Keywords
CPT2 gene - fetal renal anomalies - prenatal diagnosis - compound heterozygosity -
genetic counseling
Introduction
In today's era, it has become increasingly challenging for practicing obstetricians
to counsel regarding the prognosis of a fetus with structural abnormalities, detected
prenatally at the time of an anomaly scan. The finding of enlarged hyperechogenic
fetal kidneys during pregnancy is one such anomaly that, when identified early, requires
health care professionals to provide appropriate medical guidance and management for
the expectant parents.
If the renal parenchyma of the fetal kidneys is more echogenic than the fetal liver,
they are considered to be hyperechogenic.[1] It becomes more apparent with associated loss of corticomedullary differentiation.
As there are various etiologies associated with it, the outcomes also become variable.
Whether the etiology is obstructive, metabolic, genetic, or transient, it may be one
of the first indicators of an underlying disease, and further evaluation becomes imperative.
The incidence of hyperechogenic kidneys is reported as 1.6 per 1,000 scans,[2] with congenital abnormalities of the kidneys and the genitourinary tract (CAKUT)
representing 15 to 20% of all prenatally diagnosed congenital anomalies. Hence, evaluation
of hyperechogenic kidneys for an etiological diagnosis is important for appropriate
genetic counseling.
The differentials vary with findings related to kidney size, corticomedullary differentiation,
renal pelvis dilatation, presence of renal cysts, quantity of amniotic fluid and whether
it is isolated as in cases of autosomal dominant or autosomal recessive polycystic
kidney disease (ADPCKD/ARPCKD), or nonisolated as in overgrowth syndromes like Beckwith–Weidemann
syndrome or an aneuploidy. It may even be associated with fetal infections.[3]
[4] This makes both evaluation and counseling intensive.
Case
Our case study reports a 25-year-old primigravida who presented to us at 18 weeks
6 days at the Department of Fetal Medicine, Apollo Hospitals, Amritsar, Punjab, India,
for a detailed anomaly scan. She did not have any medical conditions or any specific
family history. They were a nonconsanguineous couple. Findings were suggestive of
bilateral hyperechogenic and enlarged kidneys ([Fig. 1A]). Associated anomalies were enlarged cerebral ventricles ([Fig. 1B]) with an enlarged cisterna magna and a hypoplastic cerebellum suggestive of Dandy–Walker
malformation ([Fig. 1C]). Invasive testing was offered; however, the pregnancy was discontinued as per the
couple's wish. Microarray (CMA 315 K) testing was performed on the products of conception
(POC). The report was normal and not suggestive of microdeletions or microduplications.
The patient came for follow-up in a subsequent pregnancy 1 year later. The nuchal
translucency scan was normal and low risk for aneuploidies. A follow-up early anomaly
scan was done at 16 weeks. Findings were suggestive of a subchorionic collection measuring
3.7 × 0.8 cm with echogenic bowel with slightly prominent looking kidneys (left kidney
measuring 10 mm in length, 8 mm in width, and 7 mm in anteroposterior diameter and
the right kidney measuring 11 mm in length, 8 mm in width, and 6 mm in anteroposterior
diameter) of normal echogenicity and no other associated gross structural anomalies.
Invasive testing was offered, but the couple wanted to wait for a follow-up detailed
anomaly scan at 20 weeks. This revealed bilateral enlarged hyperechogenic kidneys
([Fig. 2A, B]). Additional findings of prominent bilateral cerebral ventricles (left ventricle
measuring 8 mm and right ventricle measuring 9 mm) with absent cavum septum pellucidum
were present. The fetal head circumference was below the 3rd centile for gestation,
suggesting microcephaly. Bowel appeared echogenic. The family was counseled, and they
opted to discontinue the pregnancy. Trio whole-exome sequencing (WES) was performed
on the couple and the POC.
Fig. 1 (A) Bilateral hyperechogenic and enlarged kidneys. (B) Enlarged bilateral ventricles. (C) Enlarged cisterna magna and a hypoplastic cerebellum suggestive of Dandy–Walker
malformation.
