None of the authors reported a conflict of interest.
Prenatal testing for Huntington's disease in the Netherlands from 1998 to 2008
Article first published online: 27 MAR 2013
© 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
Special Issue: BRCA1 and BRCA2
Volume 85, Issue 1, pages 78–86, January 2014
How to Cite
Prenatal testing for Huntington's disease in the Netherlands from 1998 to 2008., , , , , , , , , , , , , , , .
- Issue published online: 12 DEC 2013
- Article first published online: 27 MAR 2013
- Accepted manuscript online: 25 JAN 2013 09:41AM EST
- Manuscript Accepted: 31 DEC 2012
- Manuscript Revised: 27 DEC 2012
- Manuscript Received: 6 DEC 2012
- continued pregnancy;
- exclusion test;
- exclusion-definitive test;
- grey test result;
- Huntington's disease;
- intermediate allele;
- prenatal diagnosis;
- reproductive decision making;
This study aims to give an overview of the number of prenatal tests for Huntington's disease (HD), test results, and pregnancy outcomes in the Netherlands between 1998 and 2008 and to compare them with available data from the period 1987 to 1997. A total of 126 couples underwent prenatal diagnosis (PND) on 216 foetuses: 185 (86%) direct tests and 31 (14%) exclusion tests. In 9% of direct tests the risk for the foetus was 25%. Four at-risk parents (4%) carried intermediate alleles. Ninety-one foetuses had CAG expansions ≥36% or 50% risk haplotypes: 75 (82%) were terminated for HD, 12 (13%) were carried to term; four pregnancies were miscarried, terminated for other reasons or lost to follow-up. Unaffected pregnancies (122 foetuses) resulted in the birth of 112 children. The estimated uptake of PND was 22% of CAG expansion carriers (≥36 repeats) at reproductive age. PND was used by two new subgroups: carriers of intermediate alleles and 50% at-risk persons opting for a direct prenatal test of the foetus. A significant number of HD expansion or 50% risk pregnancies were continued. Speculations were made on causative factors contributing to these continuations. Further research on these couples' motives is needed.
Huntington's disease (HD) is an incurable, autosomal dominant inherited, neurodegenerative disorder. The disease is characterized by progressive chorea, cognitive impairment and psychiatric disturbances . Onset of symptoms is usually between the ages of 35 and 44, and life expectancy at diagnosis is around 20 years. The HD gene was localized on chromosome 4 in 1983, enabling predictive linkage testing within families . A decade later the HD causing genetic defect, an expanded trinucleotide (CAG) repeat in the HTT gene, was discovered , creating the possibility of performing individual pre-symptomatic testing (PT) in persons at risk. A CAG repeat size of up to 26 trinucleotides is considered to be normal. Intermediate alleles range from 27 to 35 repeats and are generally not associated with HD symptoms, but the CAG repeat can increase if the allele is transmitted to offspring. Alleles with 36–39 CAG repeats are associated with a reduced penetrance and alleles with a repeat size ≥40 are invariably associated with HD.
Preventing transmission of HD to offspring can be facilitated by prenatal testing of foetal DNA obtained through chorionic villus sampling or amniocentesis. The most frequently applied method is direct testing of the repeat size with a view of terminating the pregnancy if the foetus shows a CAG repeat size associated with HD (>35 repeats). If the at-risk parent prefers not to have his/her HD expansion status defined, exclusion testing can be performed by linkage analysis. The presence of one of the chromosome 4 haplotypes of the affected grandparent in the foetus is associated with a 50% risk of developing HD, equal to the at-risk parent . If the foetus carries this so-called ‘50% risk haplotype’ the pregnancy is expected to be terminated. If, on the other hand, a couple objects to terminating a 50% risk pregnancy, an exclusion-definitive test may be used to determine the actual HD status of the foetus and at-risk parent . Pre-implantation genetic diagnosis (PGD) may be an alternative for couples who are reluctant to terminate the pregnancy (TOP) for late-onset disorders such as HD or a 50% HD risk. By selecting in vitro fertilization (IVF) embryos without the CAG repeat expansion or the 50% risk haplotype (exclusion PGD), transmission of HD to offspring can be avoided [6-8].
