Neurological manifestations and molecular genetics of acute intermittent porphyria in North Western Russia

Abstract

Acute intermittent porphyria (AIP, MIM #176000) is an inherited metabolic disease due to a partial deficiency of the third enzyme, hydroxymethylbilane synthase (HMBS, EC: 4.3.1.8), in the haem biosynthesis. Neurological symptoms during an acute attack, which is the major manifestation of AIP, are variable and relatively rare, but may endanger a patient's life.

In the present study, 12 Russian and two Finnish AIP patients with severe neurological manifestations during an acute attack were studied prospectively from 1995 to 2006. Autonomic neuropathy manifested as abdominal pain (88%), tachycardia (94%), hypertension (75%) and constipation (88%). The most common neurological sign was acute motor peripheral neuropathy (PNP, 81%) often associated with neuropathic sensory loss (54%) and CNS involvement (85%). Despite heterogeneity of the neurological manifestations in our patients with acute porphyria, the major pattern of PNP associated with abdominal pain, dysautonomia, CNS involvement and mild hepatopathy could be demonstrated. If more strict inclusion criteria for biochemical abnormalities (>10-fold increase in excretion of urinary PBG) are applied, neurological manifestations in an acute attack are probably more homogeneous than described previously, which suggests that some of the neurological patients described previously may not have acute porphyria but rather secondary porphyrinuria. Screening for acute porphyria using urinary PBG is useful in a selected group of neurological patients with acute PNP or encephalopathy and seizures associated with pain and dysautonomia.

Clinical manifestations and the outcome of acute attacks were used as a basis for developing a 30-score scale of the severity of an acute attack. This scale can easily be used in clinical practice and to standardise the outcome of an attack. Degree of muscle weakness scored by MRC, prolonged mechanical ventilation, bulbar paralysis, impairment of consciousness and hyponatraemia were important signs of a poor prognosis. Arrhythmia was less important and autonomic dysfunction, severity of pain and mental symptoms did not affect the outcome. The delay in the diagnosis and repeated administrations of precipitating factors were the main cause of proceeding of an acute attack into pareses and severe CNS involvement and a fatal outcome in two patients. Nerve conduction studies and needle EMG were performed in eleven AIP patients during an acute attack and/or in remission. Nine patients had severe PNP and two patients had an acute encephalopathy but no clinically evident PNP. In addition to axonopathy, features suggestive of demyelination could be demonstrated in patients with severe PNP during an acute attack. PNP with a moderate muscle weakness was mainly pure axonal. Sensory involvement was common in acute PNP and could be subclinical. Decreased conduction velocities with normal amplitudes of evoked potentials during acute attacks with no clinically evident PNP indicated subclinical polyneuropathy. Reversible symmetrical lesions comparable with posterior reversible encephalopathy syndrome (PRES) were revealed in two patients' brain CT or MRI during an acute attack. In other five patients brain MRI during or soon after the symptoms was normal. The frequency of reversible brain oedema in AIP is probably under-estimated since it may be short-lasting and often indistinguishable on CT or MRI.

In the present study, nine different mutations were identified in the HMBS gene in 11 unrelated Russian AIP patients from North Western Russia and their 32 relatives. AIP was diagnosed in nine symptom-free relatives. The majority of the mutations were family-specific and confirmed allelic heterogeneity also among Russian AIP patients. Three mutations, c.825+5G>C, c.825+3_825+6del and c.770T>C, were novel. Six mutations, c.77G>A (p.R26H), c.517C>T (p.R173W), c.583C>T (p.R195C), c.673C>T (p.R225X), c.739T>C (p.C247R) and c.748G>C (p.E250A), have previously been identified in AIP patients from Western and other Eastern European populations. The effects of novel mutations were studied by amplification and sequencing of the reverse-transcribed total RNA obtained from the patients' lymphoblastoid or fibroblast cell lines. The mutations c.825+5G>C and c.770T>C resulted in varyable amounts of abnormal transcripts, r.822_825del (p.C275fsX2) and [r.770u>c, r.652_771del, r.613_771del (p.L257P, p.G218_L257del, p.I205_L257del)]. All mutations demonstrated low residual activities (0.1-1.3 %) when expressed in COS-1 cells confirming the causality of the mutations and the enzymatic defect of the disease.

