Preclinical diagnosis of pseudoxanthoma elasticum – methodological restrictions and ethical problems

ARTICLE

Pseudoxanthoma elasticum (PXE), an inherited disorder affecting mainly elastic fibres of the skin, eyes, and cardiovascular system, was first described by Darier about a century ago [1]. Cutaneous changes involve the skin of neck, axillae, and other body folds and present as yellowish papules and plaques. Ocular features include the so-called angioid streaks which represent cracks of Bruch's membrane behind the retina due to alterations of elastic fibres. Scotomas can result from retinal haemorrhage or from development of subretinal membranes. Potentially life-threatening manifestations of PXE are associated with blood vessel involvement. Splitting of the internal elastic lamina leads to arteriosclerosis or thrombosis of peripheral or cardiac vessels, and myocardial ischemia may cause early death [2-5]. Other complications like gastrointestinal bleeding have been reported as well [6]. In individual patients, clinical expression and sequence of symptoms may display considerable variability, and disease mechanisms are still unclarified [4, 7]. Actually, elastic as well as collagen fibrils and other matrix components are involved [8-12]. Details of their interactions and interdependence in PXE remain to be elucidated as discussed at a recent PXE-symposium [13].

Genetics and preclinical diagnosis

A locus for PXE had already been mapped to chromosome 16p13.1 [14, 15]. But only recently, different mutations in the MRP6 gene on this chromosome encoding a transmembrane ATP-binding cassette transporter have been identified in several PXE families [16, 17]. These findings represent an important step towards clarifying the pathogenesis and the clinical variability of PXE. Earlier, a mutation of the gene for elastin, the most prominently defective component in PXE, could not be demonstrated [18]. The hypothesis was favoured that the genetic defect of PXE would concern regulatory or controlling mechanisms of connective tissue metabolism. Supporting this suggestion are the findings that dermal fibroblasts from patients with PXE displayed abnormal cell-cell and cell-matrix interactions together with increased proliferation compared to fibroblasts from healthy donors [19].

Heterozygous carriers of the newly localized mutations could be identified, who were clinically unaffected by PXE, suggesting an autosomal recessive inheritance pattern [17]. In the literature, autosomal dominant and recessive forms of the disease have been documented, but also complex inheritance patterns [7, 20-22]. Numerous cases seem to occur sporadically or mild cases escape diagnosis [13]. On the other hand, there are families with autosomal recessive PXE with a clearly recessive inheritance of the ocular and dermatological symptoms, while the effect on the vascular system is dominant [15]. Whether additional or modifying genes are involved in the pathogenesis of PXE, remains to be elucidated. The basis for the obvious clinical variability of symptoms represents another as yet unsolved question.

The recent progress in identifying a PXE gene has yielded the prerequisites for development of predictive and preclinical testing. However, genetic counselling of affected families is difficult and even more complicated in late onset PXE and family members with a lethal outcome due to the disease. With genetic testing as yet unavailable, early preclinical diagnosis of PXE in children of affected families has been suggested to be facilitated by regular clinical check-ups, with special attention to the organs primarily involved and, in the absence of obvious signs and symptoms, ultrastructural analysis of overtly normal skin of predilection sites and in scar tissue has been proposed [3, 7, 9]. Recently, results of light and electron microscopy of skin biopsies from apparently healthy PXE family members have been compared to haplotype analysis of markers near the PXE gene locus on chromosome 16p. The individuals who were heterozygous carriers of a mutation in the PXE gene showed morphological alteration of collagen bundles and elastic fibres which were similar but less severe than in PXE patients [23]. We report here on a mother with PXE who was referred to us from the department of human genetics for early diagnosis of her three clinically normal daughters, for the purpose of subsequent genetic counselling.