This study aims to evaluate the number of prenatal tests, test results, and pregnancy outcomes in relation to repeat size of the at-risk parent in the Netherlands between 1998 and 2008 and compare these data with the preceding decade .
Materials and methods
DNA analysis for HD in the Netherlands is centrally performed in the Laboratory for Diagnostic Genome Analysis in Leiden (LDGA). The results of all prenatal tests from 1998 to 2008 were retrospectively collected from the LDGA and, if applicable, the date of PT and CAG repeat size of the at-risk parent. Eight Dutch university departments of clinical genetics offer genetic counselling around pre- and postnatal testing for HD. Additional clinical information was collected in these centres in cooperation with the clinical geneticists involved. These data included information on gender and age of at-risk parents, reproductive history, and prenatal tests during the study period, including test results and pregnancy outcomes. All results were compared with those obtained in the Netherlands from 1987 to 1997 .
To estimate the uptake of couples applying for prenatal diagnosis (PND), our study population was compared with the number of HD carriers of reproductive age (arbitrarily set at females ≤40 years and males ≤50 years) undergoing PT in an 11-year period starting in October 1996.
A total of 126 at-risk couples opted for PND in the period 1998–2008. More than half of the at-risk parents (52%) were female (Table 1). The majority, 71% (89/126) of at-risk parents, had undergone PT for HD before their first prenatal test. Four individuals had PT at the same time as their first PND with the intention of having their carrier status defined independently of the PND result. For two of these couples, the first PND (and PT) had taken place prior to the study period. Significantly more females than males had performed a PT prior to or simultaneous to their first PND (83% of females vs 63% of males, p = 0.01). Thirty-three at-risk partners (26%), preponderantly males (M/F ratio 22/11), preferred not to know their HD expansion carrier status. None of the at-risk individuals had been diagnosed as clinically affected.
n = 126
|Gender at-risk partner/carrier CAG expansion|
|At-risk parent undergoing PT prior to/or at same time as first PND|
|At-risk parents not wishing to know their HD status: male/female||22/11|
|Age men at PT (if prior to first PND)||30.3 (36)||4.9|
|Age women at PT (if prior to first PND)||26.3 (53)||5.1|
|Male age at first PNDa||33.0 (120)||9.8|
|Female age at first PNDa||30.0 (126)||4.5|
|Untested children prior to first PND||25 (19)||—|
|Pregnancies with PND prior to study period||21 (13)||—|
Fifteen percentage of couples (n = 19) had had one or more untested children before their first PND. In most cases, the HD diagnosis of a family member was not known before the pregnancy. Thirteen couples had a total of 21 prenatal tests prior to the study period.
In the study period, PND was performed in two pregnancies achieved by PGD to confirm the PGD result. The prior risk of showing HD after PGD is <1% ; therefore, these PND tests were excluded from the analysis.
Prenatal test results and pregnancy outcomes
A total of 216 prenatal tests were performed in 214 pregnancies in 126 couples (including 2 dichorionic twin pregnancies, Table 2). Direct tests were performed in 86% of PND (n = 185). In the remaining 14%, exclusion testing was performed (n = 31). For 168 direct tests (91%) the prior HD risk was 50%, 17 (9%) had a 25% prior risk for the foetus. Four at-risk parents whose carrier statuses were defined (after exclusion-definitive or direct PND testing for 25% HD) subsequently used direct PND. PND in 91 foetuses showed a CAG expansion ≥36 or a 50% risk allele (after exclusion testing). The majority of these pregnancies (82%, 75/91) were terminated, two ended in spontaneous abortion, one was terminated for congenital anomalies, and one was lost to follow-up. In all, 13% (12/91) of the foetuses with a CAG expansion or 50% risk allele were carried to term. These continued pregnancies will be described in more detail later. PND had ruled out or excluded the transmission of HD in 89% of the live born children (112/126). Two pregnancies at 50% prior risk were continued without obtaining the PND result. A total of 125 pregnancies (including one twin pregnancy) resulted in the birth of one or more liveborn children in 100 couples.
