Dizygotic twinning

The effect of birthweight on childhood cognitive development in a middle-income country

BACKGROUND

Intra-uterine growth is a powerful predictor of infant mortality and of health, developmental and socioeconomic outcomes in adulthood. The question about whether this relationship is causal rather than driven by unobserved characteristics of low-weight infants is, however, still open. We use twin models to examine the hypothesis that in utero growth has a detrimental impact on cognitive development in childhood.

METHODS

We merge birth registry information on birthweight with standardized Math and Spanish test scores for all fourth graders in Chile to create a prospective data set. Twin fixed-effects models are used to estimate the causal effect of intra-uterine growth on test scores. Fixed-effect estimates are compared with traditional regression results in a cross-section of births to gauge the omitted variable bias emerging from unobserved genetic, maternal and pregnancy-related factors in cross-sectional models.

RESULTS

Birthweight differences within twin pairs have a substantial effect on test scores. A 400-g increase in birthweight results in a 15% standard deviation increase in Math scores. The effect is larger among (estimated) monozygotic than dizygotic pairs, reaching >20% standard deviation. The effect varies across family socioeconomic status. It is strong among disadvantaged families but it nearly disappears among advantaged ones.

CONCLUSION

Scarcity of uterine resources resulting in intra-uterine growth restriction has a detrimental effect on cognitive development in childhood. This effect interacts with family socioeconomic status (SES), so that low-SES families reinforce the effect of low birthweight and high-SES families fully compensate for it. Findings are particularly relevant in the developing world, where intra-uterine growth restriction is the main determinant of low birthweight.

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.

A genome wide linkage scan for dizygotic twinning in 525 families of mothers of dizygotic twins

BACKGROUND

The tendency to conceive dizygotic (DZ) twins is a complex trait influenced by genetic and environmental factors. To search for new candidate loci for twinning, we conducted a genome-wide linkage scan in 525 families using microsatellite and single nucleotide polymorphism marker panels.

METHODS AND RESULTS

Non-parametric linkage analyses, including 523 families containing a total of 1115 mothers of DZ twins (MODZT) from Australia and New Zealand (ANZ) and The Netherlands (NL), produced four linkage peaks above the threshold for suggestive linkage, including a highly suggestive peak at the extreme telomeric end of chromosome 6 with an exponential logarithm of odds [(exp)LOD] score of 2.813 (P = 0.0002). Since the DZ twinning rate increases steeply with maternal age independent of genetic effects, we also investigated linkage including only families where at least one MODZT gave birth to her first set of twins before the age of 30. These analyses produced a maximum expLOD score of 2.718 (P = 0.0002), largely due to linkage signal from the ANZ cohort, however, ordered subset analyses indicated this result is most likely a chance finding in the combined dataset. Linkage analyses were also performed for two large DZ twinning families from the USA, one of which produced a peak on chromosome 2 in the region of two potential candidate genes. Sequencing of FSHR and FIGLA, along with INHBB in MODZTs from two large NL families with family specific linkage peaks directly over this gene, revealed a potentially functional variant in the 5' untranslated region of FSHR that segregated with the DZ twinning phenotype in the Utah family.

CONCLUSION

Our data provide further evidence for complex inheritance of familial DZ twinning.

Estimation of the contribution of non-assisted reproductive technology ovulation stimulation fertility treatments to US singleton and multiple births

Infertility treatments that include ovulation stimulation, both assisted reproductive technologies (ARTs) and non-ART ovulation stimulation, are associated with increased risks of multiple birth and concomitant sequelae and adverse outcomes, even among singletons. While a US surveillance system for ART-induced births is ongoing, no population-based tracking system exists for births resulting from non-ART treatments. The authors developed a multistage model to estimate the uncertain proportion of US infants born in 2005 who were conceived by using non-ART ovulation treatments. Using published surveillance data, they estimated proportions of US multiple births conceived naturally and by ART and assumed that the remainder were conceived with non-ART treatments. They used Bayesian meta-analyses to summarize published clinical studies on the multiple-gestation risk associated with non-ART ovulation treatments, applied a fetal survival factor, and used this multiple-birth risk estimate and their own estimate of the proportion of US multiple births attributable to non-ART ovulation stimulation to estimate the total (and, through subtraction, singleton) proportion of infants conceived with such treatments. On the basis of the model, the authors estimate that 4.6% of US infants born in 2005 (95% uncertainty range: 2.8%-7.1%) resulted from non-ART ovulation treatments. Notably, this figure is 4 times greater than the ART contribution.

