Genetic origin of recurrent hydatidiform moles

Hydatidiform mole (HM) is a non viable form of pregnancy with a cystic
appearance of chorionic villi, most often in the absence of an embryo. HM
occurs in approximately 1 in 1000 pregnancies. Hydatidiform moles can be
divided in complete and partial HM. Complete hydatidiform moles (CHM) commonly
have a diploid karyotype (46,XX, occasionally 46,XY), representing
reduplication of the haploid genome of the sperm in an enucleated oocyte or the
fusion of two sperms with an enucleated oocyte. Foetal development or foetal
erythroblasts are not observed in complete moles, trophoblastic proliferation
and hydropic villi are usually more prominent and diffuse. In contrast, partial
hydatidiform moles (PHM) are generally triploid (69,XXX or 69,XXY). This
results from fertilisation of a normal ovum by one sperm followed by
reduplication of the haploid paternal genome, or from dispermic fertilisation.
Foetal development, such as embryonic structures, stromal capillaries and
amniotic tissue are often present.

Following molar evacuation by uterine curettage, remaining trophoblastic tissue
resolves spontaneously in most cases. However some HM persist and even
metastasise. This is called persistent trophoblastic disease (PTD). PTD is a
clinical diagnosis and is determined by the production of serum human chorionic
gonadotropin (hCG). According to the FIGO criteria, PTD is defined as 1) a
plateau in weekly serum hCG concentrations for 3 consecutive weeks, 2) an
increase of the serum hCG level for 2 weeks and 3) persistence of detectable
hCG levels for more than 6 months after evacuation. PTD occurs in approximately
15% of all complete molar pregnancies. Less often (0.5-5.6%) partial moles
develop malignant sequelae.
Some non-gestational malignancies can also produce hCG. In most cases these can
be distinguished from a gestational trophoblastic tumour by histopathological
examination. In difficult cases, genetic testing can be useful in determining
the correct diagnosis.

Diploid moles are most often androgenetic with two identical (homozygous) or
different sets of paternal chromosomes (heterozygous) [9, 10]. Beside the more
common androgenetic diploid moles, in rare cases (4%) a diploid biparental
genome is observed. This is usually associated with familial recurrent
hydatidiform mole.
Recurrent hydatidiform mole occurs in about 1 - 6% of patients with a sporadic
hydatidiform mole. Familial biparental hydatidiform moles are usually complete.
Familial and non-familial recurrent hydatidiform moles are associated with
mutations in particular genes resulting in a defect in imprinting. Linkage to
chromosome locus 19q13.4 and autosomal recessive mutations of the NLRP7 gene in
that region have been found in a small number of families with recurrent
biparental diploid HM, although in other families with recurrent hydatidiform
mole no association with the NLRP7 gene was observed. Recent studies implicated
a second gene in this class of moles, KHDC3L (C6orf221). It is speculated that
NLRP7 and KHDC3L interact as components of an oocyte complex that is directly
or indirectly required for determination of epigenetic status on the oocyte
genome.

The goal of this study is to explore genetic mutations explaining HM. A
systematic screen of the described genes NLRP7 or KHDC3L will show the
prevalence of mutations in these genes in Dutch women with HM. By using exome
sequencing in blood of patients with recurrent HM, without NLRP7 or KHDC3L
mutations, we aim to identify novel genes involved in the development of HM.
Identification of additional candidate genes responsible for recurrent
(familial) HM will lead to a better understanding of the aetiology of HM, PTD
and possibly other forms of recurrent aberrant pregnancy.

This study has two objectives:
1) The first objective is to determine the frequency of mutations in NLRP7 and
KHDC3L in patients with recurrent HM in the Netherlands.
2) The second objective is to identify additional candidate genes associated
with (familial) recurrent HM.

The study design for the two objectives:
1) Women were identified who had two or more hydatidiform moles. Evacuation
tissue of these women will be retrospectively and prospectively collected from
the pathology laboratories of their hospitals and will be reviewed by an expert
pathologist to be certain of the diagnosis of HM. In addition, a blood sample
will be obtained from the patients. After isolating DNA from the blood samples
of the patients, the frequency of mutations in NLRP7 and KHDC3L in Dutch
patients with recurrent HM can be determined. In collaboration with the
department of Human Genetics we will apply Sanger sequencing to screen these
two known genes for point mutations.
2) In a second step we will investigate 5-10 of the patients without mutations
in the known genes NLRP7 and KHDC3L, preferably including patients with the
highest frequency of HMs. In order to identify novel genes for this disorder,
we will apply exome sequencing by using the blood that was obtained for the
first objective.

Since all patients have finished their treatments for HM, this research will
not interfere with the patients treatment schedule. The burden consists of
obtaining a blood sample from the patients and their parents.