The intermediate alleles in eight foetuses (Table 2) were identical to the at-risk parents' shortest CAG repeat in four cases, whereas in two cases the intermediate alleles originated from the healthy spouse. On one occasion the intermediate allele was identical to the at-risk parent's longest allele consisting of 28 CAG repeats (simultaneously tested). The origin of one intermediate allele (27 repeats) remains unknown, because the parents were not tested.
Disclosure of HD carrier status of at-risk parents
An overview of all individuals with their carrier status as well as the number of prenatal tests is given in Table 3. In the group of 91 persons with a known HD status at the time of their first PND there were 77 carriers of a full penetrance allele (85%), 10 (11%) had a reduced penetrance allele and 4 (4%) had an intermediate allele. In the latter group two males had received this ‘grey’ PT result unexpectedly . They were only prepared to receive a ‘positive’ or ‘negative’ result', and decided to undergo PND to avoid any chance of having an HD-affected child. A third male belonged to a family with one documented expansion of an intermediate allele into the full penetrance range after paternal transmission, which may be associated with an increased expansion risk for other family members. No information was available on the female intermediate allele carrier opting for PND. Two others had PT performed simultaneously with their PND, both showing intermediate alleles. The HD status of 27% (9/33) of at-risk individuals who initially did not want to know their status was defined as the result of one or more exclusion-definitive or direct PND tests (six were confirmed by PT). Five at-risk individuals directly or indirectly proved to carry a repeat expansion associated with HD, while four at-risk individuals proved to be free of HD. One of these couples had previously terminated two pregnancies because of a 50% HD risk (retrospectively unaffected) (Table 3).
|Gender at-risk parent |
|Moment of PT||PND tests |
|Prior to PND |
|Simultaneous to or after PNDa|
|CAG expansion (longest allele) if tested: ≤26||2||1/1||2b (100%)||4|
|27–35||6||3/3||4c (67%)||2 (33%)||7|
|36–39||12||7/5||10 (83%)||2d (17%)||20e|
|≥40||80||31/49||77 (96%)||3f (4%)||145g|
|HD status disclosed indirectly|
|50% HD risk (HD-status undisclosed)||23||16/7||n.a.||34|
Continued pregnancies with CAG expansion ≥36% or 50% risk haplotype
Information on the 12 continued pregnancies of foetuses showing a CAG repeat expansion associated with HD or a 50% risk haplotype is presented in Table 4. The age of the at-risk parents did not significantly differ from the age at PND of other couples (data not shown). The proportion of continued pregnancies (Table 2) varied between the different subgroups: 7 out of 71 (9.9%) full penetrance alleles, 3 out of 8 (37.5%) reduced penetrance alleles and 2 out of 12 (50%) risk alleles. No pregnancies were terminated for an intermediate allele. Two out of four pregnancies with a prior risk of 25% and CAG repeats ≥36 indicating ‘double bad news’ were continued. In the majority (8/10) of continued pregnancies with CAG expansions, the at-risk parent was female. Six couples showed a reproductive history with TOP (n = 3), miscarriages (n = 3) or threatened abortion (n = 1). Two couples had untested children.