Fig. 2 (A, B) Bilateral enlarged and hyperechogenic kidneys in coronal view and sagittal view.
Results of WES in the couple suggested a heterozygous c.28_29insAGCAAG (p.Trp10delinsTerGlnGly)
and c.1891C > T (p.Arg631Cys) variant in the CPT2 gene, present in both husband and
wife, which meant two variants of the CPT2 gene were present in the couple ([Table 1]).
Table 1
Segregation of CPT 2 variants in the couple and fetus with pathogenic classification
|
Gene
|
Location
|
Variant
|
Mother
|
Father
|
Fetus 1
|
Current fetus
|
Disease (inheritance)
|
Classification
|
|
CPT 2
|
Chr1:53662643T > TAGCAAG Exon 1NM_000098.3
|
c.28_29insAGCAAG(p.Trp10delinsTerGlnGly)
|
Heterozygous
|
Heterozygous
|
Homozygous
|
Heterozygous
|
CPT2 deficiency infantile/CPT2 deficiency lethal neonatal
|
Pathogenic
|
|
CPT 2
|
Chr1:53679181C > T Exon 5NM_000098.3
|
c.1891C > T(p.Arg631Cys)
|
Heterozygous
|
Heterozygous
|
Homozygous
|
Heterozygous
|
CPT2 deficiency infantile/CPT2 deficiency lethal neonatal
|
Pathogenic
|
Note: Couple and current fetus whole exome sequencing report suggesting heterozygous
c.28_29insAGCAAG (p.Trp10delinsTerGlnGly) and c.1891C > T (p.Arg631Cys) variant in
the CPT2 gene. Fetus 1 is homozygous for both variants.
Samples of the fetus (second pregnancy) showed a homozygous c.28_29insAGCAAG (p.Trp10delinsTer)
and a homozygous c.1891C > T (p.Arg631Cys) variant in the CPT2 gene, which meant four
variants of the CPT2 gene were present in the fetus ([Table 1]). No significant maternal cell contamination was detected on polymerase chain reaction-based
variable number tandem repeat analysis with a lower limit of detection of 10%.
A year later, in her third conception, knowing their genetic carrier status, clinical
exome sequencing was offered as an invasive antenatal diagnostic test. The couple
were counselled regarding the options of chorionic villous sampling at the time of
her nuchal translucency scan (which indicated a structurally normal-looking fetus),
for earlier diagnosis. However, they chose to wait until the early anomaly scan. The
ultrasound findings were normal, and pretest counseling was done before the amniocentesis
was performed.
The report by the genetic laboratory suggested once again an affected fetus with compound
heterozygosity. The fetus showed two variants in the CPT2 gene: c.28_29insAGCAAG (p.Trp10delinsTerGlnGly)
and c.1891C > T (p.Arg631Cys) variant. This was interpreted by the laboratory as a
compound heterozygote for the CPT2 gene and was reported as “affected.”
Compound heterozygosity suggests that the mutations occur on different copies of the
gene, in turn completely knocking out the gene function. However, this was completely
unexpected as in the current (third) pregnancy, the ultrasound was normal with no
renal or cranial anomalies as seen in her previous pregnancy. A consultation of the
couple was scheduled with the clinical geneticist and the genetic reports and data
were reanalysed.
A reanalysis of the genetic data by the clinical geneticist was done. As the parental
segregation was already known from the trio exome sequencing done posttermination
of the second pregnancy, the phase of the two variants was already identified in the
CPT2 gene. The couple was explained that both the variants were present on one allele
(cis form) in the parents, making them carriers and not affected by carnitine palmitoyltransferase
deficiency ([Fig. 3]). Similarly, in the fetus, both variants were present on the same allele as the
parents ([Table 1]). A neurosonogram was subsequently done for the fetus in the current pregnancy (third)
to reconfirm that there were no cranial anomalies in the fetus.
Fig. 3 Both the variants were present on one allele (cis form) in the parents, hence they
were carriers and not affected.
