may be attributed to triplication
and duplication of the 10q24.31q24.32 region, with triplication associated with an
increased risk[13]; however, the underlying cause of extra-skeletal abnormalities remains unexplained.
Notably, there is no correlation between the size of the 10q24 duplication and the
presence of additional anomalies.[14] The precise mechanisms underlying the phenotypic variability remain to be elucidated.
Among the genes, BTRC, FBXW4, SUFU, and FGF8 are expressed in the developing limbs. FGF8, a fibroblast growth factor expressed in the AER, is required for limb patterning.
Tandem duplications at chromosome 10q24 consistently include at least the FBXW4 gene.[15]
FBXW4 is involved in ubiquitin-mediated protein degradation and is considered the most
likely candidate gene.[13] Duplications typically encompass the BTRC and POLL genes but were initially thought to be insufficient to cause SHFM3. Recently, it
has been proposed that overexpression of BTRC (duplication of the first exon of the BTRC gene), which is involved in key signaling pathways such as the Wnt/β-catenin and
Sonic Hedgehog pathways, contributes to the phenotype.[12] In our case, the duplicated region included the genes LBX1, BTRC, POLL, DPCD, and FBXW4 ([Fig. 1F]).
As per Cova et al structural variants including inversions at the Lbx1/Fgf8 locus
disrupt chromatin architecture and result in misexpression of the FGF8 gene and activation of the LBX1 and BTRC genes, highlighting the complex regulatory mechanisms underlying SHFM3.[16] A microduplication of 120 Kb involving only the BTRC gene has been identified in a Chinese family.[17]
To date, no intragenic sequence variants have been reported in these genes.
The differential diagnosis of SHFM includes oligodactyly and digit amputation due
to amniotic band syndrome. While the diagnosis of SHFM can be made through ultrasound,
it cannot distinguish between non-syndromic and syndromic SHFM, nor can it determine
the specific type of SHFM or provide an accurate prognosis for the fetus.
Conclusion
Although ultrasound can detect SHFM, identifying the genetic etiology is crucial for
classifying the type of SHFM, especially in cases of sporadic occurrence. Some cases
are syndromic and may have additional features that are not detectable by ultrasound,
or may evolve later in gestation, or to identify mildly affected fetuses where the
anomaly is not clear on scan. In families with a history of non-syndromic SHFM and
an increased risk of recurrence, genetic testing becomes particularly useful for couples
considering reproductive options such as in vitro fertilization with preimplantation
genetic testing.