Beyond parity: association of ovarian cancer with length of gestation and offspring characteristics

Women with higher parity have a lower risk of ovarian cancer possibly because of pregnancy hormones, but the specific effect of different pregnancy hormones on ovarian cancer risk is not clear. Some clarification might be gained by considering situations where hormone levels vary between pregnancies. Study participants from an Australian population-based, case-control study of epithelial ovarian cancer (2001-2005) completed a reproductive/lifestyle questionnaire. The authors included 1,203 cases and 1,286 controls with at least 1 birth and, using multiple logistic regression, calculated odds ratios and 95% confidence intervals to investigate the effects of pregnancy-related factors on cancer risk. Women who had 1 or more preterm births had higher risks of ovarian cancer than those who had only full-term births (odds ratio (OR) = 1.48, 95% confidence interval (CI): 1.02, 2.15). The authors also found that bearing only boys was associated with a 2-fold increased risk of mucinous ovarian cancer (OR = 2.19, 95% CI: 1.15, 4.17). There was no association between multiple pregnancy and ovarian cancer (for any multiple pregnancy vs. only singleton pregnancies: OR = 1.22, 95% CI: 0.74, 2.02). The results suggest that pregnancies associated with differing hormonal milieux have different effects on ovarian cancer risk and that some of these associations may vary with histologic subtype.

Early research on human genetics using the twin method: who really invented the method?

The twin method consists of a formal comparison between the resemblance between identical (monozygotic, MZ) twins and the resemblance between fraternal (dizygotic, DZ) twins for some trait of interest. It was developed between 1900 and about 1940, as more accurate tools for diagnosis of zygosity and for statistically analyzing the resemblance between relatives were built. Its early use was in the demonstration that a trait was inherited or that part of the causation of a trait was genetical, but it has now evolved to the point that twin registries constitute an important resource for the identification of specific genes and their interactions both with other genes and with the internal and external environment. Who really invented the method is still an unsettled question, which this article explores.

Ovarian aging: mechanisms and clinical consequences

Menopause is the final step in the process referred to as ovarian ageing. The age related decrease in follicle numbers dictates the onset of cycle irregularity and the final cessation of menses. The parallel decay in oocyte quality contributes to the gradual decline in fertility and the final occurrence of natural sterility. Endocrine changes mainly relate to the decline in the negative feedback from ovarian factors at the hypothalamo-pituitary unit. The declining cohort of antral follicles with age first results in gradually elevated FSH levels, followed by subsequent stages of overt cycle irregularity. The gradual decline in the size of the antral follicle cohort is best represented by decreasing levels of anti-Mullerian hormone. The variability of ovarian ageing among women is evident from the large variation in age at menopause. The identification of women who have severely decreased ovarian reserve for their age is clinically relevant. Ovarian reserve tests have appeared to be fairly accurate in predicting response to ovarian stimulation in the assisted reproductive technology (ART) setting. The capacity to predict the chances for spontaneous pregnancy or pregnancy after ART appears very limited. As menopause and the preceding decline in oocyte quality seem to have a fixed time interval, tests that predict the age at menopause may be useful to assess individual reproductive lifespan. Especially genetic studies, both addressing candidate gene and genome wide association, have identified several interesting loci of small genetic variation that may determine fetal follicle pool development and subsequent wastage of his pool over time. Improved knowledge of the ovarian ageing mechanisms may ultimately provide tools for prediction of menopause and manipulation of the early steps of folliculogenesis for the purpose of contraception and fertility lifespan extension.