It was thus proclaimed that the fetus, too, was a carrier like the parents, unaffected.
The couple was reassured on these lines, and the third pregnancy was continued to
term to deliver a healthy baby girl weighing 2,600 g.
Discussion
Hyperechogenic kidneys are associated with renal disease in up to 94% of cases; however,
in isolated cases with normal amniotic fluid, it may represent a normal variant. In
our case, associated findings were noted in both affected pregnancies. Though the
microarray was normal in the first pregnancy, WES of the POC in the second pregnancy
was suggestive of homozygous variants of both pathogenic and likely pathogenic mutations
of the CPT2 gene, similar to the WES of parents (who were of carrier status) also
tested at the same time.
CPT2 (carnitine palmitoyltransferase) muscle deficiency is the most common form of
muscle fatty acid metabolism disorders.[5] Carnitine palmitoyltransferase 2 deficiency due to mutations in the CPT2 gene is
often diagnosed in the postnatal period after more than one terminated pregnancy for
multiple malformations and in successive stillbirth or perinatal loss cases.[6] Carnitine palmitoyltransferase 2 (CPT2) deficiency, the most common autosomal recessive
inherited disease of the mitochondrial long-chain fatty acid β-oxidation, may result
in three distinct clinical phenotypes[7]
[8]: a mild adult muscular form which is the most frequent one and is characterized
by recurrent episodes of rhabdomyolysis triggered by prolonged exercise, fasting,
or febrile illness.[9] A severe infantile hepatocardiomuscular disease[10] which is associated with hypoglycemia, liver failure, cardiomyopathy, and peripheral
myopathy and finally, the neonatal form of the disease[11] including dysmorphic features, cystic renal dysplasia, and neuronal migration defects,
in addition to symptoms observed in the infantile form. Both the latter forms are
life-threatening diseases, and prenatal diagnosis should be offered to couples for
a one-fourth risk of having an affected child[12] as these disorders are inherited in an autosomal recessive manner, with a recurrence
risk of 25%.
Rarely, presentation is antenatal with cerebral periventricular cysts and cystic dysplastic
kidneys.[13]
Diagnosis is usually achieved using either ultrasound detection of renal and brain
damage[14] or direct molecular analysis when the disease-causing mutation has been identified
in the index patient.[15]
Likewise, in our case, the suspicion of a genetic etiology arose in her second pregnancy
when recurrent cranial and renal abnormalities were noted on her scan, and follow-up
molecular testing was diagnostic for the mutation in the CPT2 gene.
Genetic counseling is extremely important to evaluate, manage, and prognosticate such
cases. Carrier status clarification, along with a detailed family history, helps assess
and identify possible patterns that might indicate a higher risk, rather than just
a 25% risk, as seen in autosomal recessive disorders.
Literature also states that despite a recessive mode of inheritance, heterozygote
carriers could not only be regarded as asymptomatic carriers but may occasionally
show mild symptoms later on in adult life.[16] These symptoms may present as recurrent attacks of fatigue, muscle pain, and muscle
weakness triggered by prolonged exercise.[17] Some may have only a few episodes of these attacks and remain asymptomatic for most
of their lives,[18] while others may experience frequent myalgia even after routine daily activities.[19] In our case, however, the couple experienced no such symptoms.
Therefore, genetic counselling not only offers insight into the above-mentioned issues
but also emphasizes the pertinence of prenatal genetic molecular testing. Posttest
counseling thereafter is not only technically informative but also an emotional support
for managing anxiety and offering coping strategies to the couple, whether it involves
dealing with a normal report or managing an affected child.
This case sets a perfect example of how the correct interpretation of reports must
be prenatal and documented prior to fetal testing in subsequent pregnancies, so that
the couple can be reassured to continue the pregnancy and to finally deliver a healthy
child.
Conclusion
In genetically complex scenarios such as CPT2 deficiency, the collaboration of obstetricians,
fetal medicine specialists, and clinical geneticists brings a complementary set of
skills and expertise that, when combined, contribute to comprehensive care for both
the mother and fetus.