|Test result||Gender at-risk parent||Reproductive history||CAG repeats foetus||Specific information on continued pregnancy|
|High-risk allele after exclusion test (n = 2)||1||M||Missed abortion after first exclusion test, discovered at the moment of preparation for TOP||—||Hospital refused to offer future exclusion PND, which was notified simultaneously with PND result|
|2||M||First pregnancy||—||No additional information|
|Reduced penetrance allele (n = 3)||3||F||TOP for neural tube defect (37 CAG repeats)||37||Same repeat size (37) as previous pregnancy and similar to at-risk parent|
|4||F||TOP without PND, unplanned pregnancy?||39||CAG repeat length identical to at-risk parent|
|5||M||≥1 untested child(ren) with previous partner. After continued pregnancy again PND with normal result||38||At-risk parent (39 CAG repeats) was simultaneously tested with PND|
|Fully penetrant allele (n = 7)||6||M||Two pregnancy terminations for HD, one unaffected child and two miscarriages (one after PGD)||44||Two previous terminations showed 47 and 52 CAG repeats|
|7||F||≥1 untested children (one with previous partner)||43||At-risk parent had not performed PT prior to PND. Very disappointed after double bad news, not able to terminate|
|8||F||Two miscarriages||40||Couple had agreed to terminate if >44 repeats(> at-risk parent)|
|9||F||First pregnancy, in a subsequent pregnancy couple requested PND without intention to terminate if affected. This request was refused||40||At-risk parent showed 42 repeats. Pregnancy occurred during PGD preparation. Couple opted for PND, but a missed abortion was seen on two successive ultrasounds. A third ultrasound showed an intact pregnancy after all: PND was performed resulting in HD. Couple was not able to terminate after the disappointment of an abortion had been replaced by the relief of having an intact pregnancy|
|10||F||First pregnancy||41||Couple had agreed to terminate if 40 or more, foetus showed a repeat size of 41 only|
|11||F||First pregnancy, in a subsequent pregnancy couple requested PND without the intention to terminate if affected, which was refused||44||No additional information|
|12||F||No information available||47||No additional information|
Annual PND tests for HD performed in the Netherlands in the period 1988–2008
Figure 1a shows the yearly number of PND tests from 1988 to 2008. After the first application in 1989 there was an increase in numbers of PND for several years. Since 1996, the number of tests has remained quite stable with approximately 20 PND tests per year. The proportion of exclusion tests has gradually decreased after the introduction of direct CAG repeat testing, but the actual number of yearly intentional exclusion tests has remained fairly stable at three to four exclusion tests per year (Fig. 1b). Before 1993, when only linkage testing was available, the number of exclusion tests performed was relatively high. In the period from 1988 to 1998, in four cases, exclusion testing was the only technical option, because the haplotypes were uninformative, or there were not enough family members available  while only 13 out of 17 couples (76%) intentionally opted for exclusion testing as they did not want to know the HD expansion status of the at-risk parent. Taking this overestimation into account, we can see an even stronger stability of the absolute number of couples requesting exclusion testing.
Uptake of PND
We estimated the uptake of PND among carriers of an expanded repeat. The HD status of the 103 at-risk persons (Table 3) was (directly or indirectly) disclosed on average 2.2 years before their first PND test (range 14.41 years before to 9.07 years after PND). From this, we concluded that the 11-year period starting October 1996 was the closest way to establish the timing of PT in our study population. In this period 1414 PT were performed, in which 962 individuals were at reproductive age at the moment of PT and 406 showed a CAG repeat ≥36 (199 males <50 years and 207 females <40 years). Additionally, three untested individuals of reproductive age turned out to be HD expansion carriers after an unfavourable outcome of exclusion-definitive testing or direct PND, resulting in a total of 409 HD expansion carriers. Four out of the 93 carriers of an HD expansion ≥36 (Table 3) were excluded because they had had PT performed abroad or had been tested diagnostically. This leads to an estimated uptake of PND of 22% (89/409) among HD expansion carriers. The age at PT of males and females not opting for PND was significantly higher (34.3 and 29.4, respectively), compared with the age at PT of couples opting for PND (males 31.0 and females 27.0) (age differences: males p = 0.02; females p = 0.001).
In the 11 years from 1998 to 2008 a total of 126 couples underwent 216 prenatal tests in the Netherlands. Most couples opt for a direct test. The proportion of couples opting for an exclusion test has decreased from about 25% in the period 1987–1997  to 13.5% in this study. The proportion of exclusion testing PND vs direct testing varied from around 30% in Australia (1994–2003) , to about 10% in Belgium, 29% in France, 30% in Denmark, 42% in Italy and 48% in UK (1993–1998) [12-14]. In Germany, Switzerland, Austria, and Greece no exclusion tests were performed (1993–1998) [14-16]. The absolute number of Dutch couples requesting exclusion tests over the years, even since the availability of direct testing, has remained quite stable. One might conclude that for a subgroup of at-risk individuals this is an attractive option. Apparently, these couples prefer the risk of terminating a non-affected foetus over disclosure of their own HD status. The motives of couples opting for exclusion testing are described elsewhere .
The intermediate alleles found in 4% of direct PND tests (8/189 including four exclusion-definitive tests) may be considered a background/population risk. In most foetuses (6/8), the origin was clearly different from the HD-causing allele, and in the remaining two (27 and 28 CAG repeats, respectively) it seems unlikely that these alleles are the ones causing HD in a close relative. In the Western European population, the background prevalence of intermediate alleles (27–35 CAG repeats) is estimated at around 2–6% [18-20].