Case report

The 43 year old female patient presented with PXE, diagnosed only 3 years earlier after a retinal haemorrhage when typical angioid streaks were found on fundoscopy. At that period, she already suffered from arteriosclerosis of the leg arteries, with symptoms of claudicatio intermittens. Since her youth, yellowish skin changes had been present on the sides of her neck. She had lost contact with her paternal family and only knew that her father died when he was 59 years old. A sister and a brother of her mother were affected by mental retardation of unknown cause. To her knowledge, mother and father were not related. Skin changes like hers had not been noted in other family members. Two brothers had died in early adulthood - one of them at the age of 39 years during heart surgery - and might have been affected by PXE as well. Medical records were not available. A skin biopsy from the neck of the patient was processed as described previously [9] and showed the characteristic changes of PXE, with degeneration and calcification of the elastic fibres and - only evident on ultrastructural analysis - alterations of collagen fibrils as well (Figs. 1A and 1B). She strongly wanted a preclinical diagnostic procedure for her three daughters (22, 21 and 9 years old) who did not show any symptoms or signs of PXE. Skin biopsies from the neck were performed. On light microscopy, no alterations were noted. Ultrastructural examination of all biopsies (for preparation see [9]) however revealed morphological abnormalities of collagen fibrils, similar to those observed in the mother's biopsy (Fig. 2). Alterations of the elastic fibres could not be detected so that the diagnosis of PXE could neither be confirmed, nor altogether rejected. Changes of collagen fibrils, especially in bundles adjacent to calcified elastic fibres, are common in PXE [9]. These alterations of collagen fibrils may be non-specific and present a secondary phenomenon. However, in the light of the recent progress in identifying a PXE gene and heterozygous carriers it cannot be completely ruled out that the daughters might be grouped among those and thus be able to pass on the disease to the next generation. Genetic examination of the gene locus would be desirable for diagnosis of the daughters.

Discussion

Predictive DNA testing of heritable disorders with known gene defect at the molecular level is nowadays a routine procedure from the technical point of view and becomes available for an ever increasing number of inherited diseases. After identification of several mutations in the MRP6 gene on chromosome 16p13.1 in PXE families a decisive step towards predictive and prenatal DNA testing has been achieved [16, 17]. Before this newly obtained knowledge of the PXE genetics, ultrastructural alterations of the connective tissue in seemingly normal skin of the predilection sites have lead to early diagnosis in a preclinical stage in some cases of PXE [9]. But with only discrete and possibly nonspecific findings, as was the case in the family presented here, they might be of no help in genetic counselling and might even increase concern and anxiety of the potentially involved persons. Additional information resulting from recognising an underlying mutation within a PXE gene will be helpful, even if the pathogenesis of PXE remains to be clarified.

On the other hand, predictive or preclinical diagnosis of incurable heritable disorders remains controversial in principle. Possibly affected individuals have to cope with the psychological distress associated with testing, regardless of the outcome. They may be afflicted by a sense of guilt towards other family members, irrespective of whether they are carriers or not. Test results may promote speculations about carriership of relatives. In addition, questions of partnership, family planning, and abortion are closely related to the issue of preclinical testing. Furthermore, legal consequences regarding employment and insurance have to be considered. There is also a problem regarding discretion and confidentiality, particularly when tests are performed by public or commercial laboratories. These problems are discussed particularly for autosomal dominant, late-onset disorders like Huntington`s chorea, hereditary cancers, and Alzheimer disease [24-30]. As long as effective therapies are not available and prevention is confined to very limited measures or to abortion, affected individuals are at risk of genetic discrimination concerning life style, career, and legal aspects. Nevertheless, many patients strongly ask for a preclinical diagnosis for themselves or family members.

Therefore, in order to develop successful therapeutic strategies, means of early diagnosis and a detailed knowledge of the molecular gene defects and the pathogenesis of the particular disorder are a basic prerequisite. Expectations concerning gene therapy have often been diffuse and overestimated during recent years [28, 31]. Genetic intervention at the germline level still bears the risk of incalculable side effects, uncertain curative outcome, and unpredictable long term consequences. Preimplantation screening which might prevent a later abortion is not allowed in many countries. Both methods implicate ethical issues concerning genetic instrumentalisation and manipulation. These cannot be solved by prohibitive legislation but must instead be thoroughly discussed from the scientific, ethical, and political point of view before distinctive and appropriate legislative measures may minimise the risk of abuse of genetic engineering.

Article accepted on 19/7/00

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