BMP15 mutations associated with primary ovarian insufficiency cause a defective production of bioactive protein

Bone morphogenetic protein-15 (BMP15) is selectively synthesized by oocytes as a pre-proprotein and is considered an ovarian follicle organizer whose adequate function is critical for female fertility. Missense mutations were reported in primary ovarian insufficiency (POI) but their biological impact remained unexplored. Here, screening of 300 unrelated idiopathic overt POI women with primary or secondary amenorrhea (SA) led to the identification of six heterozygous BMP15 variations in 29 of them. All alterations are nonconservative and include one insertion of three nucleotides (p.L262_L263insL) and five missense substitutions. Except for the p.S5R located in the signal sequence, the other variants (p.R68W, p.R138H, p.L148P, and p.A180T) localize in the proregion, which is essential for the processing and secretion of bioactive dimers. The mutations p.R68W, p.L148P, and the novel p.R138H lead to marked reductions of mature protein production. Their biological effects, evaluated by a novel luciferase-reporter assay in a human granulosa cell (GC) line, were significantly reduced. Cotransfection experiments of defective mutants with equal amounts of wild-type BMP15 cDNA, thus reproducing the heterozygous state seen in patients, did not generate a complete recovery of wild-type activity. No or minor deleterious effects were detected for the variants p.L262_L263insL, p.A180T, or p.S5R. In conclusion, heterozygous BMP15 mutations associated with the early onset of overt POI lead to defective secretion of bioactive dimers. These findings support the concept that an adequate amount of BMP15 secreted in the follicular fluid is critical for female fertility. We propose to consider the screening of BMP15 mutations among the analyses for the prediction of POI risk.

Lamotrigine and twin pregnancy, incidental event or possible correlation?

Lamotrigine is frequently used to manage epilepsy during pregnancy. However, some major and minor birth defects are reported with lamotrigine monotherapy. This study is a case series report on 20 epileptic women who were receiving lamotrigine in their pregnancy. Apart from the normal pregnancies and births, in our study three patients gave birth to healthy twins, two vanished twin syndromes occurred and one patient had abortion. Twin pregnancy in association with lamotrigine therapy has not been clearly described before. Our observational study raises the idea that lamotrigine may induce twin pregnancy but larger studies need to investigate the presence or absence of such association more decently.

Twinning on the brain: the effect on neurodevelopmental outcomes

Twinning is currently considered a complex multifactorial trait. Few studies have explored how the unique genetic and environmental influences that create twinning affect phenotypes and outcomes. Previous data has shown that twins account for a significant proportion of preterm and low-birth-weight infants, who are at risk for long-term neurodevelopmental disabilities such as cerebral palsy and cognitive impairment. More recently, it has been postulated that even without these co-morbidities, twinning in and of itself may incur a neurodevelopmental disadvantage even among term newborns. The purpose of this review is to report primarily on neuromotor outcomes of twins compared to singletons. In addition, we describe specific environmental risk factors among twins which are associated with poorer outcomes. Several putative neurodevelopmental modulators are explored, including death of a co-twin, chorionicity, birth weight discordance, and twin-twin transfusion. By teasing out environmental influences that potentially influence neurocognitive outcomes, families can receive more specific counseling and developmental services can be provided to those twins at especially high risk.

Traces of embryogenesis are the same in monozygotic and dizygotic twins: not compatible with double ovulation

Common knowledge of over a century has it that monozygotic and dizygotic twinning events occur by unrelated mechanisms: monozygotic twinning 'splits' embryos, producing anomalously re-arranged embryogenic asymmetries; dizygotic twinning begins with independent ovulations yielding undisturbed parallel embryogeneses with no expectation of departures from singleton outcomes. The anomalies statistically associated with twin births are due to the re-arranged embryos of the monozygotics. Common knowledge further requires that dizygotic pairs are dichorionic; monochorionicity is exclusive to monozygotic pairs. These are fundamental certainties in the literature of twin biology. Multiple observations contradict those common knowledge understandings. The double ovulation hypothesis of dizygotic twinning is untenable. Girl-boy twins differ subtly from all other humans of either sex, absolutely not representative of all dizygotics. Embryogenesis of dizygotic twins differs from singleton development at least as much as monozygotic embryogenesis does, and in the same ways, and the differences between singletons and twins of both zygosities represent a coherent system of re-arranged embryogenic asymmetries. Dizygotic twinning and monozygotic twinning have the same list of consequences of anomalous embryogenesis. Those include an unignorable fraction of dizygotic pairs that are in fact monochorionic, plus many more sharing co-twins' cells in tissues other than a common chorion. The idea that monozygotic and dizygotic twinning events arise from the same embryogenic mechanism is the only plausible hypothesis that might explain all of the observations.