Compared with Maat-Kievit et al. , two new subgroups of PND applicants appeared. First, carriers of an intermediate allele represented 4% of individuals requesting PND. After receiving their ‘grey’ PT result , the main reason to perform PND was to eradicate HD completely. To our knowledge, PND has not been described before for intermediate alleles. However, Decruyenaere et al. described one PND test in a woman with an equivocal repeat size (27–39 repeats) which was continued because the number of CAG repeats, the exact sizes of which were not mentioned , was lower than 40. The chance of intermediate alleles expanding into a reduced penetrance allele or very rarely into the full penetrance allele ranges from 1% to 20% of transmissions [19, 21-25]. Expansion risk is associated with a longer repeat length of the at-risk parent and mainly with paternal transmission.
Second, individuals at 50% HD risk opting for direct PND (25% prior risk for the foetus) represented about 8% of PND applicants . This group represented 5% in a UK study and 37.5% in Germany, Austria and Switzerland [12, 15]. For individuals who object to terminating a 50% risk pregnancy but do accept the 25% chance of disclosure of their own HD status, this could be a valid alternative . This method may also be applied to save time [compared with the time frame needed for successive PT and PND (or exclusion-definitive testing)] if couples do not present themselves until during the pregnancy . The main disadvantage is the chance of ‘double bad news’ should the foetus turn out to carry the CAG expansion, and indirectly confirms its at-risk parent's HD status [5, 27, 28].
The majority of CAG expansion or 50% risk pregnancies was terminated; however, a substantial 13% were continued to term. According to international guidelines, continuing an CAG expansion or at-risk pregnancy of a late-onset disorder like HD can be considered an early form of predictive testing and therefore violates the future child's right not to know [8, 28, 29]. In the study period 1987–1997, all HD positive and high risk pregnancies (n = 28) were terminated. Continuation of a pregnancy with a CAG expansion or 50% HD risk has been described previously [13, 15, 30-32]. One may speculate about the risk factors making the decision to terminate such a pregnancy difficult. For some couples, the ‘double bad news’ of an HD test result in a 25% prior risk pregnancy might have complicated the decision to terminate the affected pregnancy. Furthermore, a reduced penetrance allele in a foetus may induce a sense of hope despite the disappointing test results as a whole. Individuals at risk of transmitting HD to their offspring do show a tendency to hold on to objectively rather small levels of hope as part of their coping strategy [33-35]. The observation that some couples do eventually continue a pregnancy with a smaller CAG repeat than their own – yet still a full penetrance allele – supports this idea. Therefore, if couples are determined to eradicate HD from their family, and if they expect difficulties to terminate when knowing the exact repeat length, especially in a certain range, they might prefer not to know the exact repeat size. These couples can agree with the counsellor to set a specific cut-off CAG repeat level, above which a ‘HD carrier’ result will be communicated. Naturally, the exact way of communicating the test result should be thoroughly discussed with each couple prior to the PND test. In individual cases, this form of non-disclosure of the exact CAG repeat size may reduce the reluctance of parents to terminate a pregnancy. According to Dutch law, parents are entitled to receive test results in full . Moreover, a reproductive history of miscarriages or pregnancy terminations, as well as the presence of untested children within the same family, may increase parents' reluctance to terminate another pregnancy. Confronted with an HD positive or 50% risk result, the couple may have the complex association of rejecting an already existing child or the at-risk parent when opting for a termination of the pregnancy . Furthermore, the time transpiring between PND intake and test result may affect couples' moral judgement on the acceptability of TOP . The motives of these couples and the long-term consequences of this unfavourable outcome after PND will be the subject of future study.
Compared with the period studied by Maat et al. (1987–1997) , the number of 92 prenatal tests in the Netherlands has more than doubled to 216. A similar twofold increase (11–22%) was observed in the approximated uptake of PND by HD expansion carriers . We assume that the uptake estimate in the study of Maat-Kievit et al. showed an underestimation of the actual uptake of PND because of the reported time lapse of 19 months between PT and first PND, as was reflected in the bar charts of PT applicants and prenatal tests in the Netherlands (1987–1997) [5, 39]. The uptake of PND among PT applicants with reproductive motives, in our study and in the previous study period, may be even higher; however, these motives were not centrally collected.
