An enzymatic marker in mothers of trisomy 21 children: neutrophil alkaline phosphatase

Down syndrome screening marker levels in women with a previous aneuploidy pregnancy

BACKGROUND

In Down syndrome screening programmes, women with a previous affected pregnancy are assumed to have the same marker distribution as those without a family history. This assumption needs to be tested.

METHODS

Information on previous aneuploidy pregnancies was routinely sought on the test request forms in three centres, Leeds, Romford and the Fetal Medicine Centre, London. For each woman with a previous aneuploidy (case), five unaffected pregnancies to women without a history were selected as controls. The markers tested included maternal serum free beta-human chorionic gonadotrophin (hCG), pregnancy-associated plasma protein A (PAPP-A), alpha-fetoprotein, unconjugated estriol and ultrasound nuchal translucency thickness.

RESULTS

There were 375 cases: 303 with previous Down syndrome, 63 with Edwards syndrome and 9 with Patau's syndrome. There was a statistically significant difference between cases and controls, in the distribution of free beta-hCG and PAPP-A levels, adjusted for gestation. On average, free beta-hCG was increased by 10% in a subsequent pregnancy after aneuploidy (p < 0.005, Wilcoxon rank sum test) and for PAPP-A the increase was 15% (p < 0.0001). No other marker was significantly different.

CONCLUSION

Risk calculation algorithms need to be modified to take account of the increased marker levels. Until data from sufficient affected pregnancies are available for study, it would be prudent to assume that the same increase as in unaffected pregnancies applies.

Atypical neutrophil alkaline phosphatase associated with impaired neutrophil functions

We report the case of a healthy young man presenting with atypical neutrophil alkaline phosphatase (NAP) and reduced neutrophil chemotactic activity, but with no susceptibility to infection. NAP activity was low, kinetic parameters were modified and immunoreactive properties and subcellular distribution were abnormal. Neutrophil morphology was normal. A similar pattern was observed in the patient's healthy brother. The profile of the observed anomalies offers some similarity to that previously described in patients with chronic myelogenous leukaemia. However, in the present case, the NAP deficiency with impaired neutrophil function was present in two brothers with no haematological symptoms and is probably related to a non-acquired neutrophil abnormality. This observation of a primary NAP variant reinforces the hypothesis of a direct link between NAP activity and functional properties of neutrophils.

Interpretation and clinical significance of alkaline phosphatase isoenzyme patterns

Alkaline phosphatase (ALP, EC 3.1.3.1) is a membrane-bound metalloenzyme that consists of a group of true isoenzymes, all glycoproteins, encoded for by at least four different gene loci: tissue-nonspecific, intestinal, placental, and germ-cell ALP. Through posttranslational modifications of the tissue-nonspecific gene, for example, through differences in carbohydrate composition, bone and liver ALP are formed. Nowadays, most commercially available methods for separating or measuring ALP isoenzymes are easy to perform and sensitive and allow for reproducible and quantitative results. As more isoenzymes and isoforms have been characterized, confusion has arisen due to the many different names they were given. For the sake of simplicity and because of structural analogies, we propose an alternative nomenclature for the ALP isoenzymes and isoforms based on their structural characteristics: soluble, dimeric (Sol), anchor-bearing (Anch), and membrane-bound (Mem) liver, bone, intestinal, and placental ALP. Together with lipoprotein-bound liver ALP and immunoglobulin-bound ALP, these names largely fit the many forms of ALP one can encounter in human serum and tissues. The clinically relevant isoenzymes are sol-liver, Mem-liver, lipoprotein-bound liver, and Sol-intestinal ALP in liver diseases, and Sol-bone and Anch-bone ALP in bone diseases. Many different isoenzyme patterns can be found in malignancies and renal diseases. This test provides the clinician with valuable information for diagnostic purposes as well as for follow-up of patients and monitoring of treatment. However, ALP isoenzyme determination will only provide clinically useful information if the patterns are correctly interpreted. In this respect, care should be taken to use the proper reference ranges, taking into account the age and sex of the patient. A normal total ALP activity does not rule out the presence of an abnormal isoenzyme pattern, particularly in children. Separating ALP into its isoenzymes adds considerable value to the mere assay of total ALP activity.

Kinetics and characteristics of bovine neutrophil alkaline phosphatase during acute Escherichia coli mastitis

Alkaline phosphatase activity of isolated bovine blood neutrophils was investigated before and during experimentally induced Escherichia coli mastitis. Activities markedly increased 1 wk after infection in neutrophils of cows suffering from moderate or from severe disease. Elevated neutrophil alkaline phosphatase activity did not correlate with a specific stage of maturating postmitotic neutrophils appearing in circulation during mastitis. Neutrophil alkaline phosphatase from healthy cows before infection and from mastitic cows was characterized by means of thermostability, specific inhibitor patterns, slab gel electrophoresis, and kinetic parameter analysis. The leukocyte enzyme from healthy and mastitic cows displayed very similar characteristics, suggesting that the increase in activity during mastitis is most probably related to the enhanced expression of the normal alkaline phosphatase enzyme.

Heat resistance, immunological and quantitative changes of neutrophil alkaline phosphatase in trisomy 21 pregnancies

Neutrophil alkaline phosphatase (NAP) was analysed in 25 pregnant women with trisomy 21 foetuses whose chromosomal aberration was recognized by cytogenetic study after amniocentesis. Enzyme investigation was performed at 20-22 weeks of gestation using cytochemical and biochemical techniques. Twenty-nine women at the same stage of normal pregnancies were selected as controls. In parallel, each mother was karyotyped. Ten subjects from each series underwent biochemical and immunological investigation: measurement of enzyme levels, thermostability study and immunological tests with alkaline phosphatase isoenzyme antibodies. NAP from pregnant women with trisomy 21 foetuses was characterized by: (1) a lower rate of enzyme activity, (2) a large amount of heat-stable enzyme (T = 56 degrees C for biochemical assays, T = 85 degrees C for cytochemical tests), and (3) a marked loss of liver antigenicity. These findings suggest the presence in trisomy 21 pregnancies of a non-specific alkaline phosphatase isoenzyme which appears as an "enzyme marker" in maternal circulating neutrophils.