Epidemic of plurality and contributions of assisted reproductive technology therein

A commentary on contributions of ART to the pandemic of multiple gestations is presented and mechanistic aspects therein are explored.

Familial twinning and fertility in Dutch mothers of twins

We studied twinning and fertility indices in mothers with spontaneous monozygotic (MZ) and dizygotic (DZ) twins and in mothers who conceived their twins after the use of assisted reproduction techniques (ART). Participants in this study consisted of 8,222 and 5,505 women with spontaneous DZ and MZ offspring and 4,164 and 250 women with ART DZ and MZ twin pairs, respectively. Women were compared with respect to the number of sibs and offspring, the presence of other relatives with twins and the time it took to conceive the twins. We also compared familial twinning between a younger and an older age group. Women with spontaneous DZ twins more often reported female relatives with twins than those with spontaneous MZ twins. The proportion of DZ versus MZ twin offspring in relatives was also larger in women with spontaneous DZ offspring than in women with MZ offspring. The first group of women reported a shorter time to conceive. Women with ART twins had fewer sibs and offspring and less often reported relatives with twins. We did not observe that DZ twinning was more familial in women who had their twins before age 36 years compared to older women. Familial DZ twinning is clearly present in mothers of spontaneous DZ twins. The mechanisms underlying spontaneous and non-spontaneous DZ twinning are different and fertility treatment should be taken into account in any study of twinning. Twinning is not more familial in women who have their twins at a younger age.

Local effects of inbreeding on embryo number and consequences for genetic diversity in Kerguelen mouflon

A classical paradigm in population genetics is that homozygosity or inbreeding affects individual fitness through increased disease susceptibility and mortality, and diminished breeding success. Using data from an insular population of mouflon (Ovis aries) founded by a single pair of individuals, we compare embryo number of ewes with different levels of inbreeding. Contrary to expectations, ewes with the highest levels of homozygosity showed the largest number of embryos. Using two different statistical approaches, we showed that this relationship is probably caused by heterozygosity at specific genes. The genetics of embryo number coupled with cyclic dynamics could play a central role in promoting genetic variation in this population.

Relationships between multiple births and autism spectrum disorders, cerebral palsy, and intellectual disabilities: autism and developmental disabilities monitoring (ADDM) network-2002 surveillance year

Since the 1970s, the prevalence of multiple births (MBs) in the United States has increased significantly. This has been attributed, in large part, to iatrogenic MBs resulting from infertility treatments that include ovulation stimulation. A past study has indicated that children from MBs have an increased prevalence of cerebral palsy (CP). Other studies also have suggested an association between MBs and intellectual disabilities (ID) and autism spectrum disorders (ASDs); however, results have been inconsistent. From the Autism and Developmental Disabilities Monitoring (ADDM) Network, a surveillance project among several US populations, we obtained MB estimates among children born in 1994 and classified by 8 years of age as having: an ASD (n=1,626 total children from 11 sites; 50 born as part of an MB); CP (n=302 total children from 3 sites; 25 born as part of an MB); or ID (n=1,195 total children from 3 sites; 45 born as part of an MB). All three MB estimates were notably higher than age-adjusted expected estimates of naturally conceived MBs derived from 1971 US natality data. However, when MB estimates from the ADDM Network were compared with expected MB estimates derived from 1994 natality data for the states corresponding to the relevant ADDM Network sites, we observed no association with ASDs (observed/expected=1.08 [0.78-1.38]), a moderate, but not statistically significant association with ID (observed/expected=1.34 [0.95-1.73]), and a strong association with CP (observed/expected=2.96 [1.80-4.12]). Further investigation of specific types of MBs (natural vs. iatrogenic) is warranted.