Penetrance and predictive value of genetic screening in acute porphyria

Penetrance, predictive value and female patients' perspectives on genetic testing were evaluated among Finnish patients with acute porphyria. We conducted a retrospective study to evaluate prognosis among at-risk female family members depending on the primary method of diagnosis.

The penetrance was calculated among 23 genetically heterogeneous families selected from the Finnish porphyria registry (n = 515, AIP 333; VP 182). We included kindreds with ≥9 patients in a family (range 9-23 patients, total 216 AIP; 129 VP). In 2015, the registry included 164 living female subjects between 14 and 85 years of age. A questionnaire was sent to 143 women, of whom 107 (75%, AIP 67; VP 40) replied. Female at-risk relatives (AIP 54; VP 30) were divided into two groups based on the primary method of diagnosis: mutation analysis (Group A, n = 40) or biochemical analysis (Group B, n = 44).

Mean penetrance for all acute symptoms was 35% among AIP and 40% among VP families. In both study groups, the penetrance was higher among female (AIP 50%; VP 44%) than male patients (AIP 17%; VP 33%). Penetrance for hospitalized attacks was 30% among AIP families (range 10-80%, for women 41%) and 25% in VP (range 0-50%, for women 27%) demonstrating wide variations among families even with the similar genotype. Acute porphyria was diagnosed at the median age of 26 years (range 0-76 years) among female patients, commonly after the onset of acute symptoms. Diagnostic delay was an average of 7.4 years (range 1-30 years). Acute symptoms occurred at the median age of 24 years (range 10-57 years) and the first hospitalization at the median age of 26.5 years (range 15-57 years). At the onset of symptoms, 38% of the women were ≤ 20 years of age. According to the life table analysis, acute attacks occurred mainly during the following five years after the diagnosis and the attack risk diminished after 35 years of age. The annual risk for hospitalization due to an acute attack during fertile years was lower in Group A than Group B (0.002 vs. 0.010, p = .018), but the risk of all subsequent acute symptoms did not diminish (Group A 0.017 vs. Group B 0.019, p = .640). The cumulative risk of acute symptoms among asymptomatic patients at the time of diagnosis was 26.7% for Group A and 58.3% for Group B. The cumulative risk of the first subsequent attack requiring hospitalization after the diagnosis among all at-risk relatives was similarly less frequent in Group A than in Group B (OR 0.180; 95% CI 0.041-0.789, p = .041). If attacks were followed among symptomatic patients only, attack-free years were more frequent in Group A than in Group B. Patients preferred genetic screening before puberty to minimize the risk of acute symptoms and genetic discrimination was rare. 44% of the patients reported social, psychological or physical impairment due to acute hepatic porphyria, emphasizing the importance of supporting patients' emotional and resilience capacity.

Among female at-risk relatives the annual risk for hospitalization due to an acute attack is <1% and for acute symptoms <2% during the fertile years. Genetic testing of relatives diminishes the risk of acute attacks. Diagnosis before symptom onset is key for subjects to remain asymptomatic during follow-up, and genetic screening should be done earlier than currently.