The clinical outcome, prognosis and correlation between the HMBS genotype and phenotype were studied in 143 Finnish and Russian AIP patients with ten mutations (c.33G>T, c.97delA, InsAlu333, p.R149X, p.R167W, p.R173W, p.R173Q, p.R225G, p.R225X, c.1073delA) and more than six patients in each group. The patients were selected from the pool of 287 Finnish AIP patients presented in a Finnish Porphyria Register (1966-2003) and 23 Russian AIP patients (diagnosed 1995-2003). Patients with the p.R167W and p.R225G mutations showed lower penetrance (19% and 11%) and the recurrence rate (33% and 0%) in comparison to the patients with other mutations (range 36 to 67% and 0 to 66%, respectively), as well as milder biochemical abnormalities [urinary porphobilinogen 47±10 vs. 163±21 mol/L, p<0.001; uroporphyrin 130±40 vs. 942±183 nmol/L, p<0.001] suggesting a milder form of AIP in these patients. Erythrocyte HMBS activity did not correlate with the porphobilinogen excretion in remission or the clinical of the disease. In all AIP severity patients, normal PBG excretion predicted freedom from acute attacks. Urinary PBG excretion together with gender, age at the time of diagnosis and mutation type could predict the likelihood of acute attacks in AIP patients.

Neurologiset löydökset ja molekyyligenetiikka äkillisessä jaksottaisessa porfyriassa Lounais-Venäjän alueella. Äkillinen jaksottainen porfyria kuuluu hemin biosynteesihäiriöihin, jotka ovat perinnöllisiä aineenvaihdunsairauksia. Lääkkeet, paasto tai hormonaaliset muutokset elimistössä lisäävät hemin muodostumista maksassa ja entsyymin puutteesta kärsivät potilaat keräävät porfyriinien esiasteita kehoonsa. Nämä esiasteet ovat myrkyllisiä hermokudokselle. Tauti ilmenee siten äkillisenä kohtauksena, joka alkaa usein lievillä psyykkisillä oireilla kuten unettomuudella ja levottomuudella ja etenee autonomisen hermoston häiriöihin, jossa tyypillisimpänä oireena on voimakas vatsakipu. Jos kohtaus etenee, potilaalle voi kehittyä lihasheikkoutta, kouristuksia ja tajunnantason laskua, ja siten se voi olla henkeä uhkaava tilanne. Nykyisin varhaisen diagnostiikan ja hoidon seurauksena neurologiset ilmentymät ovat harvinaisia, mutta osa äkillistä porfyriaa sairastavista potilaista on selvästi alttiimpia halvauksille. Akuutti porfyria varmistetaan virtsanäytteestä, josta mitataan porfyriinien esiasteet. Kohtauksen aikana ne ovat selkeästi koholla ja diagnoosi varmistetaan myöhemmin geenitestillä. Tämän tutkimuksen tarkoituksena oli selvittää äkillistä porfyriaa sairastavien esiintyvyys Pietarin alueella. Seuloimme 108 :sta äkillistä hermotulehdusta sairastavasta potilaasta 12 äkillistä jaksoittaista porfyriaa sairastavaa potilasta porfyrian esiasteita mittaavan virtsakokeen avulla. Tutkimme nämä potilaat huolellisesti kliinisen ja hermojohtumistutkimuksen avulla ja teimme heille myös aivojen tietokone- tai magneettitutkimuksen selvittääksemme taudin hermohäiriöiden mekanismeja. Potilaiden hermotulehduksessa kärsii useinmiten sekä hermoimpulssin johtuminen mutta myös sen muodostuminen. Kohtauksen aikana potilaille voi kehittyä ohimenevä aivojen takaosiin painottuva nestekertymä, joka voi aiheuttaa epileptisiä kohtauksia ja tajunnan tason laskua. Kaksi potilasta menehtyi kohtauksen aikana sydän pysähdykseen ja keuhkokuumeeseen.

Löysimme myös uusia porfyriaa sairastavia sukulaisia geenitestin avulla. Yhdeksän mutaation tunnistamisen jälkeen selvitimme miten geenivirhe vaikuttaa lähettiRNA:n ja muodostuvan entsyymin toimintaan koeolosuhteissa. Mutaatiot olivat kolmessa perheessä paikallisia ja aikaisemmin tuntemattomia, mutta kahdeksassa suvussa oli jo muualla Euroopassa ja Suomessa tunnistettuja mutaatioita. Länsi- eurooppalaisten mutaatioiden kulkeutuminen Pietariin selittynee sillä, että se on ollut kansainvälinen kaupunki jo 300 vuotta. Kun yhdistimme suomalaisten ja lounais-venäläisten potilaiden kliiniset tiedot, biokemialliset tulokset ja geenivirheiden analyysit, saatoimme luokitella potilaat mutaatioiden perusteella myös kliinisiin ryhmiin. Jotkut mutaatiot aiheuttivat selvästi vähemmän oireita kuin toiset. Lisäksi virtsan porfyriinien esiasteiden määrää oireettomassa vaiheessa ja mutaation laatua voidaan käyttää ennustettaessa taudin luonnollista kulkua yksittäisen potilaan kohdalla.