As PGD can be regarded as an alternative to PND it is interesting to note that after the introduction of PGD for HD in the Netherlands in 1999, the use of PND in terms of absolute figures did not decline (van Rij et al. submitted). In contrast, in France PND is only rarely performed after the introduction of PGD .
The uptake of PND by HD expansion carriers in the Netherlands is relatively high compared with France, Canada, the United States, Germany, Austria, Switzerland, Greece, Australia, and Northern-Ireland [8, 11, 14, 16, 26, 40-42] but is roughly comparable with UK, South Africa (Johannesburg), Belgium, and Denmark [12-14, 31, 43]. Since 2010, PND for HD has been prohibited in Germany; prenatal testing and termination of a pregnancy for HD are considered a form of genetic discrimination because of the late onset of HD . In our study, the motives for performing a PT were not registered systematically; therefore, we assume that the actual uptake among individuals with reproductive motives for PT will most probably be higher. Additional support for this idea is presented in another paper in the uptake of PND and PGD in the Netherlands (van Rij et al. simultaneously submitted article).
Difficulties in calculating the absolute uptake of PT with respect to the at-50% HD-risk population complicate an accurate calculation and comparison of the uptake between these populations [42, 45].
Implications for good practise
Reproductive counselling issues should be discussed according to the international guidelines . Parents opting for PND should be aware that they are expected to terminate a pregnancy should the foetus show a CAG repeat expansion associated with HD. Sequential use of PND does not imply a moral obligation to use PND in future pregnancies. The willingness and ability of couples to undergo (another) pregnancy termination should be discussed in every single PND intake. Hesitation, at any time during the procedure, should be taken seriously. Couples should be given the opportunity to withdraw from PND at any time, preferably before performing chorionic villus sampling, or at the latest before receiving the PND result.
In some cases non-disclosure of the exact CAG repeat size may reduce the reluctance of parents to terminate an affected pregnancy; this should be discussed prior to PND. Alternative reproductive options such as PGD should be considered during reproductive counselling.
The use of PND for HD in the Netherlands for both direct testing and exclusion has remained reasonably stable over the years. PND was used by two new subgroups: carriers of intermediate alleles, and couples opting for direct testing for a 25% HD risk for the foetus. A considerable number of affected or at-risk pregnancies were continued, for which we identified some risk factors. Couples' willingness to undergo TOP on account of HD should be explored thoroughly prior to and during the PND procedure.
- 1Huntington's disease. New York, NY: Oxford University Press, 2002., , , (eds).
- 10“Grasping the Grey”: patient understanding and interpretation of an intermediate allele predictive test result for Huntington disease. J Genet Couns 2012: [Epub ahead of print]., , .
- 14Predictive and prenatal testing for Huntington's disease in Greece, Germany, Austria, Switzerland and Denmark. In: Evers-Kiebooms G, ed. Prenatal testing for late-onset neurogenetic diseases. Oxford: BIOS Scientific Publishers Ltd, 2002: 69–82., , .
- 17Evaluation of exclusion prenatal and exclusion pre-implantation genetic diagnosis for Huntington's disease in the Netherlands. Clin Genet 2012: [Epub ahead of print]., , et al.
- 28Recommendations for the predictive genetic test in Huntington's disease. Clin Genet 2012: [Epub ahead of print]., , et al.
- 36Wet Geneeskundige Behandel Overeenkomst, Dutch Civil Law, BW Art. 7:458. 2011.
- 40Predictive and prenatal testing for late-onset neurogenetic diseases in North-America. In: Evers-Kiebooms G, Zoeteweij MW, Harper PS, eds. Prenatal testing for late-onset neurogenetic diseases. Oxford: BIOS Scientific Publishers Ltd, 2002: 191–201..
- 44Gesetz über genetische Untersuchungen beim Menschen (Regulation on genetic testing in humans). Art. 74 Abs. 1 Nr. 26 GG. Germany, 2009.